Military Diving Medicine

Military Diving Medicine

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Chapter 26

MILITARY DIVING MEDICINERICHARD S. BROADHURST, MD, MPH; LELAND JED MORRISON, MD; CHARLES R. HOWSARE, MD; ANDANDREW F. ROCCA, MD

INTRODUCTION

MILITARY DIVING OPERATIONS

HISTORY OF DIVINGEarly Diving EndeavorsEvolution of Diving Equipment

DIVING PHYSICS AND DECOMPRESSION THEORYApplication of Gas Laws to DivingHistory and Development of Decompression ProceduresCurrent Decompression Procedures and Tables

GASES AND DIVINGOxygen DisordersCarbon Dioxide ToxicityCarbon Monoxide PoisoningNitrogen NarcosisHelium Diving

ENVIRONMENTAL HAZARDSThermal DangersAcoustic HazardsBlast InjuriesElectrical HazardsRadiation HazardsDangerous Marine Life

DYSBARIC ILLNESSDecompression SicknessPulmonary Over-Inflation SyndromeBarotraumaDysbaric Osteonecrosis

EPIDEMIOLOGY OF DIVING CASUALTIES AND DECOMPRESSION ILLNESSCivilian ExperiencesMilitary Data

PREVENTION OF DIVING-RELATED ILLNESSES AND INJURIESThe Fit DiverPersonnel and Their Roles in PreventionDiver TrainingResearch

INJURED DIVER ASSESSMENTThe EvaluationMonitoring the DiveSummary

DIVING ACCIDENT MANAGEMENTRecompression ChambersDiving Accident Investigation

SUMMARY

Military Preventive Medicine: Mobilization and Deployment, Volume 1

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R. S. Broadhurst; Colonel, Medical Corps, North Carolina Army National Guard; Commander, Company C, 161st Area SupportMedical Battalion; formerly, Lieutenant Colonel, Medical Corps, US Army, Senior Diving Medical Officer, US Army Special Opera-tions Command, Fort Bragg, NC 28307

L. J. Morrison; Captain, Medical Corps, US Navy (Retired); Chief Physician Naval Forces, United Arab Emirates; formerly, SeniorMedical Officer, Naval Diving and Salvage Training Center, 350 S. Crag Road, Panama City, FL 32407

C. R. Howsare; Medical Director, Healthforce; 210 W. Holmes Avenue, Altoona, PA 16602; formerly, Lieutenant Commander, Medi-cal Corps, US Navy, Training Medical Officer, DMO Course Coordinator, Naval Diving and Salvage Training Center, 350 S. CragRoad, Panama City, FL 32407

A. F. Rocca; Lieutenant Commander, Medical Corps, US Navy; Senior Resident, Department of Orthopedics, National Naval Medi-cal Center, Bethesda, MD 20889-5600; formerly, Director of Clinical and Hyperbaric Medicine, Naval Diving and Salvage TrainingCenter, 350 S. Crag Road, Panama City, FL 32407


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INTRODUCTION

a specialized type of occupational medicine, con-siders the medical issues involved in working inthe unique conditions of the undersea environ-ment, much as aviation medicine is concernedwith similar issues in the hypobaric realm offlight. This chapter will address the issues thatmilitary preventive medicine specialists shouldunderstand about military diving, including rel-evant physics and physiology associated withdiving, approaches to the prevention of diving ill-nesses and injuries, and the epidemiology andmanagement of diving medicine problems.

MILITARY DIVING OPERATIONS

Military diving is divided into two generalmissions, fleet diving (eg, underwater shipshusbandry, salvage, underwater construction)and specialized diving (eg, special operations,explosive ordnance disposal [EOD], saturation).Military diving is generally performed at depthsof less than 60 ft, except for some salvage, EOD,and saturation (prolonged duration) diving op-erations. Fleet diving is conducted throughoutthe US Navy and by US Army Engineer Corpsdivers, while specialized diving is performed byall four services in the Department of Defense.Special Operations Forces (SOF) in the Army in-clude Rangers and Special Forces (Green Be-rets). Combat divers in the Special Forces per-form inland search and rescue and combat searchand rescue, as well as coastal infiltration andexfiltration, reconnaissance, and demolitions.Army Ranger combat divers are currently usedonly to perform reconnaissance. The SOF ele-ments in the Air ForcePararescue Teams andCombat Controllersalso must be capable ofconducting downed-aircraft recovery and infil-tration into denied territory. Navy Sea/Air/

Land (SEAL) forces conduct all aspects of SOFdiving, including maritime direct-action opera-tions (eg, ship attacks, ship boardings, harborpenetrations, hydrographic reconnaissance,clearance of beaches for amphibious assaults,shallow-water mine countermeasures). Divers inthe Marine Corps Force Reconnaissance Unitssurvey landing beaches and conduct covert un-derwater insertions. The Coast Guard, a Navalaugmentee in time of war, uses divers for shipshusbandry, navigational buoy maintenance, andocean search and rescue. As part of what is themost extensive diving program in the US mili-tary, the Navy operates the Naval ExperimentalDiving Unit, the Naval Submarine Medical Re-search Laboratory, and the Naval Medical Re-search Center. These organizations conductmanned and unmanned testing of military div-ing equipment, decompression algorithms, andhyperbaric physiology. The Naval Safety Centersupports all diving in the Department of Defenseby managing diving data, issuing safety mes-sages, inspecting diving lockers, and investigat-ing accidents.2,3

HISTORY OF DIVING

Early Diving Endeavors

The first recorded example of military divingwas in the 5th century BC when a Greek divercut the anchor lines of Persian warships to createconfusion between vessels and cover his escape.4

Salvage diving is nearly as old, traceable to the1st century BC in the eastern Mediterranean. Overtime, the use of divers in combat operations has

evolved to include reconnaissance, covert infil-tration, ordnance disposal, obstacle removal, anddemolitions, in addition to underwater sabotage.US divers disposed of ordnance during the CivilWar, investigated the sinking of the USS Maine in1898, and performed submarine rescue and sal-vage in the early part of the 20th century. It wasthe period during and after the second World War,however, that saw rapid growth in military div-

All services in the Department of Defense haveelements actively involved in diving operations.Historically the US military, particularly the USNavy, has been a world leader in the developmentof diving and undersea technology, including div-ing medicine.1 Diving Medical Officers (DMOs),both physicians and physician assistants, andDiving Medical Technicians (DMTs) are specifi-cally trained in military diving medicine and per-form fitness-to-dive evaluations, prevent andtreat diving-related injuries, and monitor ongo-ing diving operations for safety. Diving medicine,


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ing, including the development of all types of spe-cialized diving and significant improvements infleet diving.5 All of these advances were madepossible by revolutionary improvements in div-ing technology.

Evolution of Diving Equipment

Humanitys success underwater has always de-pended on the ability to procure air or oxygen andto survive in the environment. Although the an-cients initially relied on simple breath-hold div-ing, they soon recognized the need to spendgreater time working at depth. One solution wasthe diving bell, an inverted container lowered byrope to provide air at depth and increase bottomtime. Alexander the Great is reported to have usedone in the 4th century BC.4 Diving bells were im-proved and came into widespread use from the16th to the 19th centuries to support the growingbusiness of salvage diving. Then in 1840, AugustusSiebe improved on existing diving suits, designedto protect divers from temperature and pressureextremes and other hazards, with new technologythat allowed air to be pumped from the surface todivers at depth (Figure 26-1). This surface-supplieddiving apparatus was the prototype for deep seadiving rigs, which are still used today in situationsrequiring prolonged bottom time, stationary work,or other specific circumstances.5

The next significant technological breakthroughwas the development of scuba (self-contained un-derwater breathing apparatus). In 1878, Fleussdemonstrated the first practical scuba, which wasa closed-circuit (no expired gas released into thewater), 100% oxygen rebreather rig. This technol-ogy was used to develop oxygen rebreather sys-tems for submarine escape and rescue in the early20th century. The first scuba used to a significantdegree in the US was invented in 1940 by Dr. Chris-tian Lambertsen and was a semi-rebreather rigwith steady-flow oxygen. This physician was alsoinstrumental in military diver training during andafter World War II and was an early authority indiving medicine. In World War II, scuba was usedby Navy Underwater Demolition Teams and com-bat divers from the Office of Strategic Services,both forerunners of todays SOF divers. Finally in1947, Frenchmen Cousteau and Gagnon developedthe first successful open-circuit (releases expiredgas into the water) air scuba rig, including a de-mand regulator, which supplied air only as thediver breathed. This Aqua-Lung rig conserved air

Fig. 26-1. Siebes Diving Dress and Helmet. AugustusSiebe developed the first practical diving suit. He im-proved on earlier attempts by attaching the helmet to therest of the suit and adding an exhaust valve.Source: US Department of the Navy. US Navy DivingManual. Rev 4. Washington, DC: Naval Sea Systems Com-mand; 1999: 1-5.

and increased diving time, while the use of air as abreathing gas was a safer and cheaper alternativeto oxygen. The Aqua-Lung not only provided moreopportunities in military diving, it set the scene forthe development of diving for sport and recreation.4

After World War II, researchers became inter-ested in testing human endurance under thewaves. Saturation diving grew out of experimentsdesigned to test whether or not divers could sur-vive at depth for days and weeks with their tis-sues saturated with the gases they breathed.Neither air nor pure oxygen could be used at theseextreme depths because they caused nitrogen nar-cosis and oxygen toxicity, respectively, which areexplained later in this chapter. Helium was theprincipal gas added to combat these conditions.In 1924, the US Navy began to experiment withhelium-oxygen breathing mixtures, and this tech-nology was successfully used in the famous sal-vage of the USS Squalus in 1939. The Navy con-tinued to pioneer advances in mixed-gas diving,which was a necessary step to allow divers to divelonger and deeper. The US Navy performed itssaturation diving experiments with personnel inan artificial habitat, SEALAB, which remained atdepth for extended periods but required signifi-


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Principle of Operation:

1. Self-contained, closed-circuit oxygen system

Minimum Equipment:

1. MK-21 Helmet2. Harness3. Weight belt (if required)4. Dive knife5. Swim fins or boots6. Surface umbilical7. EGS bottle deeper than 60 fsw

Principal Applications:

1. Search2. Salvage3. Inspection4. Underwater Ships Husbandry and enclosed

spamce diving

Advantages:

1. Unlimited by air supply

cant support from the surface. An improved ap-proach, the Deep Diving System, allowed diversto operate at depth out of a tethered capsule, whichwas then pressurized to that depth and returnedto the surface at the completion of the dive. Thecapsule was mated to a pressurized decompres-sion chamber, which provided a larger living areaand was mounted on the deck of a ship where thedivers and capsule could be easily monitored.5

Since the early 1980s, saturation diving in the Navyhas been deemphasized and attention targeted to-ward missions in shallower water, particularly spe-cialized diving.

Current fleet divers use open-circuit scuba pri-marily but sometimes dive with surface-suppliedrigs, such as the MK 20 and the MK 21 (Figure 26-2),which provide more protection and allow commu-nication with the surface. Combat and EOD divers

2. Head protection3. Good horizontal mobility4. Voice and/or line pull signal capabilities5. Fast deployment

Disadvantages:1. Limited mobility

Restrictions:1. Work limits: 190 fsw2. Emergency air supply (EGS) required deeper than

60 fsw or diving inside a wreck or enclosedspace

3. CurrentAbove 1.5 knots requires extra weights4. Enclosed space diving requires an Emergency Gas

Supply (EGS) with 50- to 150-foot whip andsecond stage regulator

Operational Considerations:1. Adequate air supply system required2. Standby diver required

Fig 26-2. General Characteristics of the MK 21 Diving System.Source: US Department of the Navy. US Navy Diving Manual. Rev 4. Washington, DC: Naval Sea Systems Command;1999: 654.


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use both open- and closed-circuit scuba. The LARV (Figure 26-3) is often used for long, shallow, clan-destine insertions, while the MK 16, which sup-plies oxygen at a fixed partial pressure, is used at

TABLE 26-1

PRESSURE CHART

Depth Atmospheric Absolute(gauge pressure) Pressure Pressure

0 1 atm 1 ata (14.7 psi)33 fsw + 1 atm 2 ata (29.4 psi)66 fsw + 1 atm 3 ata (44.1 psi)99 fsw + 1 atm 4 ata (58.8 psi)

fsw: feet of seawateratm: atmosphereata: atmosphere-absolutepsi: pounds per square inchAdapted from US Department of the Navy. US Navy DivingManual. Vol 1 and 2. Rev 3. Washington, DC: Naval Sea Sys-tems Command; 1993.

DIVING PHYSICS AND DECOMPRESSION THEORY

Application of Gas Laws to Diving

To function safely in the undersea environment, adiver must possess a fundamental knowledge of basicphysics and how its laws predict the behavior of matterunder a wide range of temperatures, pressures, and vol-umes. Pressure is defined as the amount of force appliedper unit of area and is commonly expressed in terms ofpounds per square inch (psi) or atmospheres (atm).Divers more frequently use feet of seawater (fsw), whichis not a pressure measurement in the strict sense but isuseful when actually working in the undersea environ-ment. For calculations, feet of seawater can be easilyconverted to pounds per square inch or atmospheresbecause 1 fsw exerts a pressure equal to 0.445 psi, andevery 33 fsw is equivalent to 1 atm5,6 (Table 26-1).

Fig. 26-3. General Characteristics of the LAR V Diving System.Source: US Department of the Navy. US Navy Diving Manual. Rev 3. Washington DC: Naval Sea Systems Command; 1993: 10-19.

Principle of Operation:

1. Self-contained, closed-circuit oxygen system

Minimum Equipment:

1. Secumar life jacket2. Dive knife3. Swim fins4. Face mask5. Dive watch6. Depth gauge(080 fsw type and/or depth

timer)7. Whistle8. Emergency flare (open water)9. Weight belt (as required)

Principal Applications:

1. Special warfare operations2. Shallow search and inspection

Advantages:

1. No surface bubbles2. Minimum support3. Long duration4. Portability5. Excellent mobility

Disadvantages:

1. Limited to shallow depths2. Central nervous system O2 toxicity hazard3. No voice communications4. Limited physical and thermal protection

deeper depths. Both are closed-circuit diving rigsthat allow extended operating times, remove orscrub carbon dioxide from the exhaled gases andemit no bubbles.5

Restrictions:

1. Working limits: 190 fsw2. Normal20 ft for 240 min3. Maximum50 ft for 10 min4. No excursions are allowed when using Single

Depth Dive Limits

Operational Considerations:

1. Buddy diver required if diver not tethered2. One safety boat required for diver recovery


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TABLE 26-2

GAS LAWS

Boyles Law: Daltons Law: Henrys Law:P(A)V(A) = P(B)V(B) P(total) = ppA+ppB+ppC V(A) = ppA x S(A)

P(X) = pressure of gas X P(total) = total pressure in mixture V(A) = volume of gas AV(X) = volume of gas X ppX = partial pressure of gas X ppA = partial pressure of A

S(A) = solubility of A

Source: US Department of the Navy. US Navy Diving Manual. Vol 1 and 2. Rev 3. Washington, DC: Naval Sea Systems Command; 1993.

Fig. 26-4. Gas volume and bubble diameter as a func-tion of depthBoyles Law.Reprinted with permission from: Somers LH. Divingphysics. In: Bove AA, Davis JC. Diving Medicine. 2nded. Philadelphia: W.B. Saunders; 1990: 13.

At sea level, the absolute pressure on a diver is 1 at-mosphere-absolute (ata), and so the absolute pressureexerted on a diver at depth is always equal to theweight of the water plus the weight of the atmosphere.The weight of the water acts as a force on the divercalled hydrostatic pressure, which is measured asgauge pressure.

Given this overview of pressure, the basic phys-ics definitions and gas laws attributed to Boyle,Henry, and Dalton (Table 26-2) can be used to ex-plore the behavior of gases and liquids. Boyles Lawstates that if the temperature of a fixed mass of gasis kept constant, the volume of the gas will vary in-versely with the pressure, such that the product ofthe pressure and the volume will remain constant.Boyles Law is important to divers because it relateschanges in the volume of a gas to the change in pres-sure (or to changes in depth) such that when pres-sure doubles, volume decreases to half its originalvalue (Figure 26-4). This law is most important inthe first 33 ft of water because as the diver movesthrough this pressure differential of 1 ata at the sur-face to 2 ata at 33 fsw, the volume of gas in the diverundergoes its greatest absolute change in volume.This relationship is integral to appreciating thepathophysiology of both pulmonary over-inflationsyndrome and barotrauma.

Daltons Law, or the law of partial pressures,dictates that the total pressure exerted by a mixtureof gases is equal to the sum of the pressures of eachof the different gases making up the mixture, witheach gas acting as if it were alone and occupying theentire container. Divers usually use a mixture ofgases rather than a single pure gas, so this law en-ables a diver to calculate the individual pressuresexerted by component gases, to predict potentialranges for individual gas toxicity, and to preventsuch diving hazards as oxygen toxicity.

Henrys Law states that the amount of any givengas that will dissolve in a liquid at a given tempera-ture is a function of the product of the solubility coeffi-cient for that gas and the partial pressure of the gas in

contact with the liquid. Furthermore, the solubility co-efficient depends on both the properties of the solute(the gas) and solvent (the liquidusually plasma)and is different for each combination. Simply stated,since water constitutes such a large portion of thehuman body, as a person dives deeper, more gas willdissolve in body tissues, which must then be releasedupon ascent. If a diver remains at depth for enoughtime, his or her body tissues will ultimately reach

Sea Level

79.3%Dia

69.3%Dia

63.3%

58.5%

33 ft

66 ft

99 ft

132 ft

297 ft

Vol = 1 or 100%100%Dia

1 ATM ABS or 14.7 PSIA

Vol = 1/2 or 50%

2 ATMS or 29.4 PSIA

Vol = 1/3 or 331/2%3 ATMS or 44.1 PSIA

Vol = 1/4 or 25%

4 ATMS or 58.8 PSIA

Vol = 1/5 or 20%

5 ATMS or 73.5 PSIA

Vol = 1/10 or 10%

10 ATMS or 147.0 PSIA

Diving Physics

46.2%

O

O

O

O

O

O


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equilibrium with the ambient breathing gas such thateventually no more gas can be dissolved, a state knownas saturation. When a diver whose body tissues aresaturated with a dissolved gas ascends toward the sur-face, less ambient pressure is exerted on the diver, andthere is more release (off-gassing) than acceptance (on-gassing) of the inspired gas. A simple example is re-moving the cap from a bottle of soda. Carbon dioxideis under pressure, and the liquid is saturated with it.When the cap is removed and the ambient pressurereduced, carbon dioxide comes out of solution asbubbles. If a diver ascends too quickly, he or she maynot off-gas fast enough and may transiently enter asupersaturation state where so much gas is dissolvedin the tissues that it cannot all naturally escape throughexhalation, and gas molecules coalesce to form bubbles.This theory has become the dominant hypothesis inexplaining the development of decompression sicknessor the bends. Determining how a diver could avoidthis malady and predictably ascend safely to the sur-face became the impetus for the development of theUS Navy Diver Decompression Tables.57

History and Development of DecompressionProcedures

Diving and medicine have been closely linkedthrough history, as is demonstrated by the crucialrole physiology has played in the study of decom-pression sickness (DCS). Pol and Watelle first sug-gested in 1854 that gas bubbles might be involved inthe development of what is now known as DCS andrealized that there was a relationship between pres-sure, duration of exposure, rapidity of decompres-sion, and onset of illness. They also noted that reliefof symptoms occurred during recompression. In1857, Seyler demonstrated that bubbles blocked thepulmonary circulation. Then in 1878, Bert demon-strated bubbles in blood and tissue after decompres-sion and noticed that these bubbles consisted pre-dominately of nitrogen. He recommended usingrecompression with oxygen as therapy for DCS andalso recommended decompression with oxygen, buthe did not specify decompression rates.7,8

In 1900, Heller, Mager, and Von Schrotter pro-posed the first decompression procedure, noting thatpartial pressures of inert gases at depth could be cal-culated using Daltons Law and that nitrogen couldbe assumed to be inert. Using Henrys Law as aguide, they developed a gas elimination equation todescribe the movement of an inert gas into and outof solution at varying partial pressures and the move-ment of this dissolved gas into and out of tissues.Their simple model assumed no additional tissuediffusion barrier for nitrogen but predicted that the

rate of on- or off-gassing of dissolved nitrogen woulddepend on three factors: (1) the diffusion gradientof nitrogen into the blood, a function of partial pres-sure and depth, (2) tissue blood flow, and (3) the ra-tio of inert gas solubility in blood to its solubility intissue. The model described tissue gas characteris-tics in terms of half-times and an exponential timeconstant, as in drug pharmacokinetics. The calcu-lated decompression rate of 20 min/ata, when testedin occupational scenarios, proved adequate for shortdives to 5 to 6 ata but not for longer dives.7

In 1908, Boycott, Damant, and Haldane, who wereevaluating caisson workers, noticed that the inci-dence of DCS increased as the duration of the diveapproached 5 hours, but the incidence appeared toremain constant for exposures longer than 5 hours.Their experiments with goats confirmed these ob-servations.9 Additionally, the researchers felt that thebody should not be considered one compartment butrather a series of tissue compartments, due to thevariability in tissue homogeneity and gas solubility.The time it took for half the gas to leave a saturatedtissue compartment was termed a half-time. Formathematical convenience, Haldane arbitrarily se-lected half-times of 5, 10, 20, 40, and 75 minutes tocover the spectrum of possible exchange rates, butthese rates were not assigned to specific real tissues.The gas elimination equation demonstrated that the75-minute tissue would be almost 95% saturated atthe 5-hour point.7,10

Haldane is also credited with the observation thatmen could be decompressed to 1 ata after completesaturation at 2 ata without developing symptoms butcould not tolerate greater drops in pressure withoutdeveloping DCS. Experimentally he found that goatscould perform a 2:1 drop in ambient pressure with-out developing DCS and confirmed this in the hy-perbaric chamber by observing decompression dropsfrom 2 to 1 ata, from 4 to 2 ata, and from 6 to 3 atawithout clinical sequelae. It appeared, therefore, thatboth men and goats could tolerate a 2:1 pressure dropor a 1.58:1 ratio of PN2/PB, (where PN2 is the partialpressure of nitrogen in tissue and PB is the absolutepressure) at least to a depth of 3 ata. Based on thesedata, Haldane and his associates proposed decom-pression procedures designed to ensure that the ra-tio of PN2 to PB never exceeds 1.58:1 in any tissuecompartment. For convenience, however, decom-pression stops were proposed in increments of 10 fsw.This was the beginning of staged decompression,which can be likened to intermittently opening andclosing the cap of a shaken soda bottle to avoid thesodas bubbling over. The Royal Navy adopted theHaldane Tables in 1908, and these early tables pro-vided adequate decompression for dives as deep as


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200 fsw for as long as 30 minutes, but they were notrecommended for exposures at deeper depths.7,10

The first tables developed for US Navy use weredevised by French and Stillson in 1915 and are calledthe Bureau of Construction and Repair tables (C andR tables). They used the Haldanian ratio of 1.58:1,employed oxygen decompression for dives between200 and 300 fsw, and were used successfully in 1915to salvage the sunken submarine F-4 at a depth of 306fsw.11,12 Since 1915, various modifications have beenmade to these tables by US Navy researchers to ex-tend bottom time while minimizing DCS. Hawkins,Shilling, and Hansen in 193513 and Yarborough14 againin 1937 performed retrospective mathematical analy-ses of nitrogen uptake and elimination for nearly 3,000dives. Tissues with short half-times (called fast tissues)appeared to tolerate very high ratios and off-gasquickly during ascent, allowing the decompressionstops for these tissues to be deleted. Based on thelonger 20-, 40-, and 75-minute half-times, Yarboroughcalculated new decompression tables, which were is-sued in 1937 and resulted in an acceptable annual in-cidence of DCS of only 1.1% in Navy divers.7,10

In 1951, while developing surface decompressiontables for dives using air, Van der Aue conducted anextensive evaluation of the Yarborough tables on longworking dives. For these dive profiles, he discoveredthat the 5- and 10-minute tissues did sometimes re-quire deep decompression stops for off-gassing, whileto achieve adequate decompression at the shallowerstops, it was necessary to factor in another hypotheti-cal tissue with a half-time of 120 minutes.15 This workdemonstrated that as dive depth or duration, or bothincreased, deeper decompression stops were requiredbecause of the greater saturation of tissues.

Before revising the Yarborough Tables, DesGranges,Dwyer, and Workman in 1956 analyzed inert gas up-take and elimination during 609 working dives usinghalf-times of 5, 10, 20, 40, 80, and 120 minutes. Theyconfirmed that the Haldanian ratio was not constantbut that the ratios for slow and fast tissues differed andthat they decreased exponentially with depth. The au-thors graphed the safe and unsafe nitrogen supersatu-ration ratios for each decompression stop and con-structed a table of safe ratios on the basis of both tissuehalf-time and depth of stop.16 Tables based on theseconcepts were adopted by the US Navy.

Workman in 1957 took a different approach to thedecompression table, which previously had focusedon time required for off-gassing at certain stops. Sincethe degree of allowable supersaturation differed forevery tissue and every depth, he produced a tablethat simply stated what the maximum allowable in-ert gas pressure or saturation was for each tissue ateach decompression stop.17 These maximum values,

called M-values, can be used to develop tables forair and mixed-gas diving. After considerable addi-tional research, Workman eventually also added tis-sue half-times of 160 and 240 minutes and reducedthe allowable ratios to provide for adequate decom-pression on long, deep exposures.18 Despite ex-tremely limited testing of these final additions, thesetables were promulgated for emergency use.

The decompression tables currently used by theUS military were developed through careful calcu-lations and were largely verified with human test-ing. Berghage and Durman19 evaluated the incidenceof DCS in divers using the US Navy Tables from 1971to 1978. During this period, 16,120 dives involvingdecompression were reported by the Navy, yielding202 cases of DCS and an overall incidence rate of1.25%. The majority of Navy dives are shallow (< 50fsw) and do not involve decompression, so only 43of a possible 295 depth/time combinations on thetables had more than 100 dives (data points) thatcould be used in analysis. On these schedules, theoverall incidence of DCS was 1.1%, but it was as highas 4.8% with some schedules, particularly for thosedive profiles of 100 fsw for 60 minutes and longer.Recent analysis of Naval Safety Center diving data20

for more than 600,000 dives during the period 1990through 1995 demonstrated identical rates for thetwo types of DCS at 1.3 per 10,000 dives.

Current Decompression Procedures and Tables

The US Navy Diving Decompression Tables (Fig-ure 26-5) have been improved and modified to applythe safest decompression procedures to all types ofdiving. Specific populations whose dive profiles maybe unpredictable (eg, special operations divers) havebenefited greatly from these changes. The nature ofSOF diving is to spend a considerable period of timeat shallow depths with the occasional need to per-form deeper excursions. For an extra margin of safety,the standard Navy decompression tables are basedon the square dive concept, which means if the to-tal dive time is 30 minutes, the table considers thatthe diver has spent the entire 30 minutes at the deep-est depth, even though the diver may have been atthat depth for only a few minutes.5 Using this con-cept, the decompression obligation for SOF divers wasunnecessarily long. So the Combat Swimmer Multi-level Dive decompression procedures were developedby Butler and Thalmann21 for air diving in 1983 andexpanded to include Mark 15 and 16 oxygen rigs in1985. LAR V divers can also use these procedures,which consider the dive profile as multiple squaredive segments and acknowledge off-gassing credit forthe long intervals in shallow water, just like off-


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gassing credit is awarded during a surface interval inbetween repetitive diving (multiple dives within 12hours). The standard air decompression tables for re-petitive diving, as developed by DesGranges, are usedwith the following four exceptions: (1) in place of the10-minute surface off-gassing time required in nor-mal repetitive diving, a 30-minute shallow intervalat depths of less than 20 ft can be used, (2) for safety,all segments of the dive at 30 ft or less are rated onrepetitive tables in decompression groups that are onegroup higher than the standard tables, (3) divers can-not move to lower repetitive groups over time, and(4) a diver can stay at depths of 30 ft or less indefi-nitely without ever exceeding repetitive group O.21

Special Warfare divers currently have an additionaloption, the Naval Special Warfare Dive Planner, toplan dives and calculate decompression obligations.22

The Dive Planner was developed because of the per-ception that the current tables were not equally safeacross all depth and bottom-time (approximate dura-tion of on-gassing) combinations, which was sup-ported by the variable DCS rates noted earlier in thoseusing standard decompression tables. Tissue satura-tion models, which previously had been the focus ofdecompression theory, contained too many param-eters to be determined precisely and simultaneouslyand could not exactly predict decompression obliga-tion. It was suspected that long, deep dives requiredsubstantially more decompression time than the stan-dard tables indicated. Scientists at the Naval MedicalResearch Institute (now the Naval Medical ResearchCenter, Silver Spring, Md) developed a probabilisticnitrox (nitrogen and oxygen mixture, with more oxy-gen than normal air) decompression algorithm named

USN-93 that used formal statistical techniques andmaximum likelihood modeling to form safe decom-pression tables. USN-93 is capable of calculatingdecompression obligations given an endless array of pos-sible depths, bottom times, and travel rates. In addition,the model adjusts for divers who switch breathing me-diums between air and constant partial pressure oxy-gen breathing apparatuses (ie, the MK 16) during thesame dive.23 This statistical model was developed usinga large database of well-documented experimentalchamber, air, and nitrox dives correlated with cases ofDCS. Using the results of these safe chamber (or dry)dives, an algorithm for computing decompressionschedules in real time was developed and was thensuccessfully validated in a prospective trial of morethan 700 wet dives, including multilevel dives anddives where the breathing gas was switched betweenair and a constant partial pressure of oxygen of 0.7ata during different segments of the same dive. SEALSare equipped with real-time depth and time meterscapable of recording and downloading very accuratedive profile information. Given a computer with at leasta 486-level microprocessor, a math coprocessor loadedwith USN-93, and the specific multilevel dive profile, aDMO can calculate a decompression table for almost anydive. This computer is an adjunct to the sources of de-compression information already available, includingthe Combat Swimmer Multilevel Dive procedures andStandard Air Decompression Tables, which are used bythe DMO to predict the best possible decompression plangiven the specific operational diving scenario.24,25

Standard Navy decompression tables are designedfor use at altitudes less than 2,300 ft (700 m) above sealevel. Diving gauges may not compensate for altitude

Fig. 26-5. A Portion of the US Navy Standard Air Decompression Tables.Source: US Department of the Navy. US Navy Diving Manual. Rev 4. Washington, DC: Naval Sea Systems Command;1999: 9-56.

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Timefirst stop(min:secc)

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12.1

30

9.1

20

6.0

10

3.0

Totaldecompression

time(min:sec)

Repetitivegroup

Decompression stops (feet/meters)

8024.3

Depth

feet /meters


585

or for fresh water being less dense than seawater.These two factors contribute to inadequate decom-pression by divers at altitude who are unfamiliar withthe unique decompression requirements there. The

TABLE 26-3

SYMPTOMS OF CENTRAL NERVOUS SYSTEMOXYGEN TOXICITY

Vision

Ears

Nausea

Twitching

Irritable

Dizziness

Convulsions

Vision is reported to tunnel or close in

Ringing and increased perception ofsound

Feeling of vague discomfort in theabdomen

Particularly the muscles of the face(cheeks)

Sense of agitation and uneasiness

Sense of lightheadedness, vertigo, orboth

Tonic-clonic seizures, usually with nowarning

Source: US Department of the Navy. US Navy Diving Manual.Vol 1 and 2. Rev 3. Washington, DC: Naval Sea Systems Com-mand; 1993.

Navy Experimental Diving Unit maintains altitudedecompression tables for US military use.26 Civiliandivers venturing into bodies of water above 700 mcan consult Buhlmanns Swiss dive tables.27

GASES AND DIVING

Oxygen Disorders

Oxygen deficiency, or hypoxia, leads to uncon-sciousness and death. The majority of military div-ing uses air, the absence of which is easily andquickly recognized by the diver. But unlike suddenstrangulation or suffocation, a gradual decrease inoxygen content may not be detected. Unconscious-ness can occur without anticipated warning signals.Any diving system that utilizes either surface-sup-plied mixed gas or rigs that carry their own oxygen(eg, LAR V, MK 16) can produce a hypoxic state dueto malfunction or incorrect gas mixing. The treatmentof suspected hypoxia in the water is to shift to analternate gas supply. Incoherent or unconsciousdivers should be placed on 100% oxygen, if avail-able, from 40 fsw or shallower until the diver reachesthe surface and can be monitored appropriately.5

Just as too little oxygen can create difficulty fordivers, excessive amounts can cause oxygen toxicity,specifically in the tissues most sensitive to high levelsof oxygen, including the lung, the eye, and the brain.Pulmonary oxygen toxicity is seen at oxygen partialpressures of 0.5 ata (the equivalent of breathing air at46 fsw or deeper).6,7 The symptoms include discomfortor a burning sensation deep in the throat or substernalarea, accompanied by cough or burning on inspiration.Findings include a decrease in vital capacity as measuredby pulmonary function tests. Pulmonary oxygen toxic-ity requires several hours or days to develop and maybe seen if a stricken diver requires several iterations ofrecompression treatment. If repetitive treatments areneeded for persistent symptoms, pulmonary functiontests are required. If there is a decrement in vital capac-ity of more than 10%, treatment with oxygen must beinterrupted and a transition to other treatments consid-ered. After daily exposures to high levels of oxygen (eg,daily treatments for 2 to 4 weeks), ocular oxygen toxic-ity may produce transient myopia, which will resolveover the course of 4 to 6 weeks if there are no furtherexposures.28 Central nervous system (CNS) oxygen tox-icity has been observed at oxygen partial pressures above1.3 ata (the equivalent of breathing air at 171 fsw orbreathing 100% oxygen at 9 fsw or deeper).6,7 Duringdives with enhanced oxygen rigs and chamber runs thatuse oxygen, CNS toxicity must be a constant consider-ation because, unlike pulmonary oxygen toxicity, CNSoxygen toxicity can strike within minutes of exposure.

Sensitivity to oxygen toxicity varies between individu-als and within the same individual, making it impos-sible to predict which divers are at increased risk. Thewarning symptoms associated with CNS toxicity aretaught using the acronym VENTID-C, with convulsionsbeing the most common presentation (Table 26-3). Treat-ment is straightforward. Since high partial pressures ofoxygen created the problem, the goal in therapy is toreduce oxygen tension. This can be done by several meth-ods, including removing the oxygen mask from a cham-bered diver, ascending a few feet or switching to a gasof lower oxygen content, if available. The seizures of CNSoxygen toxicity are not epilepsy and usually do not re-quire medicinal intervention; they are simply a toxic re-action. There is no increased predisposition for furtherseizure disorder throughout life. The cold water immer-sion and hypercarbia seen on long, arduous combatswimmer missions predispose divers to oxygen seizures,which if suffered at depth, place divers at great risk fordrowning and developing an arterial gas embolism.5

Carbon Dioxide Toxicity

Carbon dioxide is a by-product of human metabo-lism, and its serum concentration is maintained within


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the relatively narrow range of approximately 35 to 45mmHg by both lung and tissue functions. Despite theefficiency of these systems, excessive carbon dioxide lev-els can develop during diving due to overproduction(from increased duration and intensity of exercise), poorventilation, or failure of carbon dioxide removal systemsin rebreather rigs.5,6 Inadequate ventilation results frommissed breathing or breath-holding during a dive, in-creased breathing resistance in the equipment, or exces-sive dead space in the breathing system. Hypercapnia,an elevated level of carbon dioxide, can present in a va-riety of ways. An increased rate and depth of breathingmay be noticed first, followed by a nagging frontal head-ache or such symptoms as nausea, confusion, or uncon-sciousness. Alternatively, too little carbon dioxide pro-duces painful muscle spasm of the hands and feet(tetany), paresthesias, numbness, lightheadedness, con-fusion, and, again, loss of consciousness. Low levels ofcarbon dioxide, or hypocapnia, usually occur second-ary to hyperventilation. Treatment is straightforward:efforts should be made to slow the respiratory rate, and,if necessary, abort the dive. In cases of suspected hyper-capnia, the diver should stop work and allow time foradequate ventilation. If the cause of the hypercapnia isthe equipment, the dive should be aborted.5,6

Carbon Monoxide Poisoning

Divers will encounter carbon monoxide poisoningthrough exposure to a contaminated air supply. Contain-ers of breathing gas, such as scuba tanks, must be pres-surized to deliver the gas underwater to divers. Com-pressor intakes must be distant to any exhaust sources,which typically contain carbon monoxide. A shiftingbreeze can create a problem if the system is not moni-tored carefully. Headache, nausea, confusion, and uncon-sciousness are symptoms of high levels of carbon mon-oxide, which is tasteless and odorless. If more than onediver complains of symptoms consistent with carbonmonoxide poisoning, contaminated gas should be sus-pected. Treatment involves removal from the gas sourceand treatment with 100% oxygen at an elevated partialpressure.5,6,28

Nitrogen Narcosis

Nitrogen makes up approximately 79% of air and at1 atm serves essentially as a diluent for oxygen. Nitro-

genas with any of the diluting gases used in divingwill become toxic if the partial pressure is excessive.Humans note an anesthetic or sedating effect associatedwith nitrogen as partial pressure is increased. At depthsmuch below 50 fsw, alterations in function can be de-tected by psychological and motor skill testing; below200 fsw, symptoms progress to loss of fine motor con-trol, inattention to tasks, and poor judgment. Treatmentis straightforward: the partial pressure must be de-creased by ascending or surfacing.57

Helium Diving

Helium is used as a breathing gas in saturation div-ing to reduce breathing resistance at deep depths, aswell as to prevent the occurrence of nitrogen narcosis,which otherwise limits diving depth to approximately200 ft. Helium can present three primary problemswhen employed in diving operations. The first is hy-pothermia, because helium has a thermal conductancethat is seven times that of air. Divers can become hy-pothermic quickly at depths, so care must be taken toheat breathing gases, as well as to provide adequatethermal protection from the surrounding water (ie, drysuit or hot water suit). The second potential problem isthe distorting effect of helium on oral communication,the so-called Donald Duck effect, which can makecommunicating with a diver at deep depths very diffi-cult. Finally, high pressure nervous syndrome is a de-rangement of central nervous system function that oc-curs during deep helium-oxygen dives, particularlysaturation dives. The cause is unknown, but it is thoughtto be an effect of depth, not a specific effect of the breath-ing gas. The clinical manifestations include nausea, finetremor, imbalance, incoordination, loss of manual dex-terity, and loss of alertness. In severe cases, a diver maydevelop vertigo, extreme indifference to his or her sur-roundings, and marked confusion. The nervous syn-drome is first noted between 400 and 500 ft, and itsseverity appears to depend on both depth and compres-sion rate. With slow compression, depths of 1,000 ft maybe achieved with relative freedom from symptoms. At-tempts to make the syndrome less severe have includedthe addition of nitrogen or hydrogen to the breathingmixture, but no method has been entirely successfulbelow depths of 1,000 ft.57 These gases may influencehigh pressure nervous syndrome because their narcoticproperties may provide a partial anesthetic effect.

ENVIRONMENTAL HAZARDS

The aquatic environment is at once both incrediblybeautiful and unforgivingly dangerous to those whodo not respect it. Survival here requires an awarenessof hazards, such as temperature extremes and animal

life, as well as an appreciation of physics to understandthe behavior of gases under pressure, the significanceof light refraction, and the differences in blast effectsand sound conduction in water versus air.


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Thermal Dangers

Divers may be exposed to thermal extremes on thesurface, in a hyperbaric chamber, and in the water. Heatloss can occur through convection, conduction, radia-tion, and evaporation, including significant respiratoryloss in dehumidified air and in diving gases such as he-lium. Heat loss in still water is 25 times that in air, whileheat loss in moving water increases to a factor of 200.5,6

Heat is produced by basal metabolism and exercise (shiv-ering) and may be absorbed from environmental sources.Hypothermia is defined as a core temperature of 35C(95F) or less, which requires various treatments depend-ing on whether it is mild (35C32C, 95F90F), mod-erate (32C28C, 90F82F), or severe (< 28C, < 82F).Cardiac rhythms degrade into ventricular fibrillationvery easily if divers are jostled while severely hypother-mic. Unfortunately, the rhythms are also quite refrac-tory to the usual acute cardiac life support protocols.5,29

At the other extreme is hyperthermia. A resting diverin a swim suit will remain euthermic in 33C (92F) wa-ter, but if working in water warmer than 30C (86F), heor she may develop hyperthermia. Types of injuries in-clude heat cramps, heat exhaustion, and heat stroke,which may present in the absence of dry, hot skin. Evenpersonnel riding in a chamber can be thermal casual-ties. The Navy Diving Manual gives guidelines for whichtreatment tables can be used if the ambient temperatureis greater than 29C (85F), and the chamber is notequipped with an environmental package to cool thegas inside the chamber.5

Fig. 26-6. Reflected shock waves can increase or decrease the total effect of the blast on a diver.Source: Noonburg G. US Army Special Forces Diving Medicine Manual. Key West, Fla: Special Forces Underwater Op-erations School; 1995: SS-1.

Acoustic Hazards

Sound is transmitted much more efficiently in waterthan in air, so sound that would normally be inaudible inthe air can be heard at great distance underwater. Unfor-tunately, this rapid conduction also makes localization ofsound underwater more difficult. Humans are able todetect the small difference in the time of arrival of air-borne sound (traveling at about 1,000 ft/s) reaching oneear as compared to the other, thus allowing us to localizethe source. With waterborne sound (traveling at nearly5,000 ft/s), however, it is impossible for humans to iden-tify the direction of sound transmission.5,6 Sonar and lowfrequency sound have been explored as offensive weap-ons by which to keep unfriendly divers away from mili-tary vessels because injuries, including vestibular dis-turbances, cardiac arrhythmia, and tissue destruction,occur given certain conditions of frequency and volume.Low-frequency vibration can be perceived by the sub-merged diver, just as vibration due to sound can be feltby the music lover when touching a stereo speaker.30,31

Blast Injuries

Underwater explosions are very dangerous to a sub-merged diver. The type of the explosive used, the sizeof the device, its location in the water column, the sur-rounding seabed topography, and the location of thediver all affect the outcome of the exposure (Figure 26-6). Damage occurs as the shock wave of blast overpres-sure leaves the dense water and passes through the

Explosion

Surface

Bottom

Ship's Hull

Wall


588

divers less-dense, air-filled compartments (eg, lung,intestine, sinus, middle ear). At this interface, the wavedecreases in velocity and shreds the tissue, an effectreferred to as spalling. Hemorrhage, compartmentrupture, air embolism, and even bone fractures havebeen reported in submerged blast victims.5,32

Electrical Hazards

Military divers are exposed to electrical hazards intheir daily duties. Fleet divers, for example, are requiredto operate 220-volt equipment manually to cut and weldunderwater. Wet divers move about the deck, and thedive station itself is in contact with wet decking, therebyenhancing the potential for inadvertent electrocution.Shipboard systems have both alternating and direct cur-rent (AC and DC), as well as high- and low-voltagesources. Dry skin offers approximately 10 times (10,000ohms) the resistance to electricity afforded by wet skin.Once contact with a wet diver is established, alternatingcurrent levels of only about 15 mA are sufficient to pro-duce tetany, manifested by strong and sustained musclecontraction. Direct current does not exhibit this let gothreshold (also recognized as the inability to let go of anobject) but produces heat in tissues instead. Neverthe-less, a single, violent contraction may occur, throwingthe diver away from the direct current source. Exposureto direct current may cause cardiac asystole, while con-tact with alternating current may induce ventricular fi-brillation, and both may cause respiratory arrest.5,33

Radiation Hazards

All US Navy submarines are now nuclear pow-ered, as are many surface ships. Potential radiationexposure is maximized near the bottom of the sub-marine where shielding is thinnest. Divers must of-ten dive there, examining the submarine for vari-ous reasons. The water acts as a very good radiationshield, but divers must wear thermal luminescencedosimeters and be monitored for radiation exposure.5

Dangerous Marine Life

Animal Bites

Despite media attention devoted to this subject,bites are rare. Shark encounters are commonplace, butshark injuries are not. It has been stated that humansare far more likely to experience a lightning strike thana shark attack and more likely to survive a shark at-tack than a lightning strike. Moray eels are found inmost tropical and subtropical oceans and are com-monly encountered by military divers. Divers fear eelsbecause they move like snakes, bare their many sharp

teeth, and appear threatening. In actuality, the eel isa shy and very near-sighted animal. A military divermust go out of his way to provoke an encounter. Eelsdo not have fangs and do not envenomate.5,6,27,34 Bar-racuda bites likewise are rare for divers, althoughsurface swimmers and boaters with arms danglingin the water may attract undue attention, particu-larly if they are wearing shiny objects. Barracuda arefound in most tropical waters of the world.5

Envenomation

Envenomation, in contrast to bites, is common-place. Numerous creatures capable of inflicting dis-comfort on humans exist in the sea (Figure 26-7). Themajority of these encounters are nonfatal. All areavoidable. The following is a short list of the morenotable hazards.

The sea wasp, a member of the Coelenterate family,which includes jelly fish, hydroids, sea anemones, andcoral, has a very fatal venom in the thousands of nema-tocysts aligned on its meter-long tentacles. Reaction isswift, with shock and death possible within 4 to 6 min-utes of the encounter. Antivenin is available but oftenadministered too late. These hydroids are found aroundAustralia and the Great Barrier Reef.27,34

Sea snakes provoke the same primitive, exagger-ated, and unnecessary fear response as their land-bound cousins. Sea snakes are most commonly foundin the warm waters of the Pacific and Indian Oceans.One member of the family, Pelamis platurus, is foundin unusual areas, such as the cooler waters along thewest coast of Latin America to just south of Califor-nia and from southern Siberia to the cold waters offTasmania. It is doubtful that any venomous snake hasyet crossed into the Caribbean waters, though manybanded Caribbean eels, often mistaken for sea snakes,offer divers an opportunity to swear to the contrary.Sea snakes are air breathers and are often seen in shal-low water or within the protection of a rocky sectionof beach. They move moderately well on dry beach.They do not hiss but rather create a snoring or gur-gling sound. They are often seen as curious, not ag-gressive, and are not afraid to approach an unusualobject for closer observation. A diver must generallygo out of the way to provoke a negative encounter.The sea snake is more venomous than a cobra, butthe sea snakes fixed fangs make envenomation moredifficult because of the shallower penetration as-sociated with the bite. Antivenin is available.27,34

Stone fish are widely distributed, including in coldcoastal waters of the United States. Aside from thebeautiful, feathered-looking fish found in warm wa-ters and referred to as lionfish, turkeyfish, or zebrafish,the members of this family are really quite ugly. They


589

Fig. 26-7. Marine life that can be dangerous to divers in-cludes (a) the sea wasp, (b) the sea snake, (c) thezebrafish, (d) the blue-ringed octopus, (e) the cone shell,and (f) the anemone.Images from Envenomations and Poisonings Reference.Bethesda, Md: Naval School of Health Sciences; 2000.CD-ROM.

c

a b

d

f

e

are masters of camouflage and are easily stepped on.The dorsal spines can puncture normally protective foot-wear and inject venom into the wound. In addition tosevere pain, tissue destruction can be impressive anddeath is not uncommon. Antivenin is available.6,27,34

The blue-ringed octopus, found in the warm waters of

Australia, is unique within the family of octopi. Aswith its cousins, it will attempt to defend itself if mis-handled by biting with its beak-like mouth. But un-like its larger cousins (this one spans only about 6inches), the blue-ringed octopus secretes a venominto the usually painless bite. Death can occur.6,27,34


590

Cone shells have a very wide distribution, from USwaters off California and Hawaii to the warm waters ofthe world. Cone shells usually lie buried in the sandybottom during the day and hunt during the night, crawl-ing about like a garden snail. Cone shells are often beau-tifully marked and colorful, making them a temptingtarget for the aesthetically inclined military diver. Un-fortunately, they can kill. They extend a proboscis untilthey touch fingers or skin and then quickly impale thecontacted tissue with a small, harpoon-shaped radicu-lar tooth that injects the venom.5,6,27,34

Stingrays are found in a wide range of oceans andin a moderate variety of colors, but all are describedas flat, disc-shaped animals with whip tails, whichappear to fly through the water. Military divers en-counter them most often while exercising, such as jog-ging, in the shallow water along sandy beaches. Theray, buried in the warm sand and protected by thefew inches of sun-warmed water, feels the force of ahuman foot landing on its back. In a rapid reflex, thetail strikes the offending human ankle and implants aformidable barb, which tears and envenomates theleg. The diver immediately steps off of the rays back,and the ray swims away. The human, on the otherhand, requires pain medication and surgical debride-ment of the wound to remove the barb and other for-eign material. These envenomations are not lethal, butdeath has been attributed to freak accidents in whichthe chest or abdomen, usually of a child, was pen-etrated by a barb from a large ray. Rays are not ag-gressive and will do anything they can to avoid adiver. A cousin of the stingray, the torpedo ray, alongwith the South American eel and the African catfish,produces a nonlethal electric current. The torpedo raydoes not frighten away as easily as the stingray, isstockier, and has a more fin-shaped tail.5,6,27,34

Sea urchins, the porcupines of the sea, are found inmany colors and sizes, are very slow moving, and willnot swim into a diver. To effect an injury, the diver muststep or kneel on the urchin or grab at an object withoutnoticing the urchin. The spine of an urchin can thenpenetrate rubber soles, gloves, neoprene, and skin.Because the pain associated with a penetrating spineis greater than can be explained by simple mechanicaltissue damage, the discomfort is attributed to a venom.Experts disagree as to whether or not all urchins possessvenom, as well as the mechanism of envenomation. The

removal of a spine is tedious because the spines, orpedicellariae, crumble like sand. Deaths do not occur.6,34

Fire coral, a hard coral, is a ubiquitous organ-ism and a problem for divers. Its appearance var-ies with locale but generally is brownish with veryshort, white, hair-like fuzz on the surface. Thatfuzz penetrates the skin as invisibly as does fiberglassinsulation but contains thousands of microscopicnematocysts, which are miniature envenomators.Stinging hydroids are similar to fire coral in effect butappear more like a feather or plume.5,6,27,34

Sea anemones are giant versions of the microscopiccoral animal itself, but unlike coral, which builds arock-like calcareous skeleton, the anemone remainssoft. Some reach 3 ft across, undulating in the watersurge like a huge flower in a breeze. Their colors canvary from pure white to pinks to violet to deep scar-let reds. The sea anemones tentacles are home tonematocyts, which can sting the careless diver whotouches them with bare skin.34 Divers must maintainrespect for the reef as the home of abundant marinelife, including anemones. Hundreds of years of workare required for the coral animals to group togetherand create the stone-like skeletons that remain aftertheir death, forming the base for the next generation ofthe colony to attach and deposit their own calcareousskeletons. Nature teaches the careless diver to avoidcoral contact by way of lacerations and abrasions,which are slow-healing and easily infected wounds.5,6,34

Puffer or porcupine fish are rather bulky, awkwardlymoving animals, but they are considered delicacies insome Asian countries. When disturbed (a favorite pas-time of night divers), they will inflate or puff up, erect-ing normally flat-lying spines. In this state, the fish haslittle control over its own movement but is suspendedin the water like a huge prickly burr, making it a hazardfor its tormentors to handle any further. Puncturewounds result from slow learning.

Bristle worms are underwater caterpillars, amaz-ingly similar in appearance to their cousins on land.The bristle worms small tufts of fuzz can cause a con-tact dermatitis reaction in some divers.6,34

Sponges are commonplace in warmer waters andare found in several varieties, from huge barrelsponges to iridescent tube sponges. After touching themucous surface of a marine sponge, some divers de-velop a contact dermatitis.5,6,34

DYSBARIC ILLNESS

Various diving-related maladies are associated withalterations in pressure. These dysbaric illnesses includeDCS and barotrauma. In 1991, Francis and Smith35 ad-vocated altering the classification of dysbaric illness

including DCS, to emphasize symptoms rather thanassumed phsysiology of injury. The system is notwidely accepted, so this chapter will focus on the tra-ditional categorization of dysbaric illness.


591

Decompression Sickness

TABLE 26-4

COMPARISON OF SYMPTOMS IN DECOM-PRESSION SICKNESS TYPES I AND II

DCS Type I DCS Type II

Pain in bilateral or multiplejoints or muscle groups

Pulmonary symptoms(chokes)

Inner ear symptoms(staggers)

Central nervous systemsymptoms

Cardiac symptoms

Pain under pressure

Joint and/or muscle pain(bends)

Skin itch and rash

Cutis marmorata

Lymph node swellingFig. 26-8. A 27-year-old male with cutis marmorata onhis abdomen.Photograph by permission of the Center for HyperbaricMedicine and Environmental Physiology, Duke Univer-sity Medical Center, Durham, NC.

DCS probably results from the formation of bubblesin tissues and blood vessels, with resultant ischemiaand tissue injury.36,37 This occurs when a diver ascendsfrom higher to lower pressure at a rate that exceeds thebodys ability to eliminate the gas, and the partial pres-sure of dissolved gas in tissue overcomes the pressureto keep it in solution. Despite the existence of decom-pression schedules for varying depths and bottomtimes that describe the limits of no-decompressiondives, NO dive is risk-free. Tissue responses to pres-sure vary between divers and even daily within thesame diver and are not understood well enough toeliminate all risk. A wide range of signs and symptomsmay accompany the initial episode of DCS. Some of thesemay be so pronounced that there will be little doubt asto the cause, while others may be subtle and easily over-looked in a cursory examination.57 Classifying DCS intoType I or Type II (Table 26-4) permits the treating facilityto recognize or anticipate serious, even life-threatening,consequences. Divers at risk for DCS often delay report-ing symptoms and wait for them to resolve. While thebody is very forgiving, the consequences of misjudginga diving incident can be permanent and costly, so alldivers must be advised to report any symptoms. If DCScannot be eliminated as a potential diagnosis, treatmentwith recompression is recommended.5

Type I Decompression Sickness

Symptoms of Type I, also called pain-only orsimple, DCS include skin marbling, lymph nodeswelling, joint pain, and muscle pain. The most frequentsymptom of Type I DCS is joint pain, usually seen in

the shoulder, elbow, wrist, hand, hip, knee, or ankle.Type I pain may be excruciating but is usually mildwhen first noticed and becomes more intense as timepasses. It may be located in a muscle group and is usu-ally described as a progressive, deep, dull ache that isoften very difficult to localize and unchanged by move-ment. In contrast, typical mechanical injuries, such asa strain or bruise, are often more painful with move-ment. Several methods of differentiating DCS painfrom mechanical injury have been described, such asapplying pressure to the site manually or with a bloodpressure cuff (DCS pain reportedly diminishes) orwarming the area (DCS pain increases), but none isreliable.57 Medical history is the best way to distin-guish DCS Type I pain from trauma, but if confusionexists and circumstances permit, the casualty shouldbe treated presumptively for DCS. Another area of con-fusion arises when evaluating truncal pain. There isalways the possibility that shoulder or hip pain doesnot originate in that specific joint and may be referredfrom the CNS. It is important to differentiate the originof the pain because Type I problems may be treatedwith a shorter recompression table than Type II DCS.5

Divers with Type I DCS symptoms, which are not con-sidered life threatening, must be monitored carefullybecause they may progress to Type II DCS.

Cutis marmorata, a mottling or raised marbling ofthe skin, is a presentation of Type I DCS, which beginsas itching but quickly progresses to redness and then apatchy, bluish discoloration of the skin (Figure 26-8).The condition is uncommon but should be differenti-ated from simple pruritus, which may be seen in dry(chamber) dives and is not DCS. Cutis marmorata


592

sometimes precedes evidence of neurological deficits.A rare presentation of Type I DCS involves localizedpain and swelling of lymphoid tissue. Though recom-pression may provide symptomatic pain relief, reso-lution of the swelling may require several days orweeks of watchful waiting after recompression.57

Type II Decompression Sickness

Type II or serious DCS is potentially life threat-ening and includes symptoms involving the inner ear,the lungs, multiple joints, the brain, and the spinalcord. Inner ear DCS, or the staggers, results frombubble production in the inner ear (seen almost ex-clusively when switching breathing gases during satu-ration or deep mixed-gas diving) and involves symp-toms of vertigo, tinnitus, and hearing loss.6,7 Staggerscan sometimes be difficult to distinguish from innerear barotrauma, which is typically preceded by middleear barotrauma and a forceful attempt to equalize thepressure in the middle ear (Valsalva maneuver)against a closed Eustachian tube during descent. Innerear barotrauma is discussed in detail later in this chap-ter. In addition to neurological symptoms, divers withsevere DCS may suffer cardiovascular collapse orother symptoms from profuse venous bubbling, whichis analogous to shaking a soda bottle then suddenlyremoving the cap. Large volumes of intravenous gasdisrupt the pulmonary vasculature, causing diffusepulmonary injury. This pulmonary DCS, referred toas the chokes, starts as chest pain followed by in-creasing respiratory rate, dyspnea, and hemoptysis.There is rapid progression to complete circulatory col-lapse, loss of consciousness, and death. Immediate on-site recompression therapy is the divers only hope.Finally, divers with Type I symptoms located bilater-ally or in more than one joint or muscle group mayrepresent the beginning of serious DCS and thereforeshould be treated as if they had Type II DCS. Similarly,pain in the chest, spine, or hip requires special evalua-tion. If a DMO is not immediately available, thesesymptoms should be regarded as Type II and the af-flicted diver should be recompressed accordingly. Ad-ditionally, any DCS symptom that occurs during theascent phase of a dive is considered pain under pres-sure, and should be treated as Type II DCS.5

In the early stages, CNS symptoms of Type II DCSmay not be obvious, and the stricken diver may con-sider them inconsequential. The diver may feel fa-tigued or weak and attribute the condition to overex-ertion. Even as weakness becomes more severe, thediver may not seek treatment until walking, hearing,or urinating becomes difficult. Symptoms must beanticipated during the period after the dive andtreated before they become too severe. Many other

findings can be attributed to the effects of DCS on theneurological system, including numbness, tingling,feelings of pins and needles, weakness, frank pa-ralysis, loss of bladder or bowel control, mental sta-tus changes, motor performance decrement, ringingin the ears, vertigo, dizziness, hearing loss, vision dis-turbance, tremors, personality changes, and amnesia.Attempting to differentiate such findings withnondive etiologies from those that are dive-related canbe problematic. A diver with a headache has an ex-tensive list of differential diagnoses other than DCS,and fatigue in a diver may reflect only a lack of sleepthe previous evening. Divers with unexplained focalneurological symptoms following a dive must betreated with recompression therapy. Expedient deliv-ery of recompression therapy is mandatory for TypeII DCS, unless otherwise directed by a DMO.5

Aviators and flight crews exposed to the hypobaricenvironment at altitude may experience symptoms ofDCS also, but symptoms usually resolve as they descend(see Chapter 25, Aviation Medicine). To avoid DCS, diversare prohibited from flying above 2,300 ft (commercial air-liners are pressurized to 6,000 ft) for a minimum of 12hours after a dive that requires decompression stops.Patients who are treated for DCS Type I must not flyfor 24 hours. Both of these limitations will be extendedfor more complicated situations, such as aviators whodive (must wait 24 hours to fly) or patients treated onTreatment Table 4 (must wait 72 hours).32 Treatmenttables are reviewed toward the end of this chapter.

Pulmonary Over-Inflation Syndrome

Pulmonary over-inflation syndrome (POIS) is ex-plained by Boyles Law and can result when gas trappedin the lung expands with decreasing pressure duringascent and forces its way out into the interstitial tissues.This syndrome can be life threatening, depending onwhere the escaping air travels. POIS can be the mostdangerous form of barotrauma because of the possibil-ity of gas embolism. It is separated into four distinctentities for diagnosis and treatment:

1. pneumothorax;2. mediastinal emphysema;3. subcutaneous emphysema; and, the

most serious,4. arterial gas embolism (AGE).

A pneumothorax results when air escapes from theruptured alveoli into the pleural space. If it is severeenough, the lung will be compressed and displacedby the air. The patient may describe sudden chest painand may experience shortness of breath, rapid, shal-low breathing, or a cough. The examination may re-


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veal an absence or diminution of breath sounds. Asimple pneumothorax may be transformed into a ten-sion pneumothorax, particularly during the ascentphase of recompression chamber treatments, andmedical personnel should be prepared for these emer-gencies. If a diver complains of sudden chest painduring the ascent phase of the chamber run, the as-cent should be stopped and the diver should descendto a depth where he or she has relief of symptoms asa temporary measure. A chest tube should be placedto provide permanent relief.5 A spontaneous pneu-mothorax, one that occurs without obvious reason, ispermanently disqualifying for dive duty because ofits potential for recurrence. Asthma is also disquali-fying, since a plug of mucus may create an area oftrapped gas distal to it. A traumatic pneumothorax,one associated with rib fracture, gun shot wound, orother trauma, is temporarily disqualifying.38

Mediastinal emphysema is caused by gas dissect-ing into in the tissues behind the sternum. Patients mayreport a dull ache or tight feeling in the front of thechest, with pain made worse by deep breathing, cough-ing, or swallowing. The pain may radiate to the shoul-der, jaw, or back. Subcutaneous emphysema resultswhen air leaks from lung tissue and accumulates be-tween tissue planes of the upper chest and neck. Thesepatients seldom complain of pain but often note achange in voice, may have a full or bloated look aboutthe neck, and feel a popping or crepitus when the neckis palpated. Recompression is not the recommendedtreatment for mediastinal emphysema, subcutaneousemphysema, or pneumothorax. Definitive treatmentsare the same as for injuries not related to diving, exceptthat a diver should have a detailed neurological exami-nation to ensure an AGE is not also present.

An AGE is a bubble that has forced its way out ofthe alveoli and into the pulmonary venous circula-tion via an alveolar capillary. It then travels to the heartand is pumped through the arterial system until itreaches a capillary that is too small to accommodateits size. The AGE occludes further blood flow and thenecessary delivery of oxygen to the tissues beyondthe occlusion, creating an area of ischemia. In divers,AGE may occur while they hold their breath duringascent, following trauma such as blast injury, as a re-sult of occluded airways due to allergies or infections,or in those with venous DCS bubbles that passthrough occult or congenital cardiac shunts. AGE isgenerally dramatic in presentation with obvious,possibly severe, neurological symptoms occurringwithin seconds to minutes of the injury. Any organsystem can be involved, but the most striking arethe CNS and the cardiovascular system, where pre-sentations mimic, respectively, classic stroke andmyocardial infarction. Treatment requires immedi-

ate recompression in an attempt to diminish the sizeof the bubble and deliver oxygen to ischemic tis-sues. If recompression is accomplished before hy-poxic damage, hemorrhage, or edema develops, arapid and complete recovery is likely.57

Barotrauma

The most common diving-related injury is baro-trauma, which results when pressure within a bodycavity and the ambient pressure do not equilibrate. Tobe subject to barotrauma, a cavity must usually meetseveral criteria: (a) be gas filled, (b) have relatively rigidwalls, (c) be enclosed, and (d) be subjected to change inthe ambient or surrounding pressure. Ambient in-creases in pressure will cause the gas trapped within acavity to compress and attempt to occupy less space.This creates a relative vacuum, which draws in thetissue forming the walls of the cavity to equalizepressure and fill the space created by the compressedgas. If the pressure difference continues, vessels inthe tissue will engorge and eventually rupture, fill-ing the space with blood and equalizing the vacuum.When this phenomenon occurs in conjunction withdescent during a dive, it is termed a squeeze. Areverse squeeze results from tissue damage causedby overpressurization of a cavity during ascent, aswith POIS. This pathophysiological model can be ap-plied to any gas-filled cavity, and Table 26-5 is a list-ing of common forms of squeezes. Prevention of

TABLE 26-5

EXAMPLES OF NEGATIVE PRESSUREBAROTRAUMA (SQUEEZE) AND CAUSES

Squeeze Cause

Eustachian tube dysfunction

Hood or piece of equipmentcovering the external ear passage

Blockage of the duct that normallyvents a sinus

Failure to equalize air in the maskby nasal exhalation

Faulty filling with trapped airbeneath

Pocket of air in a dry suit thatbecomes trapped under a fold or fitting and pinches the skin

Extremely rare but seen with deepbreath-hold diving

Failure of the air supply in a drysuit to balance water pressure

Middle ear

External ear

Sinus

Face mask

Tooth

Suit

Lung

Whole body


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barotrauma is relatively simple and includes notdiving with a cold or during a flare-up of aller-gies, using proper-fitting equipment, and not cre-ating air-filled cavities that cannot be vented, aswhen using goggles or ear plugs.

The most common site for barotrauma of de-scent is the middle ear (Figure 26-9). Normally,the only connection between the ambient air andthe air inside the middle ear is the Eustachiantube. This tube may not be patent due to inflam-mation from allergies or an infection or due tocompression during the dive from a large pres-sure gradient that has already developed betweenthe middle ear space and the upper pharynx. Asa diver descends, he or she must perform a Val-salva maneuver to equalize the pressure in themiddle ear space. If the Eustachian tube is notoperating properly, middle ear pressure cannotbe increased and there will be a pressure differ-ential across the tympanic membrane, drawing itinward and causing pain. With continued descent,the middle ear may fill with blood from surround-ing tissues in an attempt to equalize pressure orthe tympanic membrane may rupture. As the larg-est pressure-volume change occurs within thefirst additional atmosphere (33 fsw) of descent,once a diver clears that distance, barotrauma ofdescent becomes uncommon. Symptoms ofmiddle ear squeeze include pain (caused by theextreme tympanic membrane stretch), temporaryvertigo, decreased hearing, or tinnitus.

Inner ear barotrauma, although relatively un-common, usually presents in the context of con-

comitant middle ear barotrauma and requiresmedical attention. The majority of these injuriesare produced during descent by an excessive Val-salva against a blocked Eustachian tube. Whenthe pressure differential from the middle ear tothe environment exceeds 90 mmHg or at a mere 4fsw, the Eustachian tube will not open regardlessof the force of the Valsalva. The relative vacuumin the middle ear will draw the eardrum, and theround and oval windows into the middle earspace. A Valsalva maneuver may raise the intrac-ranial pressure several hundred millimeters ofmercury, which is transmitted through the co-chlear duct, exploding the round window into themiddle ear. This force may cause tears in any ofthe membranous windows or vestibular apparatior cause inner ear hemorrhage. The trauma oftenproduces a severe vertigo, with sensorineuralhearing loss and tinnitus, and may be confusedwith other ear conditions.6,7 All persistent symp-toms of vertigo or hearing loss in a diver shouldbe meticulously evaluated. If a diagnosis of in-ner ear barotrauma is suspected, the diver shouldbe placed in a semi-sitting position and kept asstill as possible; intravenous diazepam (Valium)may be useful to control vomiting. The patientshould be evaluated expeditiously by an otolaryn-gologist.57

Sinus squeezes are less common than middleear barotrauma. The frontal sinuses are the onesmost frequently involved, but any air-filled sinusmay be injured. Pain in the face over the affectedsinus is the most common presentation, but thereis sometimes a bloody nasal discharge in themask. A blocked sinus ostium prevents normalventing of the sinus to release air.

Middle ear oxygen absorption syndrome canpresent as a middle ear squeeze, but its patho-physiology is different. Ear pain may follow longoxygen dives (as with a LAR V rig) after gas witha very high oxygen percentage enters the middleear cavity and the oxygen is absorbed through themucosa of surrounding tissues. If the middle earspace is not actively vented to replace the oxy-gen with air, a negative pressure relative to am-bient may result from the oxygen being absorbed.Without an inert gas such as nitrogen in themiddle ear, there will be insufficient volume sup-port to the tympanic membrane. The diver maynotice ear fullness the morning after a long oxy-gen exposure, which may be accompanied bysharp pain with ear-clearing maneuvers and tran-sient hearing loss. Equalizing the pressure will

Fig. 26-9. The anatomy of the ear in frontal section.Source: US Department of the Navy. US Navy DivingManual. Rev 4. Washington, DC: Naval Sea Systems Com-mand; 1999: 3-22.


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eventually relieve the symptoms, although some-times a serous effusion persists.5

Treatment of barotrauma of descent is straightfor-wardstop the descent. The diver should be broughtup a few feet; if this does not relieve the symptoms,the dive should be terminated. Once on the surface,the patient should be seen by medical personnel toevaluate the degree of trauma. If the diagnosis of asqueeze is confirmed by examination, the divershould be kept out of the hyperbaric environmentuntil the tissue damage heals. This period of time isvariable depending on the extent of injury; a rup-tured tympanic membrane may require weeks to healbut blood in the middle ear may require only daysto resolve. A mild squeeze may not stop a diver fromdiving the next day, yet a severe squeeze may keepthe diver out of the water for weeks. Each case ofbarotrauma of descent must be individually evalu-ated, but no form of barotrauma is treated by recom-pression except AGE.

Dysbaric Osteonecrosis

Dysbaric osteonecrosis is not a diagnosis that is madein the acute setting; it is primarily a chronic disease ofcareer divers and is strongly associated with satura-tion diving. The pathogenesis for the disease is un-known, but it is suspected to be a result of extreme pres-sures causing direct damage to bone (ie, gas osmosis)or multiple microinfarctions from asymptomatic DCS.6,7

The bone lesions present as aseptic necrosis, which canmanifest as deep bone pain or be entirely asymptom-atic, depending on the location of the lesion. Juxta-articular lesions are most painful, but midshaft lesionsare mild and most commonly discovered on routineradiographs. Treatment is nonspecific, and nonsteroi-dal anti-inflammatory drugs and, for debilitating con-ditions, joint arthroplasty are effective. Personnel withjuxta-articular lesions are permanently disqualifiedfrom diving, while those with asymptomatic lesionsare considered on a case-by-case basis.38

EPIDEMIOLOGY OF DIVING CASUALTIES AND DECOMPRESSION ILLNESS

AGE and DCS may be difficult to distinguishclinically, due to the similarity of their signs andsymptoms. Decompression Illness (DCI) is nowused to describe cases which have characteristicsof both AGE and DCS. Military diving, when com-pared with civilian sport diving and commercialdiving, is relatively safe, but comparisons are diffi-cult because accurate population-based data on DCIand diving fatalities are difficult to obtain. Militarydiving medical personnel can gain valuable insightsby reviewing underwater accident information fromboth sport and commercial diving. The Naval SafetyCenter (NSC) collects diving accident and mishapdata for the Department of Defense, which has aknown and quantified population at risk.2,20,39 Allmishaps, cases of DCI, and fatalities must be re-ported. In addition, the computerized Dive Report-ing System provides excellent data for population-based analysis of military diving accidents.2,3

Civilian Experiences

Sources of Data

Various organizations, including the NationalUnderwater Accident Data Center, Diver Alert Net-work (DAN), and the Australian Diving MedicineCenter, have compiled diving accident statisticssince the early 1980s for sport, open-circuit, and airscuba diving.7 DAN, a nonprofit diving safety as-

sociation affiliated with Duke University MedicalCenter, Durham, NC, depends on a network of 247hyperbaric chambers in the United States andaround the world to report DCI and diving fatali-ties.7,40 DAN annually solicits participation from hy-perbaric facilities and requests standardized DANforms for each DCI case or diving fatality. All casesmust meet strict inclusion criteria to be used inDANs annual Report on Diving Accidents andFatalities.38 These data are excellent for their char-acterization of factors leading to recreational scubafatalities and cases of DCI, but they are not adequateto compute accurate diving accident rates becausethere is no precise count of either the total numberof dives or divers to be used as denominators. Inaddition, civilian DCI cases are notoriouslyunderreported, and there is no autopsy protocol fordiving fatalities that is widely accepted and prac-ticed throughout the United States.

Risk Factors

Most fatalities and DCI cases involve multiplefactors that interact as confounders, yet understand-ing these factors is crucial to the development ofrisk management strategies. The DAN data suggesta very crude fatality rate of 2 to 4 deaths per 100,000sport divers annually from 1989 to 1994.40 Olderdivers (older than 50 years) and younger divers(younger than 24 years) have higher-than-expected


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fatality rates. Cardiovascular disease has been aleading cause of death in DAN fatality case reports,especially among older divers. Fatigue as the resultof poor physical fitness is an aggravating factor.Inexperience, risk taking, and alcohol use within 12hours of their fatal dive often are contributing fac-tors in all divers but particularly those younger than24 years of age. DCI rates were increased in diversdoing repetitive daily dives and dives deeper than80 fsw. Interestingly, over 80% of the 566 cases ofDCI reported to DAN for 1994 developed neuro-logical symptoms. This high percentage of moreserious decompression sickness may be due tounderreporting of pain-only DCS, to more liberaldecompression procedures, or to time delays in ini-tiating recompression treatment. Only two thirdsof the reported 55 cases of AGE and less than 40%of the 513 cases of DCS were treated with hyper-baric oxygen within 12 hours of surfacing. This rep-resents a significant delay to recompression therapyand adversely affected clinical outcomes. Causes fortreatment delays include patient denial of symp-toms, failure to recognize the signs and symptomsof DCI, long transportation times from remote div-ing locations to a chamber, and failure of symptomsto spontaneously resolve with other therapies.40

Decompression computers and decompressiontables increase awareness of the need for slow as-cent rates and decompression, but sometimes diversassume that since they follow tables or their com-puter, DCS will not occur. To the contrary, many DCIcases follow no-decompression dives or those whichare not expected to require decompression.39,40

Accident Rates

Commercial diving accident rates are more un-reliable than sport diving rates. No central data

collection organization exists, and commercial div-ing accident information is often sketchy. There areno accurate estimates of the number of commercialdivers at risk, let alone the total number of divesmade. Rough estimates from the United KingdomDepartment of Energy suggest diving mortalityrates in the North Sea are similar to, or better than,the mortality rates in the construction industry.7

While many sport divers follow US Navy no-decompression limits, commercial divers often dodeeper dives, including mixed-gas or saturationdives, and do not follow military decompressiontables when they complete their underwater work.Nevertheless, more attention is given to dive plan-ning, equipment selection, and medical screeningof divers by industry than by sport divers. Theemphasis on safety in industry is driven somewhatby financial considerations because accidents de-lay completion of work, increase personnel andmaterial costs, and increase insurance premiums.The military has similar concerns.

Military Data

US military diving operations are conducted ina disciplined manner by well-trained divers. TheNSC studies all military diving mishaps in the con-tinuing interest of accident prevention and empha-sizes investigation procedures, reporting require-ments and lines of responsibility for diving mishapand diving fatality inquiries. Unlike sport divingand commercial diving, near misses and nonfa-tal accidents are also evaluated for risk managementand corrective action. NSC accident data can be re-garded as reasonably accurate but not applicableto diving outside operational military dives.2,7,39,41

Table 26-6 shows a comparison of the use of vari-ous diving rigs in the US Navy from 1991 through

TABLE 26-6

TOTAL NAVY DIVES BY DIVING APPARATUS, 19911995

RIG 1991 1992 1993 1994 1995

SCUBA 55,499 57,133 53,605 44,990 26,324MK 20 8,488 9,388 13,204 12,420 8,705MK 21 10,923 13,793 13,865 11,489 9,972LAR V 16,912 17,458 15,802 15,169 11,474MK 16 4,862 3,607 4,154 3,450 2,674CHAMBER 5,625 6,099 5,222 4,913 4,668OTHERS 12,729 10,152 1,648 730 334TOTALS 115,038 117,630 107,500 93,161 64,151

Data sources: (a) US Naval Safety Center. Naval Safety Center Diving Database. Norfolk, Va: US Department of the Navy; 1997. (b)Butler FK, Thalmann ED. A Procedure for Doing Multiple Level Dives on Air Using Repetitive Groups. Panama City, Fla: US NavalExperimental Diving Unit; 1983. NEDU Report 13-83.


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1995.20,42 During this period, 84% of Navy diveswere shallow (between 10 and 50 fsw). An evengreater percentage of dive profiles in the Army, AirForce, and Marine Corps, where Special Operationsdiving constitutes a larger share of the total dives,are in this range. Most EOD and Special Operationsdives use closed-circuit scuba, either the LAR V orthe MK 16 closed-circuit oxygen rebreather rigs. Forthe 5-year period mentioned, 25% of diving mis-sions were Special Operations, 25% were ships hus-bandry, and 20% were EOD. Inspections, searches,and underwater construction each made up 5% ofthe total number of dives. Salvage operations, whichare often at 50 fsw or deeper, constituted only 1.8%of dives during this period, although there was anincrease in the total percentage of deeper dives (>50 fsw) in years when there were deeper salvagejobs or research protocols with deeper dives. Thesedata are representative of the evolution of US mili-tary diving away from deeper, mixed-gas, bouncediving, or saturation diving to shallower, open- andclosed-circuit scuba diving.20,39

The NSC Dive Reporting System maintains a data-base for epidemiologic studies of military diving.Mandatory reporting requirements, regular safety in-spections of diving units, and computerized local dataentry were established to collect a reasonably com-plete set of data.2,3,39,41 The NSC diving database haslogged more than 2.3 million military dives from 1985through 1995, but good quality injury data were notcollected until 1990.2,3,20,41 During the period 1 Janu-ary 1990 to 31 December 1995, there were 382 cases ofDCI reported to the NSC in divers fr

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Military Diving Medicine