Neuromuscular blockade: Electromyographicand mechanical versus visual interpretationMAJ. ( ; L K N N A. I I A R D K S T Y , CRNA, MSN, USAK, NCFairficUL Califonmi

Ancsthcttsts frcifucnlly providetntraoperative musctc relaxation in additionto general anesthesia. However, visualinterpretation of the effect of neuromuscularbtockuig drugs is not always possible.

This study examined two alternativetnethods felectromvographv/electrocardiographv[EMG/ECG\ and mechanical/ECG)of interpreting ucuromuscular btockade andcompared these methods to vtsualinterpretation. EMG/ECG andmechanical/ECG ttiethodologies were foundio provide rctiabte valid intraoperativeinterpretation of nondepolarizingneuromuscular btockade for single-twitch andtrain-of-four stimuli.

EMG/ECG and mechanical/ECGmeasures of neuromuscular hlockade wereperformed with an electrocardiographicmonitor and a pressure transducer,respectively. Both EMG/ECG andmechanical/ECG. when compared to visualinterpretation, were found to be equally, andusuatty more, vatid indicators ofneuromuscular blockade. The clinicalsignificance of this study is itscontribution to ijuality care and patient safety.When visual monitoring of neuromuscutarbtockade is not feasible, either EMG/ECG ormechantcal/ECa provide an alternativemethod of momtonng neuromuscularbtockade.

IntroductionIt is common practice to use adjunctive neuromus-cuiar biocking drugs during general anesthesia toprovide muscle relaxation. However, determiningthe degree of blockade can be difficult. The periph-eral nerve stimulator (PNS) is used to determinethe degree of residual muscle activity in the pres-ence of neuromuscular blocking agents. In stan-dard ciinicai anesthesia practice, the degree of neu-romuscular blockade is visually evaluated throughinterpretation of the evoked muscular response tothe PNS. Interpretation of the evoked PNS responseallows the anesthetist to titrate neuromuscularblocking drugs, thereby ensuring adequate musclerelaxation as well as prompt reversal of neuromus-cular blockade.' ^

Although visually monitoring the degree ofneuromuscular blockade is the clinical standard,certain surgical procedures preclude visual accessto the site stimulated hy the PNS. The researchproblem addressed by this study involved the needto validate alternative methods for monitoring thedegree of neuromuscular blockade in patients forwhom visual access is not available. The purpose ofthis study was to determine if two nonvisual mea-sures (electromyography and mechanical) ofnondepolarizing neuromuscular blockade could besubstituted and prove a reasonable alternative tovisual interpretation of PNS stimuli.

The proposed alternative methods were devel-oped from concepts derived from a review of thephysiology of neuromuscular transmission and theaction of specific blocking agents. A review of thatmaterial will be presented, followed by a discussion

Journal of the American Association of Nurse Anesthetists

of evaluation of neuroniusculur blockade and thena full discussion ol this study.

Physiology of neuromuscular transmissionA motor nerve enters a muscle and then

branches repeatedly depending on how fine themovoment is for that particular nuiscle.'' Neuro-muscular transmission starts with a nerve actionpotential at the nerve terminal and terminates withdepolarization of the postjunctional membrane. Insummary, neuromuscular transmission has multi-ple components. First, acetylcholine (ACH) is syn-thesized in the nerve terminal from choline andacetate. Depolarization of the nerve terminal re-sults in the influx of sodium and calcium and theefflux of potassium ions. The effect of the actionpotential is a quanta release of ACH molecules.ACH diffuses across the synaptic cleft and interactsthrough a complex process with the muscle at thepostjunctional membrane end-plate receptors toproduce depolarization and muscle contraction.ACH is rapidly hydrolyzed by the enzyme acetyl-cholinesterase. and the neuromuscular junction'scapacity to be depolarized is reestablished.

Neuromuscular blockade pharmacologyAnesthetists use two major classes of neuro-

muscular blocking drugs to counter depolarizationat the neuromuscular junction and produce muscu-lar relaxation. The depolarizing neuromuscularblocking drugs (e.g., succinylcholine) act like a mas-sive dose of ACH. Therefore, by the law of massaction, the neuromuscular junction remains depo-larized and muscles remain relaxed until hydroly-sis of the depolarizing muscle relaxant drug occurs.-

The nondepolarizing neuromuscular blockingdrugs (e.g., atracurium, vecuronium, pancuronium,d-Turbocurarine) were the primary focus of thisstudy. The nondepolarizing muscle relaxants com-pete with ACH for the end-plate receptor sites andprevent depolarization of the postsynaptic mem-brane (i.e., the muscle). The nondepolarizing mus-cle relaxant drugs are reversed with anticholines-terase drugs. Anticholinesterase acts to increase theconcentration of ACH. By the law of mass action,ACH displaces the nondepolarizing drug whichallows ACH to act at the end-plate receptor site.'

Interpreting neuromuscular blockadeMovement of respiratory muscles, surgical re-

laxation and sustained head lift are indices of thedegree of neuromuscular blockade; however, dur-ing tbe intraoperative period the most effectivemonitoring technique to determine the effect ofnondepolarizing muscle relaxant drugs is interpre-tation of the evoked response to PNS stimuli.' The

slimuhis from u I*NS, wlu-n a[>pli<'fl to a motornerve wilh intact ncuroniusc ular transmission, willresult in the contraction of those mu.sclcs .sup|)liedby the sliinulatcd nerve' 7 he stimulus from thePNS must be of supramaxinial intensity, that is, ofsufficient intensity to ensure that all muscle fiberssupplied by the nerve will contract. As muscle re-laxant drugs are administered, the force of musclecontraction will be reduced. The measured reduc-tion in contractile force at unchanged supramaxi-nial stimulation is an expression of the degree ofneuromuscular blockade.' The adductor pollicismuscle is innervated solely by the ulnar nerve.*^This study included recording the mechanical forceand electromyographic (EMG) activity of the pa-tient's adductor pollicis muscle.

• Tx'pe of stimulation. Clinical nerve stimulatorsare capable of eliciting three basic types of supra-maximal stimuli: single twitch, tetanic and train-of-lour (TOF).' Supramaximal single-twitch stimula-tion is delivered at a frequency of 0.15 Hz and theimpulse is rectangular with a duration of 0.2 milli-seconds. The twitch response is not reduced until75-80% of the nicotinic receptors in the neuromus-cular end plate are blocked; the response disap-pears completely when 90% of the receptors areblocked.'' Tetanic stimulation is a continuous rapidrate (50 Hz or 100 Hz). The most physiologic tetanicstimulation is 50 Hz for five seconds which is equiv-alent in stress to a maximal voluntary effort.' Asneuromuscular blockade increases, the tension, orsustained tetanic response to the tetanic stimula-lion, decreases. This phenomenon is known as fade.

Tetanic stimulation results in a large amount ofACH release from immediately available stores inthe nerve terminal." As these stores become de-pleted, the rate of ACH release is proportional tothe rate ACH is manufactured. Fade develops as aresult of both the decrease in rate of release ofacetylcholine and decrease in the muscle's ability torapidly respond to the tetanic stimulation.^" Te-tanic stimulation only gives a crude interpretationof degree of block when compared to TOF stimula-tion. The tetanic stimulus is primarily useful whenit is used in conjunction with post-tetanic twitch tointerpret intense nondepolarizing neuromuscularblockade in the absence of response to single-twitchor TOF stimulation. On average, post-tetanic twitchappeared 36 minutes before response to TOF dur-ing pancuronium administration and eight min-utes before response to TOF during vecuroniumadministration." " The post-tetanic twitch stimulusmay be present when the tetanic stimulus has disap-peared. The phenomena of post-tetanic twitch facil-itation is due to the mobilization of ACH from thereserve to the readily available stores."' The in-

februarv 1991/ Voi 59/No. 1 83

creased rotuintration ol ACII outlasts the periodo f U ' t a n i t ^ t i Iml l ;^^ i ( )n .

lVain-of-rovir includes the concepts rOf fade;ind rOF ratio. "• VOV is dt'fiiu'tl as four individualsuprainaxiiual slinuili al intervals of 0.5 scrtnulsover a prridd »tf 12 seconds (L' 11/). Tlu' rOK ralio iscalculated by comparing tbe ainplitudf of tin-lomtbresponse of the TOF to the first response of theTOK TOK fatU' refers to bolb the decrease in sizeand number ol subsequent stimuli following tbeinitial stimulus in one TOK

The TOF has several advantages compared tosingle-twitchor tetanic stimulation.'^-"' '-'First, theTOF allows the anesthetist to quantitatively esti-mate tbe dej;ree of neuronuiscular blockade witb-out a control response. The TOF at 2 Hz (2 stimuliper second) is not as uncomfortable to a consciouspatient as a 30 Hz (50 stimuli ]>er second) tetanicstimuli. The TOF ratio of 0.8 or greater correspondswith otber indices of adequate reversal of neuro-niuscular blockade. The degree of neuromuscularblockade ranging from 75-95% twitch inhibition de-fines satisfactory clinical muscle relaxation." Dur-ing nondepolari/ing neuromuscular block, thefourth response of the TOF is eliminated at 75%twitch inhibition: the third and fourth TOF re-sponses are eliminated at 80% twitch inhibition;and the second, third, and fourth TOF responsesare eliminated at 90% twitch inhibition.

Unlike tetanic stimulation, the TOF does notaffect the subsequent patterns of recovery from neu-romuscular blockade (i.e.. it does not causepoststinnilation facilitation of subsequent PNS stim-uli). To ensure that the PNS stimulus does not alterinterpretation of neuromuscular blockade, nervestimulation frequency limits bave been established.^I he single-twiicb, TOF^ or tetanic nerve stimula-tion should not be administered more frequentlythan every 6-10 seconds, 10 seconds or 6-10 minutes,respectively.

Intraoperatively, in order to achieve sufficientmuscle relaxation, it may be necessary to increaseneuromuscular blockade to tbe extent that tbere isno reaction to nerve stimulation. Respiratory move-ments may occur with twitch depression of 90% dueto decreased sensitivity of respiratory muscles tomyoneural blocking drugs, surgical stimulationduring superficial anesthesia or peripheral coolingwhich results in respiratory muscles recoveringfaster tban cooler peripberal muscles secondary toinhibited diffusion of myoneurai blockers."

Methods for response interpretationTbere are five metbods used to interpret the

PNS-evoked muscular response: visual, tactile, elec-tromyograpbic, mecbanical and neuromuscular

transmission analysis.^ '" Neuromuscular transmis-sion studies are primarily used in formal drug stud-ies and are not in general clinical use. As previouslymentioned, the standard method is visual interpre-tation of the PNS-evoked response. Often anesthe-tists will use tbe tactile method when visual access isimpossible. Tactile methods are generally imprac-tical or impossible whenever visual access is a prob-lem. The only two clinically feasible alternatives tovisual/tactile interpretation are the electromyo-grapbic and mecbanical methods.

• EMGfECG interpretation. Generally, EMGstudies are conducted witb sopbisticated computer-ized monitors. These expensive, complicated in-struments are not available in many operatingsuites. However, an adequate electromyographyreadinj^ can be obtained from a simple rearrange-ment of tbe leads for the standard electrocardio-graph (ECG) monitor, which is uniformly availablein operating room suites. In this study, tbis arrange-ment will be referred to as the "EMG/ECG." TheEMG records the compound action potential, thatis, tbe action potentials of many muscle fibers ratherthan a single muscle fiber.

An oscilloscope and ECG strip chart recordercan be used for electromyography.- Tbe ECG leadsare positioned at the insertion of the stimulatedmuscle, and the PNS-evoked activity is reproducedon tbe oscilloscope and strip chart recorder.

• Mechanical/ECG interpretation. Mechanicalmetbods of interpreting neuromuscular blockaderequire recording tbe evoked force of tbe muscularcontraction.- Any mecbanical force (for example,muscular contraction) can be measured with a pres-sure transducer.

Transducer is the general term for an instru-ment that converts a mecbanical force into an elec-trical signal. The pressure transducer most com-monly used in biomedical instrumentation is amechanical or displacement transducer, so calledbecause it consists of a mechanical element that isdisplaced as a result of changes in pressure. A fluidpressure wave in the dome results in displacementof a mechanical element in the transducer hub. Thedisplacement in tbe bub causes a proportionalchange in the electrical signal emanating from thetransducer." The mechanical pressure transducerused in the anesthesia clinical setting is the stan-dard strain gauge pressure transducer wbich allowsmeasurement of low-frequency parameters (e.g.,central venous pressure) when the transducer isconnected to an ECG monitor.'^ Mechanical activ-ity of neuromuscular blockade can be recorded us-xn^ the ECG and pressure transducer and will there-fore be called "mechanical/ECG."

Tbe specific purpose of this study was twofold:

84Journal of the American Association of Nurse Anesthetists

(1) to determine intraoperatively, during generalanesthesia requiring nondepolarizing muscle re-laxation, the effectiveness of hoth of the proposedEMG/ECG and mech;niical/ECG methods of inler-preting PNS stimuli (when compared to visual in-terpretation); and (2) to show that the proposedEN1G/ECG and mechanical/ECG interpretationmethods are reliable substitutes if visual interpreta-tion is impractical.

Methodology• Design and sample. A modified counterhal-

ance design was used, that is, visual evaluation ofthe response to a particular PNS stimulus served asthe control while simultaneous EMG/ECG andmechanical/ECG responses to the satne stimuluswere recorded.'-'Therefore, every PNS stimulus wassimultaneously evaluated hy visual, EMG/ECG andmechanical/ECG methods. Since each PNS inter-pretation method (visual, EMG/ECG and mechan-ical/ECG) was performed simultaneously and actedas its own control, the dosage schedules and timeperiods of the nondepolarizing drugs did not haveto be regulated. This allowed random sampling tobe done hefore, during and after the infusion of anondepolarizing muscle relaxant. All the suhjectsreceived an infusion: 11 subjects received atracu-rium (0.5 mg/cc) and 1 subject received vecuronium(0.1 mg/cc).

Four to six random simultaneous samples foreach subject were taken during the intraoperativeperiod when different degrees of neuromuscularblockade existed. There was a total of 66 samplesobtained for the 12 subjects. A sample consisted ofPNS stimuli administered in a prescribed order.Since the visual response to the PNS stimuli actedas the control, the simultaneous responses of theEMG/ECG and mechanical/ECG were recordedwhile each subject received a nondepolarizing mus-cle relaxant infusion.

Subjects were 12 adult males, ASA Class II-III,56-77 years old, 60-107 kilograms body weight withno prior history of neuromuscular disease. All sub-jects were scheduled for surgery at the ClevelandVeterans Administration Medical Center, weregiven general anesthesia with adjunct nondepolar-izing muscle relaxant drugs and were scheduled forsurgical procedures which permitted visual,EMG/ECG and mechanical/ECG interpretation ofulnar nerve neuromuscular blockade.

• Apparatus/instruments. Ulnar nerve stimula-tion was achieved using the Bard Critical Care(Model 750 Digital®) peripheral nerve stimulator.The Mennen ECG monitor (Model 742®), withbuilt-in strip recorder and ECG cable, was used tosimultaneously observe and record the EMG and

lnins(lu(<'(l mcduinical prc.s.sun* waves. Methanicalinterprelulioii (;l |)re,ssure was accomplished wilh astandard strain gauge pressure transducer (GouldP2.'iXL"'). Other materials included a dorsal sup-port arm hoard for the wrist and hand, two 250 mlbags of saline, one set of low pre.ssure and one set ofhigh pressure intravenous(IV) tubing and one pres-sure bag. Data interpretation was performed usingthe Statistical Package for the Social Sciences (SPSS).

• Procedure. Approval of the Cleveland Veter-ans Administration Medical Center's human inves-tigational and Case Western Reserve University'sSchool of Nursing review boards was obtained. Thestudy was explained in detail and informed consentwas obtained. The principal investigator's role waslimited to data collection. The investigator did notserve as anesthetist for any subject and had no rolein selecting or administering anesthesia. Generalanesthesia was provided using a balanced narcotic-inhalation technique and adjunct nondepolarizingmuscle relaxant infusion. Normal core body tem-perature was maintained.'^^-

Prior to the induction of general anesthesia,each patient was prepared as in Figure 1. Once thepatient was supine on the operating table, one armwas placed on a standard padded operating tablearm board in a supine position with the palm upand the arm positioned at approximately 80 de-grees to the torso. Ulnar nerve stimulation of theadductor pollicis muscle was achieved by attachingthePNS at the elbow with two cutaneous electrodes;the negative electrode was placed at the ulnar notchbetween the olecranon and medial epicondyle andthe positive electrode was placed approximatelyone-inch distal to the negative electrode. Place-ment of the PNS electrodes distal to the elbow re-sulted in recording the nerve stimulus rather thanthe EMG, that is, there was a "leakage of PNS cur-rent" which only permitted recording of the PNSstimulus rather than recording the desired EMGactivity. A sample consisted of PNS stimuli admin-istered in the following order: TOF, single-twitchstimulus, tetanic stimulus (50 Hz x 5 seconds) andpost-tetanic single-twitch stimulus. As previouslydiscussed, recommended time intervals amongstimuli and between samples were maintained.' '-Following placement of the PNS, the hand of thesame arm was prepared to permit simultaneousEMG/ECG, mechanical/ECG and visual record-ing of the PNS stimuli.

• Eleclromyography (EMG/ECG)procedure. Withthe ECG monitoring lead II. the positive ECG leadwas placed on the palmar surface of the thumb atlhe insertion of the adductor pollicis muscle, thenegative lead was placed on the palmar surface ofthe index finger, and the ground lead was placed on

February 1991/ Vol. 59/No. 185

t h r dorsinii of the li;iiul (I'igurt' 1). All leads werealUuIicd with cut;nu'ou.s noiuilU'igenic iidht'sivocU' t tuxlo p;uls. I h f patient 's liimd was placed in ador.sal suppor t arm hoard .

• Mcchiiniidl/F.CG pnnrdurc. VVitlt the excep-tion ol not heparini/ing the solution, (he pressuretransducer was assemblt d using the standard arte-rial pressure monitor assembly method.''' All airwas removed from a 250 ml bag of normal salineand replaced with fluid to a calibrated pressurereading ol approximately 50 torn The pressure sen-sitivity on the ECG monitor was maintained at 25torr. A l4-i;auge. 2-inch Bectin-Uickinson IV cathe-ter was used to attach the high pressure line to thebag of saline. The transduced bag of normal salinewas placed in the same hand with the PLMG/ECGelectrodes (Fignre 1). The EMG/ECG and mechani-cal/ECG apparatus were secured to the hand with adorsal arm board support.

Figure 1Placement of apparatus for simultaneousinterpretation of EMG/ECG, mechanical/ ECGand visual data

ECG Monllor

TD

TD-strain gauge pressure transducerPNS-peripheral nerve stimulator capable of elicitingsingle-twitch, tetanus and train-of-four stimuliNS-250 ml bag of normal salineECG monitor—electrocardiographic monitorMW-mechanical/ECG waveformEW-EMG/ECG waveform

• Vhual interpretation procedure. Prior to initiat-ing PNS stimuli, the monitored hand was exposedto permit the investip^ator to view all digits, espe-cially the thumb. The KCG chart recorder was acti-vated to permit simultaneous recording of EMG/ECG and mechanical/ECG activity. The PNS stim-uli were initiated and the adductor pollicis muscle

respon.se visualized was recorded and compared tothe simultaneously recorded EMG/ECG andniechanical/E(X; activity. Visual interpretationserved as the control for the EMG/ECG andmechanical/ECG methods.

Results• Waveform results. The specific neuromuscu-

lar activities measured by EMG/ECG recordingswere found to be more rapid and circumscribedthan the "mechanical" muscular contraction eventsthat are measured by the transducer. An example ofsimultaneous EMG/ECG and mechanical/ECG re-cordings for each of the three PNS stimuli (TOF,pretetanic single-twitch and post-tetanic single-twitch) are presented in Figure 2. As can be seen, theEMG/ECG waveform is very narrow and specific tothe stimulus. See tracings A, B and C in Figure 2.)The mechanical/ECG waveform is relatively wide(tracings D, E and F in Figure 2) compared to theEMG/F^CG tracings. Counting the number of wave-forms on the mechanical/ECG recording for theTOF stimulation (e.g., tracing D in Figure 2) wasoften difficult especially when there were more thantwo out of four TOF stimuli detected by themechanical/ECG method. It was interesting to notefor Figure 2 that visual interpretation of the PNSTOF stimulation saw only one of the four TOFstimuli while the simultaneous EMG/ECG andmechanical/ECG recordings (tracings A and D inFigure 2), recorded four and three of the four TOFstimuli, respectively.

• Statistical results. Employing the SPSS statisti-cal package, each PNS stimulus (single-twitchpretetanus, single-twitch post-tetanus and TOF) wascorrelated with EMG/ECG interpretation of PNSstimuli versus visual interpretation of PNS stimuli,as well as mechanical/ECG interpretation of PNSstimuli versus visual interpretation of PNS stimuli.With visual interpretation of PNS stimuli servingas the control, the correlation (percentage of agree-ment) between visual and EMG/ECG and the cor-relation between visual and mechanical/ECG in-terpretation of the PNS stimuli are presented inTable 1. Table II presents the instances when eitherEMG/ECG and/or mechanical/ECG actuallyproved more sensitive to interpreting PNS stimulithan the visual control method of interpreting PNSstimuli. As was previously noted, tetanic stimuliwere not able to be recorded by EMG/ECG ormechanical/ECG methodologies; therefore, therewere no correlations between the visual interpreta-tion with either the EMG/ECG or the mechan-ical/ECG interpretation methods.

The TOF PNS stimulus is one of the mostuseful stimuli for determining the degree of neuro-

86 Journal of the American Association of Nurse Anesthetists

Figure 2Examples of EMG/ECG and mechanical/ECG waveforms^

EMG/ECG

A. Train-of-fourstimulation^

B, Pretetanic single-twitch stimulation

C. Post-tetanic single-twitch stimulation

Mechanical/ECG

D. Train-of-fourstimulation^

E. Pretetanic single-twitch stimulation

F Ftost-tetanic single-twitch stimulation

1. EMG/ECG and mechanical/ECG recordings were simultaneous on lead 11 and pressure setting of 25 torr, respectively2. Visual saw only one of the four train-of-four stimuli.

muscular blockade. Cross tabulation comparing thenumber of individual stimuli simultaneouslycounted by the visual method versus the EMCJ/ECGmethod of PNS interpretation proved that theEMG/ECG method was just as accurate as visualinterpretation of the TOF stimulus (Figure 3). Com-paring visual interpretation of the TOF stimuluswith simultaneously recorded mechanical/ECG in-terpretation proved that the mechanical/FCGmethod of interpretation was usually as accurate asvisual interpretation of the TOF stimulus (Fiji;ure4).

EMG/ECG versus visual interpretationFor response to the PNS single-twitch (pre- and

post-tetanus) and TOF stimuli, there was perfectcongruence between visual and EMG/ECG recog-nition of neuromuscular response (Table I). Thatis, each time the PNS stimulus was delivered to theulnar nerve, the adductor pollicis muscle responsevisualized was also reflected in an oscilloscope de-flection. However, the EMG/ECG and mechani-cal/ECG methods frequently recorded responses toPNS stimuli (Table II and Figure 3) before thesesame stimuli were able to be visually recognized.

februarv 1991/ Voi 59/No. 1 87

Table IPercentage agreement: Visual comparedwith electromyography (EMG/ECG)^ andmechanical/ECG^

Type ot PNS stitvutusPNS stimulustnterpretattonmettiod

Single- Single-twitct) twitch Train-

pretetanus^ post-tetanus^ ot-four^

EMG/ECGcomparedwith visual 100% 100% 100%

Mechanical/ECGcomparedwith visual 98,5% 98.5% 97%

(n = 66)PNS-peripheral nerve stimulator1 EMG/ECG interpretation performed using lead II ofan electrocardiograph machine on the adductorpollicis muscle2 Mechanical/ECG interpretation performed bytransducing a 250 ml fluid bag3 Supramaximal stimulation 0.15 Hz. rectangularimpulse duration of 0 2 milliseconds4. Rfty Hz X 5 seconds5, Four individual stimuli at intervals of 0.5 secondsfor 2 seconds (2 Hz)

However, there was no agreement between vi-sual and EMG/KCG interpretation of the PNS teta-nus stimulus. That is, each time the tetanic stimuluswas delivered to the ulnar nerve, the adductorpoliicis muscle tetanic response was not reflected inan oscilloscope deflection.

Mechanicai/ECG versus visuai interpretationFor response to the PNS single-twitch (pre- and

post-tftanus) and TOF stimuli, there was a highpercentage of congruence between visual andmechanical/ECC; recognition of PNS stimuli atvarying degrees of neuromuscular blockade (Table1). Again, each time the stimulus was delivered tothe ulnar nerve, the adductor pollicis muscle re-sponse seen was also reflected in an oscilloscopedeflection. Mechanical/ECG interpretation of thePNS tetanus stimulus was similar to EMG/ECGinterpretation. That is, interpretation of the tetanusstimulus was not possible with either mechanical/ECG or EMG/ECG methods.

DiscussionMK' KMCi/ECG and mechanical/ECG were

found to be reliable indicators of neuromuscularactivity for both the single-twitch and TOF PNSstimuli (Figure 2). Ihe EM(i/ECG measurementnot only proved to be a reliable, valid alternative tovisual interpretation, but it was often more sensi-tive than visual interpretation (Table II and Figure

Tabie IIPercentage of observations: EMG/ECG^ andmechanical/ECG^ more sensKive thanvisual interpretation

Type of PNS stimulus

PNS stimulusinterpretationmethod

Single- Single-twitch twitch Train-

pretetanus^ post-tetanus" of-four^

EMG/ECG 15% 10.9% 13.8%

Mechanical/ECG 5.6% 9.8% 5.8%

(n = 66)PNS-peripheral nerve stimulator1 EMG/ECG interpretation performed using lead II ofan electrocardiograph machine on the adductorpollicis muscle2. Mechanical/ECG interpretation performed bytransducing a 250 ml fluid bag3. Supramaximal stimulation 0.15 Hz, rectangularimpulse duration ot 0.2 milliseconds4. Fifty Hz X 5 seconds5. Four individual stimuli at intervals of 0.5 secondsfor 2 seconds (2 Hz}

3). The mechanical/ECG waveform increased insize as neuromuscular blockade decreased. Conse-quently, it was sometimes difficult to separate indi-vidual TOF waveforms. Flowever, mechanical/ECG interpretation often proved more reliable thanvisual interpretation of PNS stimuli (Table II).

Limitations of the study included sex (all malesubjects), age (more than 55 years old), and the typeof nondepolarizing muscle relaxant drugs (atracu-rium = 11 cases, vecuronium = 1 case). Generaliz-ing these results to other types of nondepolarizingmuscle relaxants will require further study. Also,"leakage" of PNS current occurred when stimulat-ing PNS electrodes were placed distal to the elbow.PNS current leakage may affect the use of theEMG/ECG in pediatric cases due to the decreasedolecranon to adductor poliicis distance. Finaiiy,there are other sites recommended for interpreta-tion of PNS stimuli in addition to the ulnar nerve(i.e., posterior tibial nerve or the lateral poplitealnerve)."

In spite of the aforementioned limitations,findings in this study are of benefit to the patientwhen visual interpretation of PNS stimuli is im-practical. The apparatus used for EMG/ECG andmechanical/ECG interpretation of neuromuscularblockade allowed reliable, valid interpretation ofboth single-twitch and TOF stimuli. Although nei-ther EMG/ECG or mechanical/ECG recording oftetanic PNS stimulation is possible, the PNS, itself,provides a tetanic stimulation. Therefore, eitherECG method can be used to interpret and compare

88 Journat of the American Association of Nurse Anesthetists

Figure 3The comparison of the visual and electromyography(EMG/ECG) responses to train-of-four stimulation

Number of responsesrecognized visualty

0 1 2 3 4

Number of

responses

recognized

by EMG/ECG

8

7

1

15

8

1

6

4

1 15

pre- and post-tetanic stimulation. The comparisonof pre- and post-tetanic stimulation is especiallyimportant for interpreting profound neuromuscu-lar blockade when no reaction to TOF or single-twitch stimulation is realized.' It has been estab-lished that the TOF is the most advantageous of thethree PNS stimuli. Therefore, it is important tonote that both ECG methods permitted interpreta-tion of the TOF PNS stimulus (Figures 3 and 4).

The fact that the FMG/FC(J and mechanical/ECG waveforms can be measured and recorded issignificant. One can calculate a TOF ratio when theEMG/ECG and mechanical/ECG methods of inter-pretation are used. Calculation of a TOF ratio is notpossible with visual interpretation.

ConclusionsThe significance of this study lies in its poten-

tial to improve anesthesia practice and increase pa-tient safety. Insufficient relaxation of the patientunder general anesthesia can inhibit surgery or, atworst, result in injury to the patient due to unex-pected movement. Excessive neuromuscular block-ade can inhibit neuromuscular blockade reversalresulting in prolonged ventilatory support. Thepotential benefit of this study is that it presents twoalternatives to determine the degree of neuromus-cular blockade when visual interpretation of PNSstimuli is not practical.

Another benefit of using either the EMG/ECGor mechanical/ECG methods of interpretation con-

Fjgure 4Visually observed versus mechanical/ECG recordedresponses to the train-of-four stimulation

Number of responsesrecognized visually

0 1 2 3 4

Number ofresponsesrecognizedby mechani-cal/ECG

13

3

r

18

3

2

2'

4

5

15

'Three observations out of 66 (4.5%) when visual interpretationwas more accurate than mechanical/ECG (probablysecondary to artifact from fluid pressure wave)

cerns determining the TOE ratio. Adequate neuro-muscular blockade reversal is evidenced by a TOEratio greater than O.8.' ' One can calculate a TOEratio when the EMG/ECG and mechanical/ECGmethods of interpretation are used. However, cal-culation of a TOE ratio is not possible with visualinterpretation.

In addition lo the clinical significance, perma-nent recordings of F^MG/ECG and mechanical/ECG waveforms have clinical, educational and, pos-sibly, legal significance. That is, permanent wave-form recordings can be used to interpret neuromus-cular blockade, the recordings permit visualizationof neuromuscular concepts (e.g., fade and post-tetanic facilitation), and recordings provide evi-dence of adequate reversal of neuromuscular block-ade (e.g., TOF ratio greater than 0.8). Finally, thereis a financial benefit. Transducers and ECG moni-tors in anesthesia departments can be used in lieu ofpurchasing special expensive neuromuscular ana-lyzers for use when the visual analysis of neuromus-cular blockade is not possible.

In summary, this study has shown that appara-tus, currently available in most clinical anesthesiasettings, can be reconfigured to allow reliable, validEMG or mechanical interpretation of single-twitchor TOF stimuli when nondepolarizing muscle re-laxants are administered to adult males. These find-

Febmary 1991/ Vol. 59/No. 1 89

ings art' significiint in cases when j;t'n('ral ancsthc-si;i willi ;ul|U!ut nius( lc r<'Ui\;tti()n i.s rc<]nir('d andvisual intii prt'tiitioii ol tlu* I'NS-ovokcd stimulus isinipr;K tical. l^so of oitlu-r the KMC/KCXi orMU'cluinital/F.CXi nu'tliods of intcM protatioii willloniplcnu'iit tlu' iiiu'stlu'tist'.s ahility to amtrol theoffott ot Moiulrpohiri/iu^ imisclc rrUixant driig.sand onhaiue llu' (|uality ot uiu'sthcsia care.

RKKKRKNCKS( I ) .Mi H H . I't ill 1"W| I V i i r l ) , Ict i i ini ' . iiiul In i in -nf - fn i i r ;is i i u l i r c s of

r e c o v e r y f r o m n o i u l f p u h i r i / i n j ; I K U I O I I I I I M iihii bliKWiidc, ,l;jcW/jc5it»/-

,>^- yi.29\{2] Epslfin RM, Epstein KA- I','?'! I-Ici imiiiynf;i;iphy in evaliulioii ofthe response to miiNclc relii\;nil.s, K.jl/ Kl ed. Muscle Hctaxanls. NewYork: Crime &• Stniltoii, p 'JW-:(ll.',(3) Op-en P 1^8" Relaxanl revrrsal; AniichulincsU-rasc pharmacol-ogy. /H">7 Afittudl tali Aunt' Ancithct\<,t Kevwu' Cmine txcture Notes. San.•\iitonio. Texa> Lebco (ira[)lii("v Seilion 107, p, 1-18,H) \'ib\-M(»j;i'iiM'n I I'.'N'J (..liinial jssfvsnu'nl o( neui'omii.sciilar trans-mission, Bnliih Journal oj Ancsllwiui '>l:lil) V{b\ Durant NN. 19S^. The physiology'of neuromuscular transmission-Katz RL- ed- .\tusi:U' Kcla.\iiiiti San Dii-j o (Iriine k Stralton, p. 19-38.(6) .Ali HH 1 8, Ntonitnrmn nl iiriMninii^cuhir fund inn Kat/ RL, ed..\tusite /?t'/a,T<j»ils San Die^ii: Cruiu- k Stratton pp. ?t:i-68,(7) Panskv B. House KL IMWt W.-iuu of (;rt.ii ATiatnmv. 2d ed. NewYork: MacMillan(8) Waud Bt. Wjud DR. 1^71. Lhe relation between tetanic fade andreceptor occlusion in the presence of competitive neuromuscular block.A^csth€Slotog^\ i^Abff.(9) GrahamGG.etal. 1986, Relalionshipoflrain-of-fourratiolotwiichdepre!^ion during pancunniiiini-indmcd nenromuscular blockade,Ancithcs^olog^\ tib:b'i9.(10) N'iby-MnfjcnsiMi J, et ul, 1 81 I'nNt-lelanic count (PTC): A newmethod of evaUialinp an inlcnse noniiepnlari/inn neuromuscular block-ade. Anc.'ilhciUllog^' r)r>:458.(II) Muchhal K. i-l al, 1987 Kvaliiation of inUict- neuromuscularblockade caused by \ f turoniuin UMnj; pnst ti-taiiic count (PTC),Anesihesiology. SbiS-lG.

Powers S|, ] s KM I'IH7 Krlalinnsliip hctwcen singlt' twitchssioii and liani-oMour lade: Influcnic of relaxani dose durinfj

onsel and spontaneous ofhel of nvuroniU-Hcular h\ocVadc. A ne it he sio and

(li) U'vim.re ML. Kisclt' |H, I97K, Differenlial effects of dTubtJcu-rarinc nn inspiiiiloiv mint lei and twr> peripheral muscle group% ina n e s t l i i ' l i / r d i i i . t i i A i i i ' s l l i i \ i i i l i i i i \ < - I H i H i l l ,

(If) Bruner [M, l'i7H /tl.;uH'n\uirr Monilonttfr. St. Louis: C.V. MosbyCo. Chapter Ti,(Ifi) .Schroeiler |S, Daily KK. ni7(i. Tvchrmjuvs in fk'dsidv HentodvnamicMimitoyuig SI, Lmiis: C,V, M<isby (>> |), I!!-'.*!!,(Ill) Wrioils NK (iaiiLin/arn M, I'tHK, Nunitig Uvu-arch: Theory and Prac-liir St, LoiiiM (;,V, Mnsby C(., p 177 17M.

AUTHORMaj, Glenn A Hanlcsly. CRNA. MSN, TSAK NC. is associate

director of the nurse ant'stliusia clinical training site at David GrantMedical Ci'nter, Travis Air Korct- Base, California. This study was com-pleted in jiartial rer|uircnu'nt for the MSN in nurse anesthesia (FrancesPayne Boltnii Sdnml of Niirsinj;. Case Western Reserve University), Hereceived his liA in biolof^' from Ohio Dnminical CullugL'. Columbus,Ohio; his BSN from Case Western Reserve University. Cleveland; andhis nurse anesthesia education from the US, Army Anesihesiology forNur5e Corps Officers. Academy of Health Sciences.

ACKNOWLEDGEMKNTSThe author gratefully acknowledges the support and guidance of

Mary McHugh PhD. from the inception of this project and throughoutlhe entire research pnicess. The author wftuld also like to thank John G.Fraser, MD. and Mrs, Wilma Krisco for their suggestions and facilitationof data collection at the clinlLal site.

The opinions or assertions Lontained in this article arp the personalview of the author and are not to be construed as official or as reflectingthe views of the Di-partmcnt of Anesthesiology, Travis Air Force Base,the Air Korce Nursu Corps, ilu- Departnu-nl of the Air Force or theDepartment of Defense.

90 Joumat of the Ajm-ncan Assvaalwn of Nurse Anesthetists