Inhalant Anesthetics

Inhalant Anesthetics 1) Volatile liquids

Ether (prototype – not used)Halothane, Methoxyflurane (Old)Enflurane (new)Isoflurane (Newer)Desflurane (Suprane) (Newest)Sevoflurane (Ultane) (Newest)

2) GasesN2O (still used)Cyclopropane (not in use)

Physicochemical characteristics

The action and margin of safety

How they are supplied

Equipment needed for safe delivery

How they are taken up by the lung, distributed within the body, and eliminated

Pressure exerted by the molecules of the vapor phase at equilibrium of molecules moving in and out of liquid phase

Vapor Pressure dependent on temperature and physical characteristics of liquid, independent of atmospheric pressure

↑ Temperature→↑ Vapor Pressure Vapor pressure is a measure of the agent’s ability

to evaporate (volatility) .The greater is the vapor pressure, the greater the concentration of inhalant deliverable to the patient (and environment).

Boiling Point: Temperature at which vapor pressure equals atmospheric pressure

Vapour Pressure & BP

Vapour Pressure & BP

Agent BP (0C) VP (20 0C)

Halothane 50 243

Enflurane 56 175

Isoflurane 48 238

Sevoflurane 58 160

Desflurane 23 664

Nitrous Oxide -89

Xenon -107

Solubility of Inhaled Drugssolubility (partition) coefficient - the extent to which a gas will dissolve in a given solvent

Predicts the speed of induction, recovery, and

change in anesthetic depth for an inhalant.

Ideal inhaled anesthetics should have low

blood/gas and low tissue/blood solubility and low

solubility in plastic and rubber.

Low solubility means rapid induction and emergence and more precise control

Solubility of Inhaled Drugs

Solubility of Inhaled Drugs

Halo Enflur Isoflur Sevofl Desfl N2O

Blood/Gas

2.54 1.8 1.4 0.69 0.42 0.47

Brain/Blood

1.9 - 1.6 1.7 1.3 0.5

Fat/

Blood

51 - 45 48 27 2.3

MAC

Defined as the minimum alveolar concentration of

an anesthetic agent at one atmosphere that

produces immobility in 50% of patients exposed

to a noxious stimulus.

Measurement of inhalation agent potency, which

refers to the quantity of an agent required to

produce a desired effect.

methoxyflurane (MAC = 0.23) currently is the

most potent inhalant agent available.

Ether

• Properties: Colorless, highly volatile, pungent odor,

flammable, explosive, stored in cool area.Solubility 12; MAC 2-3%

• Pharmacodynamics:Lungs: Stimulates resp, increases secretion,

not good in respiratory diseasesKidney: decreases urine outputLiver: Minimum effect, decreases liver

glycogenHeart: Initially increases cardiac output, then

decreases card. output, suppresses vasomotor center.

EtherEther as an anesthetic

• Advantage: CNS depression, excellent muscle relaxant, causes surgical anesthesia

• Disadvantage: Flammable, irritates mucus membrane, breath holding, induces nausea & vomiting

• Contraindications: Resp., kidney and liver diseases

• Better agents are available now, so not used now.

HalothaneProperties: nonflammable, expensive,

colorless, nonexplosive, nonirritating, decomposes by light, Solubility 2.3; MAC 0.87%

Pharmacodynamics: Lungs: Progressive depression, acidosis, decrease pH, given with N2O, O2

Heart: Myocardial depression, decreases cardiac output (CO), hypotension, sensitizes myocardiumLiver: hepatitis by repeated administration.

HalothaneGeneral Information:

• Introduced in 1957

• Rapid induction and recovery

• Low solubility in plasma

• Sensitizes myocardium, good muscle relaxation

• 70% exhaled as such, 30% metabolized in liver

• Malignant hyperthermia in swine reported, give Dantrolene, a phenytoin derivative of sk.mus.relax.

• Exposure during pregnancy cautioned.

Methoxyflurane

Properties: clear, sweet odor, partition coefficient 13, MAC 0.23%

Pharmacodynamics: Lungs: gradually depressed, decreases tidal

volume, respiratory acidosis, ventilation required

Heart:decreases CO, BP, sensitizes myocardiumLiver: decreased hepatic function, forms free

fluoride ionsKidney:decreased flow, metabolites cause

dysfunction and renal vasoconstriction.

Methoxyflurane

•General information:

Was extensively used in large animals, better agents are available nowintroduced in 1964 slow induction and recovery Stage II is bypassed, less CNS stimulationexcellent muscle relaxation, very good analgesicvaporization difficult safe for fetus, compatible with other agents.

EtheraneProperties: colorless, nonflammable, mild sweet

odor, volatile liquid, extremely stable, no reaction with metals, Partition coefficient 1.78, MAC 2.2%

Pharmacodynamics:Lungs: nonirritating, gradually depresses, no toxic

effectHeart: less sensitization, CO decreased, less effect

on BPLiver: no adverse effect, hepatic necrosis upon

repeated administrationKidney: no adverse effects, decreases renal flow.

EtheraneGeneral information:

-Introduced in 1973-approved in horses in 1981-seizure activity at high doses-contraindicated in patients with seizure history-rapid and smooth induction-adequate muscle relaxation.

IsofluraneProperties: widely used now, an isomer

of enfllurane colorless, less soluble, nonflammable, stable, mild pungent odor, MAC 1.5%

Pharmacodynamics:Lungs: mostly exhaled as suchHeart: lesser effects, does not sensitizes, Liver and Kidney: not injurious.

IsofluraneGeneral information:

-Approved in 1985 for veterinary practice-not a convulsive agent-malignant hyperthermia in swine reported-adequate muscle relaxation-rapid and smooth induction -rapid and smooth recovery

Newest InhalantsDesflurane (Suprane)

-Needs special vaporizer-Partition coefficient 0.42; MAC 7.20-Rapid and smooth induction and

recovery-Causes least cardiovascular or cardiac

sensitization to epinephrine-Increases intracranial pressure (↑ICP)-requires temperature controlled,

pressurized vaporizer

Desflurane

•Close to isoflurane

•Reduces blood solubility almost to N2O

•Recovery is twice rapid as isoflurane

•Blood pressure decreases dose-dependently

•Does not predispose to ventricular arrhythmia

•Increase in intracranial pressure

•Resp. depression, irritation to airways

Sevoflurane(Ultane)•Nonflammable, nonirritating,

does not increase heart rate

•Rapid and smooth induction & recovery, partition coefficient 0.68; MAC 2.36

•Unstable when exposed to soda lime and toxic metabolites (compound A) are formed (renal toxicity)

Sevoflurane

•Increases intracranial pressure (↑ICP)

•Metabolized (3%) more than desflurane, (<1%) least effects on cardiovascular system

•Increases plasma and urinary fluoride ions (renal and hepatic injury)

•Being tried in Avian and exotic species.

Nitrous OxideProperties: colorless, nonirritant,

nonflammable, sweet odor, partition coefficient .45, MAC 188%

Pharmacodynamics: Lungs: least effect,exhaled as suchHeart: do not sensitize, minimum effectLiver: minimum effect, not metabolizedKidney: no effect.

Nitrous OxideGeneral information:

-used for faster induction-it reduces the dose and depressant effect of halothane on cardiopulmonary system

-requires preanesthetic medication-not a good muscle relaxant-cough reflex remains-100% O2 given during recovery to prevent diffusion hypoxia.

Inhalant Anesthetics