Inhalant Anesthetics
Inhalant Anesthetics1) 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 eliminatedPressure exerted by the molecules of the vapor phase at equilibrium of molecules moving in and out of liquid phaseVapor Pressure dependent on temperature and physical characteristics of liquid, independent of atmospheric pressure↑ Temperature→↑ Vapor PressureVapor 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
AgentBP (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 Drugs
solubility (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 controlSolubility of Inhaled Drugs
Solubility of Inhaled DrugsHalo
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 diseases Kidney: decreases urine output Liver: Minimum effect, decreases liver glycogen Heart: Initially increases cardiac output, then decreases card. output, suppresses vasomotor center.Ether
Ether 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.Halothane
Properties: 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 myocardium Liver: hepatitis by repeated administration.Halothane
General 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 myocardium Liver: decreased hepatic function, forms free fluoride ions Kidney:decreased flow, metabolites cause dysfunction and renal vasoconstriction.Methoxyflurane
General information: Was extensively used in large animals, better agents are available now introduced in 1964 slow induction and recovery Stage II is bypassed, less CNS stimulation excellent muscle relaxation, very good analgesic vaporization difficult safe for fetus, compatible with other agents.Etherane
Properties: 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 effect Heart: less sensitization, CO decreased, less effect on BP Liver: no adverse effect, hepatic necrosis upon repeated administration Kidney: no adverse effects, decreases renal flow.Etherane
General 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.Isoflurane
Properties: widely used now, an isomer of enfllurane colorless, less soluble, nonflammable, stable, mild pungent odor, MAC 1.5% Pharmacodynamics: Lungs: mostly exhaled as such Heart: lesser effects, does not sensitizes, Liver and Kidney: not injurious.Isoflurane
General 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 recoveryNewest Inhalants
Desflurane (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