PHARMACOLOGY:
Inhalation Anesthetics
Lecture 8

Jeff Ko, DVM, MS, DACVA

Overall view

Commonly used inhalation anesthetics

Less commonly used inhalant anesthetics

Clinical considerations of selecting an inhalation agent

Metabolism

% of anesthetic recovered as metabolites

 Methoxyflurane

up to 50% is metabolized by the liver and kidneys

 Halothane

up to 20-25% is metabolized by the liver and kidneys

 Sevoflurane

 Less than 3.0 % is metabolized by the liver and the kidneys

 Isoflurane

Less than 1% is metabolized by the liver and the kidneys

 Desflurane

No documented metabolism

 Nitrous Oxide

No documented metabolism

  • Major elimination route of inhalation anesthetics is through respiratory tract - i.e., via respiration and exhale the anesthetic gas to the atmosphere.
  • For a patient with hepatic dysfunction, the choice of inhalation anesthetic is isoflurane, sevoflurane or desflurane - less liver metabolism.
  • Although nitrous oxide has almost no liver metabolism, it is not commonly used in veterinary anesthesia, see below for details

Anesthetic potency

Comparison of anesthetic potency of inhalant anesthetics using MAC (volume %).

Dogs

Cats

Horses

Human

 Methoxyflurane

0.23 %

0.23 %

-----

0.16 %

 Halothane

0.87 %

0.82 %

0.88 %

0.75 %

 Isoflurane

1.28 %

1.63 %

    1.31 %

1.15 %

 Sevoflurane

2.1-2.36%

2.58%

2.31%

1.7%

 Desflurane

7.2%

9.8%

7.6%

        6.0%

 Nitrous oxide

  188 %

255 %

205%

104 %

Consideration of rate of induction, rate of change in anesthetic depth, and rate of recovery (blood/gas solubility)

Comparison of solubility, vapor pressure, and use of preservatives

 Anesthetic Agent
 Formula
 (Trade name)

Blood/gas
solubility

Vapor
Pressure
at 20oC (mmHg)

Preservatives

 Methoxyflurane
 CHCl2-CF2-O-CH3
 (Metofane®)

         12

         23

 Required

 Halothane
 CBrClH-CF3
 (Fluothane®)

        2.4

        243

 Required

 Isoflurane
 CF3-CHCl-O-CF3H
 (Forane®, IsoFlo®)

        1.4

        240

 None

 Sevoflurane
 CFH2-O-(CF3)2
 (Ultane®, SevoFlo®)

        0.69

        160

 None

 Desflurane
 CF3-CHF-O-CF2H
 (Suprane®)

        0.42

        664

 None

 Nitrous oxide
 N2O

        0.47

        ---

 None

Cardiopulmonary consideration

Overall, all inhalant anesthetics depress cardiopulmonary function a dose-dependent manner as shown by the decreases cardiac output, blood pressure, respiratory rate and increase in partial pressure in CO2 concentrations. 

 Halothane > or = methoxyflurane > isoflurane = sevoflurane = desflurane > N2O

 Halothane > or = methoxyflurane > isoflurane = sevoflurane = desflurane > N2O

 Isoflurane = sevoflurane = desflurane > methoxyflurane > or = halothane > N2O

 Isoflurane ≥ sevoflurane = desflurane > methoxyflurane > halothane > N2O

Cost consideration

Clinical use of inhalant anesthetics

Methods of inhalant anesthetic induction

Face-mask induction:

 Figure 1

facemasks 
Left
- malleable rubber facemask
 Right - transparent plastic facemask

 

Chamber induction:

Nasotracheal intubation for anesthetic induction:

 Second gas effect of nitrous oxide:

Maintenance of general anesthesia

Factors affecting MAC


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Oklahoma State University - Center for Veterinary Health Sciences all rights reserved
last updated October 19, 2007
Questions? Comments? Dr. Lyon Lee