Inhalational Anesthetics

The mechanism of action of inhalational anesthetics is unknown. The diversity of chemical structures (inert gas xenon; hydrocarbons; halogenated hydrocarbons) possessing anesthetic activity appears to rule out involvement of specific receptors. According to one hypothesis, uptake into the hydrophobic interior of the plasmalemma of neurons results in inhibition of electrical excitability and impulse propagation in the brain. This concept would explain the correlation between anesthetic potency and lipophilicity of anesthetic drugs (A). However, an interaction with lipophilic domains of membrane proteins is also conceivable. Anesthetic potency can be expressed in terms of the minimal alveolar concentration (MAC) at which 50% of patients remain immobile following a defined painful stimulus (skin incision). Whereas the poorly lipophilic N2O must be inhaled in high concentrations (>70% of inspired air has to be replaced), much smaller concentrations (<5%) are required in the case of the more lipophilic halothane.

The rates of onset and cessation of action vary widely between different in-halational anesthetics and also depend on the degree of lipophilicity. In the case of N2O, there is rapid elimination from the body when the patient is ventilated with normal air. Due to the high partial pressure in blood, the driving force for transfer of the drug into expired air is large and, since tissue uptake is minor, the body can be quickly cleared of N2O. In contrast, with halothane, partial pressure in blood is low and tissue uptake is high, resulting in a much slower elimination.

Given alone, N2O (nitrous oxide, "laughing gas") is incapable of producing anesthesia of sufficient depth for surgery. It has good analgesic efficacy that can be exploited when it is used in conjunction with other anesthetics. As a gas, N2O can be administered directly. Although it irreversibly oxidizes vitamin B12, N2O is not metabolized appre ciably and is cleared entirely by exhalation (B).

Halothane (boiling point [BP] 50 °C), enflurane (BP 56 °C), isoflurane (BP 48 °C), and the obsolete methoxyflu-rane (BP 104 °C) have to be vaporized by special devices. Part of the administered halothane is converted into hepatotoxic metabolites (B). Liver damage may result from halothane anesthesia. With a single exposure, the risk involved is unpredictable; however, there is a correlation with the frequency of exposure and the shortness of the interval between successive exposures.

Up to 70% of inhaled methoxyflu-rane is converted to metabolites that may cause nephrotoxicity, a problem that has led to the withdrawal of the drug.

Degradation products of enflurane or isoflurane (fraction biotransformed <2%) are of no concern.

Halothane exerts a pronounced hy-potensive effect, to which a negative in-otropic effect contributes. Enflurane and isoflurane cause less circulatory depression. Halothane sensitizes the myocardium to catecholamines (caution: serious tachyarrhythmias or ventricular fibrillation may accompany use of cate-cholamines as antihypotensives or toco-lytics). This effect is much less pronounced with enflurane and isoflurane. Unlike halothane, enflurane and isoflu-rane have a muscle-relaxant effect that is additive with that of nondepolarizing neuromuscular blockers.

Desflurane is a close structural relative of isoflurane, but has low lipophilic-ity that permits rapid induction and recovery as well as good control of anesthetic depth.

Low potency high partial pressure needed relatively little binding to tissue

Low potency high partial pressure needed relatively little binding to tissue

1 Anesthetic potency ,

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Responses

  • olavi
    Which has the fastest uptake into blood n2o or halothane?
    7 years ago

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