吸入麻酔薬: 概要

Inhalational anesthetics induce general anesthesia through inhaled gases and volatile liquids that diffuse rapidly across pulmonary alveoli into tissues. Most agents enhance GABA's action on GABAA receptors, decreasing central nervous system activity. Nitrous oxide uniquely inhibits NMDA receptors instead. The depth of anesthesia can be rapidly adjusted by changing the inhaled gas concentration.

Minimum alveolar concentration (MAC) is the measure of anesthetic potency, determining how much inhaled anesthetic is needed for effect. It varies based on patient factors like age, temperature, and concurrent drug use. Agents with lower MAC values are more potent. This metric helps clinicians select appropriate anesthetics and dosing for individual patients.

Modern inhalational anesthetics include volatile liquids such as isoflurane, desflurane, and sevoflurane, plus gases like nitrous oxide. Isoflurane causes dose-dependent hypotension with a pungent odor. Desflurane offers rapid onset and recovery due to low blood solubility. Sevoflurane has low pungency and is preferred for pediatric inhalation induction.

Inhalational anesthetics can cause hypotension, respiratory irritation, and organ toxicity including nephrotoxicity. Exposure to halogenated hydrocarbons may trigger malignant hyperthermia, a rare but life-threatening condition. Metabolism can generate toxic metabolites, prompting replacement with less toxic alternatives. These risks require careful patient selection and monitoring during anesthesia.

Most inhalational anesthetics are released unchanged into the atmosphere as greenhouse gases, contributing to global warming and climate change. Their environmental persistence and potency as climate agents have prompted efforts to minimize their use and atmospheric release. This environmental impact influences anesthetic selection in clinical practice.

Inhalational anesthetics distribute through alveolar wash-in followed by rapid tissue uptake. They exhibit steep dose-response curves and narrow therapeutic indices ranging from 2 to 4, with no known antagonists. Modern agents are delivered via recirculation systems to minimize waste. This distribution pattern allows rapid onset and offset of anesthetic effects.

Selection involves balancing the patient's pathophysiology against the drug's side-effect profile. Clinicians consider factors like age, temperature, concurrent medications, and the procedure duration. Sevoflurane is preferred for pediatric patients due to low pungency, while desflurane suits short procedures. The goal is maintaining optimal brain partial pressure while minimizing adverse effects.