Inhalational Anesthetics: Uptake, MAC, and Agent Profiles

The clinical effect of an inhaled anesthetic tracks its partial pressure in the brain. The alveolar partial pressure is a useful surrogate because alveolar gas, arterial blood, and brain approach equilibrium during administration.

The ratio of alveolar to inspired concentration rises faster when delivery is high and uptake into blood and tissues is low. Increasing inspired concentration or alveolar ventilation generally accelerates induction, especially for more soluble agents.

• A lower blood-gas partition coefficient means less uptake is required to raise alveolar partial pressure, so induction and recovery are faster.
• Higher cardiac output can slow the alveolar rise of a soluble agent by increasing uptake into blood.
• A right-to-left intracardiac shunt can slow inhalational induction.
• Nitrous oxide can accelerate uptake of a concurrently administered volatile agent through the second-gas effect.

Minimum alveolar concentration, or MAC, is the end-tidal concentration that prevents movement in 50 percent of subjects exposed to a standardized noxious stimulus. MAC is an ED50 measure of immobility, not a complete measure of hypnosis, amnesia, analgesia, or autonomic suppression.

MAC values are approximately additive among inhaled anesthetics and fall with increasing age, hypothermia, pregnancy, acute alcohol exposure, opioids, and many intravenous anesthetics. Hyperthermia and chronic heavy alcohol use can increase anesthetic requirement.

Sevoflurane has low blood-gas solubility and a nonpungent odor, making it useful for inhalational induction. Desflurane has even lower blood-gas solubility and permits rapid concentration changes, but its pungency can irritate the airway and abrupt concentration increases may cause sympathetic activation.

Isoflurane is more soluble than sevoflurane or desflurane and is pungent, so inhalational induction is less comfortable. Halothane is relatively soluble, depresses myocardial contractility, sensitizes the heart to catecholamines, and is associated with rare severe hepatic injury; its use has declined in many settings.

Nitrous oxide has low potency, so it cannot produce surgical anesthesia alone at normal atmospheric pressure. It provides analgesic and anesthetic-sparing effects but expands closed gas spaces and can impair vitamin B12-dependent enzymes with substantial exposure.

Volatile anesthetics cause dose-dependent depression of ventilation and blood pressure. They generally reduce the ventilatory responses to hypoxemia and hypercapnia, inhibit hypoxic pulmonary vasoconstriction, and potentiate nondepolarizing neuromuscular blockade.

• Most volatile agents increase cerebral blood flow while reducing cerebral metabolic rate; intracranial effects depend on dose, ventilation, and intracranial compliance.
• Volatile anesthetics can trigger malignant hyperthermia in susceptible patients; nitrous oxide is not a trigger.
• Postoperative nausea and vomiting, emergence effects, airway irritation, and environmental impact influence agent selection.
• Organ blood flow can fall as arterial pressure and cardiac output decline, even without direct organ toxicity.

Primary teaching source: Akram Amer, Pharmacology of Anesthesia Drugs, pages 10-15.

Concepts were reconciled with contemporary inhalational-anesthesia pharmacology; variable MAC values and formulation-specific fresh-gas-flow claims were not treated as universal constants.

This material is educational and remains a draft until reviewed by a qualified clinician. Local protocols and current product information take precedence for patient care.