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When inhaled, Halothane is absorbed through the alveoli into the bloodstream. In the bloodstream, Halothane circulates through the body to the principal site of action, the brain. Here Halothane causes a progressive depression of the central nervous system, beginning with the higher centers (cerebral cortex) and spreading to the vital centers in the medulla. This depression is reversible. However, its mode of action, like all anaesthetic agents, is unknown.
Halothane has a relatively low solubility in blood and therefore alveoli/blood concentrations equilibrate rapidly. The triexponential decline in Halothane blood concentrations following the end of administration is thought to represent distribution into three compartments; the vessel rich group (brain/heart/liver), the musculature and adipose tissue. Approximately 80% of the inhaled Halothane is eliminated unchanged by the lungs. The remaining 20% is metabolized in the liver by oxidative and under hypoxic conditions, reductive pathways. The main metabolites are trifluoroacetic acid, bromide and chloride salts (via the oxidative pathway) and fluoride salts (via the reductive pathway). The concentrations of metabolites peak 24 hours post-operatively and are eliminated by renal excretion during the following week.