Factors Affecting Elimination

FACTORS AFFECTING ELIMINATION

Recovery from anesthesia depends on lowering the concentration of anesthetic in brain tissue. Anesthetics can be eliminated by biotransformation, transcutaneousloss,orexhalation.Biotransformation usually accounts for a minimal increase in the rate of decline of alveolar partial pressure. Its greatest impact is on the elimination of soluble anesthetics that undergo extensive metabolism (eg, methoxyflu-rane). The greater biotransformation of halothane compared with isoflurane accounts for halothane’s faster elimination, even though it is more soluble. The CYP group of isozymes (specifically CYP 2EI) seems to be important in the metabolism of some volatile anesthetics. Diffusion of anesthetic through the skin is insignificant.

The most important route for elimination of inhalation anesthetics is the alveolus. Many of the factors that speed induction also speedrecovery: elimination of rebreathing, high fresh gas flows, low anesthetic-circuit volume, low absorption by the anesthetic circuit, decreased solubility, high cerebral blood flow (CBF), and increased ventilation. Elimination of nitrous oxide is so rapid that alveolar oxygen and CO₂ are diluted. The resulting diffusionhypoxia is prevented by administering 100% oxygenfor 5–10 min after discontinuing nitrous oxide. The rate of recovery is usually faster than induction because tissues that have not reached equilibrium will continue to take up anesthetic until the alveolar partial pressure falls below the tissue partial pressure. For instance, fat will continue to take up anesthetic and hasten recovery until the partial pressure exceeds the alveolar partial pressure. This redistribution is not as useful after prolonged anesthesia (fat partial pressures of anesthetic will have come “closer” to arterial partial pressures at the time the anesthetic was removed from fresh gas)—thus, the speed of recovery also depends on the length of time the anes-thetic has been administered.