PaCO₂ for Optimum Washout of Inhalational Anesthetics from the Brain. A Model Study

抄録

A hypothesis was established that, during emergence of inhalational anesthesia, hyperventilation and accompanying hypocapnia beyond a certain limit may actually disturb rather than enhance the washout of inhalational anesthetics from the brain because of a decreased cerebral blood flow. Two mathematical models were constructed and the washout of nitrous oxide, halothane and methoxyflurane were studied. In model 1, the whole body consisted of a single compartment, and blood flow to this compartment was assumed to change proportionally with the PaCO₂. In model 2, the body was divided into two compartments, brain and the rest of the body. It was assumed that the blood flow to the brain compartment varies proportionally with the PaCO₂ while that to the rest of the body remains constant. The analysis indicated that there indeed existed the PaCO₂ values at which the washout of anesthetics from the brain can be maximally achieved. In model 1, they were 49.0, 22.1 and 9.7 mmHg for nitrous oxide, halothane, and methoxyflurane, respectively. In model 2, these PaCO₂ values varied with time. While the hypothesis was proven to be valid, we conclude that it is of limited clinical significance. For halothane and methoxyflurane, these theoretically optimum PaCO₂ values are sufficiently low. For nitrous oxide, the variation of PaCO₂ makes little difference clinically, because its washout is fast enough regardless of PaCO₂