A MINIMAL-FLOW SYSTEM FOR XENON ANESTHESIA

We describe a minimal-flow system for xenon anesthesia during controlled ventilation. A computer maintained oxygen concentration in the anesthesia circle within +/- 2% of the value set by the anesthesiologist. The ventilator and the circle were connected via a large dead space, through which oxygen from the ventilator entered the circle but which prevented xenon from escaping. This arrangement simplified the computer program. The system was tested on a lung model and in six pigs (37-39 kg). The xenon expenditure and the amount of xenon washed out from the pigs after the anesthetic were measured. Additional experiments with nitrous oxide were made in three pigs. The xenon expenditure during 2 h of xenon anesthesia was 7.6 +/- 0.8 l (mean +/- 1 standard deviation). The corresponding expenditure of nitrous oxide was 16.5 +/- 2.7 l. About 75% of the xenon expenditure was in the 1st h of anesthesia; thereafter, 20-40 ml . min-1 was needed to maintain oxygen concentration at 30%. Nitrogen concentration in the circle increased to 12-16% during the xenon anesthetic, although it was preceded by a 20 min denitrogenation period. During the washout phase after the xenon anesthesia, mean expired xenon concentration decreased to below 2% within 4 min. Subsequently, washout was slower and the expired concentration remained above 0.1 % for more than 90 min. The estimated total amount of xenon washed out from the lungs and body tissues during 4 h of oxygen breathing was about 4 l. We conclude that xenon anesthesia via a fully automated minimal-flow system is feasible. About half of the xenon delivered by the system was taken up in the body. The decrease in expired concentration to less than 0.05 MAC was very rapid, after discontinuation of the xenon in halation.