Influence of the reference gas of paramagnetic oxygen analyzers on nitrogen concentrations during closed-circuit anesthesia.

Nitrogen (N2) may accumulate to unacceptable levels during closed-circuit anesthesia (CCA) when the sampled gases are redirected to the anesthesia circuit, because many gas analyzers entrain air as a reference gas to calibrate for oxygen analysis. Using oxygen instead of air as the reference gas for paramagnetic oxygen analysis could attenuate N2 accumulation. Forty-three adult ASA physical status I-III patients undergoing a variety of peripheral and abdominal procedures were assigned to one of two groups, depending on the reference gas used by a paramagnetic oxygen analyzer, either air (group I, n = 23) or oxygen (group II, n = 20). Gases sampled by the multigas analyzer were redirected to the anesthesia circuit. End-expired N2 (N2Et) was calculated as "balance gas": (100 - %O2 - %anesthetic vapor - %CO2). N2Et after 55 min (N2Et55min) was correlated with the end-expired N2 concentration when the circuit was closed (N2Et0min) and 5 min (N2Et5min) thereafter. In group I, N2Et accumulated almost linearly over time (t, min): N2Et (%) = 2.47 + 0.61 * t (r2 = 0.999). N2Et0min, N2Et5min, and N2Et55min were 1.3+/-0.8, 5.3+/-1.7, and 35.3+/-5.3%, respectively (mean +/- SD). The correlation (r2) between N2Et55min and N2Et0min was 0.19, and between N2Et55min and N2Et5min it was 0.56. In group II, N2Et increased exponentially: N2Et (%) = 1.01 + 11.9 * (1 - e(-t/43.5)) (r2 = 0.99). N2Et0min, N2Et5min, and N2Et55min were 0.87+/-0.93, 2.6+/-1.5, and 10.1+/-2.9%, respectively. The correlation (r2) between N2Et55min and N2Et0min was 0.04, and between N2Et55min and N2Et5min it was 0.40. We conclude that paramagnetic oxygen analyzers that use oxygen as the reference gas significantly attenuate N2 accumulation during CCA, which may reduce the need for frequent flushing of the anesthesia system, may provide more constant oxygen and nitrous oxide concentrations, and may simplify pharmacokinetic studies of potent inhaled anesthetics.