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

Nitrogen (N-2) may accumulate to unacceptable levels during closed-circuit anesthesia (CCA) when the sampled gases are redirected to the anesthesia ci rcuit, because many gas analyzers entrain air as a reference gas to calibra te for oxygen analysis. Using oxygen instead of air as the reference gas fo r paramagnetic oxygen analysis could attenuate N-2 accumulation. Forty-three adult ASA physical status I-III patients undergoing a variety o f peripheral and abdominal procedures were assigned to one of two groups, d epending on the reference gas used by a paramagnetic oxygen analyzer, eithe r 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 Nz (N-2Et) was calculated as "balance gas": (100 - %O-2 - %anesthetic vapor - %CO2). N-2Et after 55 min (N-2Et55min) was correlated with the end-expired Nz concentration when the circuit tvas closed (N-2Et0min) and 5 min (N-2Et 5min) thereafter. In group I, N-2Et accumulated almost linearly over time (t, min): N-2Et (%) = 2.47 + 0.61 * t (r(2) = 0.999). N-2Et0min, N-2Et5min, and N-2Et55min wer e 1.3 +/- 0.8, 5.3 +/- 1.7, and 35.3 +/- 5.3%, respectively (mean +/- SD). The correlation (r2) between N-2Et55min and N-2Et0min was 0.19, and between N-2Et55min and N-2Et5min it was 0.56. In group II, N-2Et increased exponen tially: N-2Et (%) = 1.01 + 11.9 * (1 - e(-t/43.5)) (r(2) = 0.99) N-2Et0min, N-2Et5min, and N-2Et55min were 0.87 +/- 0.93, 2.6 +/- 1.5, and 10.1 +/- 2. 9%, respectively. The correlation (r(2)) between N-2Et55min and N-2Et0min w as 0.04, and between N-2Er55min and N-2Et5min it was 0.40. We conclude that paramagnetic oxygen analyzers that use oxygen as the refer ence gas significantly attenuate Nz accumulation during CCA, which may redu ce the need for frequent flushing of the anesthesia system, may provide mor e constant oxygen and nitrous oxide concentrations, and may simplify pharma cokinetic studies of potent inhaled anesthetics.