The "second gas effect" is not a valid concept

To determine whether the "second gas effect" is valid, we determined the ph armacokinetics of 0.2% enflurane with or without 80% N2O (n = 7 each) under controlled constant volume ventilation in 14 young healthy male patients b efore their operations. The alveolar (end-tidal) concentration (F-A) and in spired concentration (F-I) at the mouthpiece and the arterial blood concent ration of enflurane were measured, and the ratio of F-A to F-I was calculat ed. The F-A/ F-I of enflurane increased rapidly during the first few minute s of administration and then increased slowly. No significant difference wa s found in the F-A/F-I between the two groups at any time point (P > 0.05). The arterial blood concentrations of enflurane increased progressively and were not significantly different between the two groups at any time point (P > 0.05). The results indicate that, at high concentrations, N2O neither facilitated the increase of F-A nor enhanced the uptake of a companion gas. The second gas effect is a nonexistent phenomenon in clinical practice bec ause the concentrating effect is very weak and the augmentation effect is n onexistent under controlled ventilation. Implications: We studied the effec ts of N2O on the ratio of alveolar (end-tidal) concentration to inspired co ncentration of the second gas (enflurane) and on its blood concentration in humans. Nitrous oxide did not affect the alveolar or blood concentration o f the second gas under controlled constant volume ventilation. The "second gas effect" is not a valid concept.