Nitric oxide (NO) in expired air at rest and during exercise

Nitric oxide (NO) was analysed in expired air from 27 healthy human subjects. At rest the NO concentration was 10.5 +/- 0.9 ng 1-1 (mean +/- SEM) corresponding to 8.6 +/- 0.7 parts per billion (ppb). The expired NO concentration did not change when the subjects were switched from breathing NO-free tank gas to room air which contained 7.7 ng l-1 NO. Repeated measurements of expired NO with an interval of 1 day showed a mean variation of 2.2 +/- 0.7 ng l-1 NO. The NO concentration in the first portion in the expired tidal volume (44%) was insignificantly higher than in the latter expired portion, 6.9 +/- 1.9 vs. 5.1 +/- 1.0 ng l-1 (n = 5). During moderately heavy exercise on an ergometer bicycle (90 W for women, n = 4, 150 W for men, n = 4) the expired concentration of NO decreased, however because of increased minute ventilation, the expired amount of NO almost doubled (from 111 +/- 12 to 209 +/- 30 ng min-1). The source of the expired NO is not clear and both the airways and the pulmonary circulation may contribute.