We previously showed that contamination of exhaled air by ambient NO could be avoided by 1 min of breathing and final inhalation of clean air (clean air procedure) prior to exhaled air sampling in balloons. This approach is, however, unsuitable for sampling large groups in epidemiological studies, because it is time consuming and laborious. We therefore discarded the initial part of exhaled air, which may contain ambient NO, in prebags of 250, 540, 775, 1000, and 2000 ml. The subsequent part of exhaled air was sampled in balloons and the NO content was measured. Inflation of a prebag of 500 ml to prevent ambient NO contamination proved to be effective only at low ambient NO levels (<20 ppb). Larger sizes of the prebag (1000 ml for adults and 775 ml for children) are, however, required so that contamination of the air sample at higher levels of ambient NO (up to 115 ppb) is excluded. Using different prebags of gradually increasing size, it was shown that the initial part of exhaled air (<500 ml) contained relatively high amounts of NO that gradually decreased, but attained a constant level in the subsequent air volumes. Using rather large prebags of 2000 and 1000 ml, respectively, in adults and children yielded exhaled NO levels even below those obtained the clean air procedure was applied in combination with a prebag of 540 ml. As this reduction also occurs at ambient NO levels of nearly zero, we suggest that this reduction was due to interference by the water vapor arising from the lowest part of the lungs. In conclusion, the use of a prebag to discard the initial volume of exhaled air ensures accurate measurement of exhaled endogenous NO in large-scale epidemiological studies not biased by ambient NO.
Copyright 2000 Academic Press.