MEASUREMENT OF HIGH LUNG-VOLUMES BY NITROGEN WASHOUT METHOD

Studies in human infants suggested that thoracic gas volume (TGV) meas ured at end exhalation may not depict the true TGV and may differ from TGV measured from a series of higher lung volumes and corrected for t he volume added. This was explained by gas trapping. If true, we shoul d expect the discrepancy to be more pronounced when functional residua l capacity (FRC) and higher lung volumes are measured by gas dilution techniques. We studied lung volumes above FRC by the nitrogen washout technique in 12 spontaneously breathing rhesus monkeys (5.0-11.3 kg wt ; 42 compared measurements). Lung volumes directly measured were compa red with preset lung volumes achieved by artificial inflation of the l ungs above FRC with known volumes of air (100-260 ml). Measured lung v olume strongly correlated with and was not significantly different fro m preset lung volume (P = 0.05; r = 0.996). The differ ence between me asured and preset lung volume was 0-5% in 41 of 42 cases [1 +/- 0.4% ( SE)]. The direction of the difference was unpredictable; in 22 of 42 c ases the measured volume was larger than the preset volume, but in 17 of 42 cases it was smaller. The difference was not affected by the vol ume of gas artificially inflated into the lungs. We conclude that, ove rall, lung volumes above FRC can be reliably measured by the nitrogen washout technique and that FRC measurements by this method reasonably reflect true FRC.