Sequence of regional filling during a tidal breath in man

Sequential portions of inspired gas were labeled with boluses of xenon 133 and inhaled at constant flow rates (0.1 to 1.0 l/s^-1) from functional residual capacity. Twelve normal subjects were studied seated upright. The distribution of boluses injected at the mouth varied with flow rate; at 0.1 l/s^-1 basal ventilation exceeded apical. At 0.4 l/s^-1 base and apex ventilation was equal and at 1.0 l/s^-1 apical ventilation exceeded basal. At comparable flow rates the distribution of boluses injected into the trachea was similar to those introduced at the mouth but when inhaled through 500 ml of added dead space, basal ventilation exceeded apical at all flow rates studied. Results suggest that the distribution of inspired gas varies with flow rate over the first part of the tidal volume but becomes more independent of flow rate as inspiration continues. During normal tidal breathing the expired alveolar gas in the trachea and upper airways is distributed preferentially to the upper zones of the lungs on inspiration.