Regional pulmonary clearance of inhaled C15O and C15O2 in man at rest and during exercise.

The regional clearance of radioactive carbon monoxide (C15O) and carbon dioxide (C15O2), after a single maximal inspiration, was investigated in six normal subjects seated on a bicycle, at rest and during exercise at 50 W. The clearances were measured sequentially during breath-holding by four pairs of scintillation counters vertically aligned over the right lung. The clearance rate (k) for C15O and C15O2 increased from apex to base and from rest to exercise. On exercise, the apex base gradient (calculated over 11 cm vertical distance) for C15O decreased by 52% because of a larger increase in the upper zones but the C15O2 gradient did not change. Thus the increase in blood flow (approximately equal to kC15O2) on exercise was accompanied by recruitment and/or distension of pulmonary capillary blood volume (approximately equal to kC15O2), most marked in the upper regions. The ratio of the clearance (C15O/C15O2) decreased in the lower zones on exercise by 25% without significant change in the upper zone. This ratio reflects the product of capillary mean transit times and peripheral vascular and extravascular volumes. Since the latter increase on exercise, capillary transit times must have shortened considerably at a moderate level of exercise at all levels in the lung.