Breath-to-breath variations of pulmonary gas exchange in resting man

Abstract

Breath-to-breath variations of ventilation and of composition of alveolar gas have been studied for periods of several minutes in resting human subjects breathing air at sea level. The dispersion of P aco 2 is much smaller than the dispersion of P aco 2. On a P co 2-P o 2 diagram, alveolar P co 2 and P o 2 points of successive breaths form an elliptical cloud which crosses the R lines. It is possible to compute the volumes of CO 2 and O 2 exchanged between the alveolar gas and the pulmonary blood during one ventilatory cycle, and then to calculate an alveoli-blood respiratory quotient. This quotient varies more than the common respiratory quotient between inspired air and alveolar gas. This is not surprising, since the variations of the gas exchange at the alveolo-capillary level are damped by the lung volume capacity. The relative variations of V̇ co 2 and V o 2 within successive ventilatory cycles are about the same. The variations are probably due to breath-to-breath changes of (1) the mean capillary blood flow, Q̇c, (2) Ca CO 2 , (3) the mixed venous blood composition. Because V̇ co 2 and V̇ co 2 are positively correlated with V̇a, the ratios of V ̇ co 2 V ̇ a and V ̇ o 2 V ̇ a are less variable than they would be without these correlations. One of several factors which may account for these relations is a relative stability of V ̇ a Q ̇ c , an increase of V̇a being accompanied by an increase of capillary blood flow Q̇c.