Dynamics of oxygen uptake during exercise in adults with cyanotic congenital heart disease

Abstract

The dynamic increase in oxygen uptake (V̇O2) at the start of exercise reflects the circulatory adjustments to metabolic changes induced by the exercise. Because V̇O2 measured at the lungs is the product of pulmonary blood flow and arteriovenous oxygen difference, pathologic conditions affecting the capacity of these factors to change would be expected to alter V̇O2 kinetics. To determine whether measurement of V̇O2 kinetics can detect conditions in which the pulmonary blood flow response to exercise is abnormal, V̇O2 was measured, breath-by-breath, during the transition from rest to exercise in 13 adults with cyanotic congenital heart disease (central venoarterial shunting) and in nine normal subjects. The increase in V̇O2 above baseline during the first 20 sec of exercise (phase I), reflecting the immediate increase in pulmonary blood flow, was diminished in the patients compared with that in normal subjects (14.8 ± 10.9 vs. 49.8 ± 19.2 ml of oxygen) (p < .001). The patients' phase I responses correlated with their reported physical activity tolerance (p < .01). In addition, the second phase of the V̇O2 response kinetics was prolonged in patients compared with normal subjects (half-time = 63 ± 13 vs 15 ± 13 sec) (p < .001). We conclude that striking disturbances in V̇O2 kinetics occur in patients with cyanotic congenital heart disease and that these measurements provide a useful noninvasive means of evaluating the degree to which the increase in pulmonary blood flow is constrained in response to exercise.