DYNAMICS OF OXYGEN-UPTAKE FOR SUBMAXIMAL EXERCISE AND RECOVERY IN PATIENTS WITH CHRONIC HEART-FAILURE

Study design and objectives: Attainment of a steady state for oxygen uptake (Vo(2)) during constant work rate exercise has been reported to take longer for patients with chronic heart failure (CHF) compared with normal. The steady state is also delayed in normal subjects during high-intensity exercise compared with moderate exercise, however, and the delay correlates with the degree of associated lactic acidosis. To determine whether prolonged kinetics of Vo(2) are attributable solely to the reduction of exercise capacity in CHF, Vo(2) kinetics were compared for patients with CHF and normal subjects, both for exercise of matched absolute work rate and for matched relative work intensity. Subjects: Eighteen men with CHF and 10 normal men. Methods and results: Subjects performed 6 min of constant work rate cycle ergometry with breath-by-breath measurement of Vo(2). Patients were studied using 25 W, and a work rate midway between the lactic acidosis threshold and maximal capacity (50 percent Delta). Normal subjects were tested similarly, and also at a work rate matched to the patients' average 50 percent Delta work rate. The Vo(2) kinetics were characterized by the mean response time (MRT) to attain the 6 min Vo(2) value. Rates of recovery of Vo(2) were analyzed for 2 min following exercise. For the same absolute work rate, Vo(2) MRTs were significantly longer for patients than controls (25 W, 67 +/- 26 vs 37 +/- 25 s; similar to 60 W, 87 +/- 20 vs 54 +/- 27 s), but there was no significant difference in Vo(2) MRT between the two groups at a matched intensity of 50 percent Delta (87 +/- 20 vs 81 +/- 18 s). However, the decrease in Vo(2) during 2 min of recovery was slower for the patients on all comparisons, even for matched exercise intensity. Conclusion: The Vo(2) dynamics for submaximal exercise are slowed in CHF. The slower dynamics are not entirely accounted for by the relatively higher intensity of a given work rate, since delayed recovery is evident even at a matched relative work intensity. Exercise intolerance in CHF is characterized not only by decreased maximal exercise capacity, but also by slower adaptations to and from submaximal levels of exercise.