Study design and objectives: Attainment of a steady state for oxygen uptake (VO2) 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 VO2 are attributable solely to the reduction of exercise capacity in CHF, VO2 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 VO2. 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 VO2 kinetics were characterized by the mean response time (MRT) to attain the 6 min VO2 value. Rates of recovery of VO2 were analyzed for 2 min following exercise. For the same absolute work rate, VO2 MRTs were significantly longer for patients than controls (25 W, 67 +/- 26 vs 37 +/- 25 s; approximately 60 W, 87 +/- 20 vs 54 +/- 27 s), but there was no significant difference in VO2 MRT between the two groups at a matched intensity of 50 percent delta (87 +/- 20 vs 81 +/- 18 s). However, the decrease in VO2 during 2 min of recovery was slower for the patients on all comparisons, even for matched exercise intensity.
Conclusion: The VO2 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.