Delayed attainment of peak oxygen consumption after the end of exercise in patients with chronic heart failure

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Abstract

Peak oxygen uptake (Vo2) is attained at peak exercise in normal subjects. Recently, it was shown that the kinetics of the Vo2 increase during exercise is slowed in chronic heart failure (CHF). We hypothesized that this may delay maximal Vo2 after the end of exercise. We studied 21 patients who attained their peak Vo2 15 s or more after cessation of a graded bicycle exercise test with breath-by-breath gas analysis (group 1). They were compared with 21 age- and sex-matched CHF patients who did not do so (group 2) and 21 normal subjects (group 3). Peak Vo2 occurred 30±10 s (15–45) after exercise and was 10±7% (3–31) higher than end-exercise Vo2 (P<0.001) in group 1. Group 1 patients had poorer functional status (NYHA class 3.0±0.2 vs. 2.4±0.5*), a smaller ejection fraction (21±6 vs. 26±8%*), a lower end-exercise Vo2 (1156±251 vs. 1535±508 ml/min), a lower anaerobic threshold (762±183 vs. 970±265 ml/min), and an identical respiratory exchange ratio (1.09±0.13 vs. 1.06±0.12) relative to group 2 patients. The ΔVo2/Δworkrate ratio was lower (9.5±2.0 vs. 11.2±1.1 ml/W) and the half-time of Vo2 recovery was longer (156±27 vs. 95±27 s*) in group 1 than in group 2 (*P<0.05, P<0.01 group 1 vs. group 2). Slow kinetics of the Vo2 increase with exercise may delay achievement of peak Vo2 beyond the maximal workrate achieved. Gas exchanges should thus be measured also during recovery so as not to underestimate the true peak Vo2, especially in severe CHF patients referred for heart transplantation.

Introduction

Exercise capacity is reduced in patients with chronic heart failure (CHF) [1]and peak oxygen consumption is recognized as one of the most important prognostic variables in this setting [2]. The accuracy of peak exercise oxygen consumption measurement is therefore critical: a 20% error may suggest different likely outcomes and have important therapeutic implications. In patients with severe CHF, we have found that oxygen consumption sometimes reaches its maximal value some time after the end of exercise. In this study, we tried to characterize these patients and to understand the reasons for these findings.

Section snippets

Patients and methods

We prospectively analyzed the exercise response of CHF patients referred to the exercise test laboratory of our cardiology department between January and May 1995, with the aim of determining whether peak oxygen consumption may be attained after end-exercise.

Results

The heart failure patients (groups 1 and 2) were older than the normal subjects (P<0.05). They was no significant difference between the two heart failure groups regarding age, body weight or maximal predicted oxygen consumption. Compared with group 2 patients, patients in group 1 had significantly poorer functional status (NYHA class 3.0±0.2 vs. 2.4±0.5) (Table 1) and lower end-exercise Vo2 (1156±251 vs. 1535±508 ml/min, corresponding to 59±13% and 70±19%, respectively, of predicted values);

Discussion

This study shows that a large proportion of CHF patients performing a graded bicycle exercise test attain their highest Vo2 value after exercise has ended; this would have been overlooked had gas analysis not also been performed during recovery. The increment in Vo2 after exercise was 10% on average above the end-exercise Vo2 and as high as 30% (Fig. 1 and Fig. 2). Thus, when a cardiopulmonary exercise test is used to guide the indication for cardiac transplantation, this underestimation of

Conclusions

Slow kinetics of Vo2 increase on graded exercise may delay the attainment of peak Vo2 beyond end-exercise in patients with severe CHF. This underlines the limitations of peak Vo2 in heart failure. Gas analysis should also be performed during the recovery phase of exercise if actual peak Vo2 is not to be underestimated, especially in patients with severe heart failure referred for heart transplantation. The relative prognostic value of end-exercise and peak-exercise Vo2 remains to be determined.

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