Oxygen cost of exercise is increased in heart failure after accounting for recovery costs

Study objectives: The oxygen cost during exercise has been reported to be decreased in patients with congestive heart failure (CHF), implying an increased efficiency (lower oxygen uptake [870 2 per Watt [VO2/W]); however, these studies ignored the oxygen debt that is increased in heart failure. Subjects: The primary aim of this research was to evaluate the total oxygen cost (work Vo(2)/W) during exercise and recovery in patients with heart failure as compared with healthy adults. Design and patients: We performed a retrospective analysis comparing the exercise Vo(2)/W, the recovery Vo(2)/W, the work Vo(2)/W, and the Vo(2)/W relationship above and below the ventilatory threshold (VT) in 11 healthy control subjects and 45 patients with CHF. Results: The exercise Vo(2)/W was decreased by 29% (p < 0.0001) in patients with CHF; however, the recovery Vo2/W was increased by 167% (p < 0.0001) and the work Vo(2)/W was increased by 14% in patients with CHF (p = 0.014). The Vo(2)/W slope increased above the VT (+ 27%, p = 0.0017) in both normal subjects and patients with CHF, suggesting a decrease in efficiency above the VT. There was an inverse correlation (r = 0.646, p < 0.0001) between exercise Vo(2)/W and recovery Vo(2)/W, implying that subjects with a low exercise Vo(2)/W were not efficient but rather accumulated a large oxygen debt that was repaid following completion of exercise. Conclusions: Heart failure is associated with lower exercise Vo(2)/W; however, the patient with heart failure is not efficient, but rather accumulating a large oxygen debt (recovery Vo(2)/W) that is repaid following exercise. In addition, the work Vo(2)/W (including both exercise and recovery) is increased in patients with heart failure in comparison to control subjects, and correlates inversely with the percentage of predicted Vo(2). The large recovery Vo(2)/W is likely due to impaired oxygen delivery to exercising muscle during exercise. The increase in the work Vo(2)/W is probably due to changes in skeletal muscle fiber type that occur in patients with heart failure (type I to type IIb).