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 [V̇o2] per Watt [V̇o2/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 V̇o2/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 V̇o2/W, the recovery V̇o2/W, the work V̇o2/W, and the V̇o2/W relationship above and below the ventilatory threshold (VT) in 11 healthy control subjects and 45 patients with CHF.
Results: The exercise V̇o2/W was decreased by 29% (p < 0.0001) in patients with CHF; however, the recovery V̇o2/W was increased by 167% (p < 0.0001) and the work V̇o2/W was increased by 14% in patients with CHF (p = 0.014). The V̇o2/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 V̇o2/W and recovery V̇o2/W, implying that subjects with a low exercise V̇o2/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 V̇o2/W; however, the patient with heart failure is not efficient, but rather accumulating a large oxygen debt (recovery V̇o2/W) that is repaid following exercise. In addition, the work V̇o2/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 V̇o2. The large recovery V̇o2/W is likely due to impaired oxygen delivery to exercising muscle during exercise. The increase in the work V̇o2/W is probably due to changes in skeletal muscle fiber type that occur in patients with heart failure (type I to type IIb).