Chest
Volume 124, Issue 2, August 2003, Pages 572-579
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Clinical Investigations
CARDIOLOGY
Oxygen Cost of Exercise Is Increased in Heart Failure After Accounting for Recovery Costs*

https://doi.org/10.1378/chest.124.2.572Get rights and content

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 [

o2] per Watt [
o2/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

o2/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

o2/W, the recovery
o2/W, the work
o2/W, and the
o2/W relationship above and below the ventilatory threshold (VT) in 11 healthy control subjects and 45 patients with CHF.

Results

The exercise

o2/W was decreased by 29% (p < 0.0001) in patients with CHF; however, the recovery
o2/W was increased by 167% (p < 0.0001) and the work
o2/W was increased by 14% in patients with CHF (p = 0.014). The
o2/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
o2/W and recovery
o2/W, implying that subjects with a low exercise
o2/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

o2/W; however, the patient with heart failure is not efficient, but rather accumulating a large oxygen debt (recovery
o2/W) that is repaid following exercise. In addition, the work
o2/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
o2. The large recovery
o2/W is likely due to impaired oxygen delivery to exercising muscle during exercise. The increase in the work
o2/W is probably due to changes in skeletal muscle fiber type that occur in patients with heart failure (type I to type IIb).

Section snippets

Subjects

Healthy adult volunteers (n = 11) were recruited from the Seattle area. Normal subjects were not trained athletes and were not receiving any medicines. Patients with heart failure (n = 45) were referred for clinical exercise testing from the End Stage Heart Failure Clinic (Table 1). All subjects signed an informed consent form approved by the Human Subjects Committee at the University of Washington. Subjects were between the ages of 23 years and 65 years. Criteria for heart failure were

Results

Table 1 outlines subject characteristics. The patients with heart failure were on average 13 years older than the healthy subjects, and most were male. The patients with heart failure were 9 kg heavier but similar in height. At peak exercise, the patients with heart failure had a 29% lower peak heart rate, a 60% lower peak oxygen consumption, a 60% lower peak workload, and a 34% lower oxygen pulse compared to control subjects (Table 2). Peak ventilatory efficiency (

e/
co2) was 23% higher, but

Discussion

Previous investigators have reported that patients with heart failure have a lower

o2 at a given workload during maximal ramp exercise. They suggested that patients with heart failure were more efficient than normal subjects2356; however, none to our knowledge have accounted for the increase in recovery
o2 in the evaluation of efficiency. We found total oxygen cost is greater when oxygen consumption during recovery is included for New York Heart Association class II-IV patients with heart

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    Often, however, calculations of efficiency did not account for the total oxygen cost of exercise, including the Vo2during recovery. Patients with heart failure were reported to have a decreased oxygen cost during exercise compared to control subjects, implying an increased efficiency, until the oxygen cost during recovery was considered (1,2). Similarly, the exclusion of recovery data from our analysis would have led to the very different result of finding no difference in efficiency with age.

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Dr. Mitchell and Dr. Steele are supported by a grant from the American Federation for Aging Research.

Dr. Levy is supported by National Institutes of Health grant K12 AG00503.

Supported in part by the Geneva Foundation.

Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (e-mail: [email protected]).

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