Cardiovascular and adrenergic response to exercise in obese subjects

In obese subjects a decreased work capacity has been described and abnormalities of left ventricular diastolic filling have been observed at rest. The aim of this study was to estimate the cardiovascular and adrenergic response of obese, otherwise healthy subjects to increasing work loads on a bicycle ergometer compared to normal subjects. At first we examined 18 obese subjects (9 males) aged 17 to 42 years and mean body mass index (BMI) 40 kg/m² and 18 non-obese control subjects (9 males) aged 19 to 39 years and BMI 22 kg/m² who performed an incremental exercise test with steps of 20 W every four minutes up to exhaustion. Oxygen consumption (VO₂), heart rate (HR), maximal peak of activity and ventilatory anaerobic threshold (AT) were measured, looking for differences of gender between the obese group and the control group. Afterwards, we studied 12 subjects (6 males) from both groups of control and obese subjects in which we also assessed plasma epinephrine (E), heart rate blood pressure product and CK-MB isoenzyme. On a separate occasion, cardiac output was measured in these two groups of 12 subjects at four increasing steps below ventilatory anaerobic threshold (AT) using a CO₂ rebreathing method. Among the obese as well as the control subjects, males demonstrated a higher work capacity due to a higher AT. The similar slope of VO₂ vs. watts in all cases, indicated an identical net mechanical efficiency between male and female subjects of the two groups. Considered as a whole, obese subjects of the analysed subgroups had, for the same work load, a similar cardiac output, a greater oxygen consumption, a greater arterio-venous oxygen difference and a smaller stroke volume. The estimated ratio of blood flow to fat free body mass was higher at any submaximal work load in non-obese compared to obese subjects. The increase of heart rate during incremental exercise was lower in the obese group and well correlated with plasma E levels. The heart rate-systolic blood pressure product, representing an indirect index of myocardial oxygen consumption, was higher, at any work load in obese compared to control subjects. The creatinphosphokinase cardiac isoenzyme (CK-MB) plasma concentrations after 5 min of recovery was significantly higher in obese subjects compared to controls. In summary, the data indicate that obese subjects have a decreased working capacity compared to non-obese people with a peculiar adrenergic answer to progressive physical exercise. In the absence of medical problems, they may be regarded as less fit individuals, probably with a reduced cardiac efficiency at heavier workloads.