Objective To investigate whether physiological cardiac reserve can be measured in man without invasive procedures and whether it is a major determinant of exercise capacity.

Design Development of method of measurement and an observational study.

Setting A regional cardiothoracic centre.

Subjects 70 subjects with a wide range of cardiac function, from heart failure patients to athletes.

Results Cardiac power output (CPO_max) at peak exercise was found to be significantly related to aerobic capacity: CPO_max(W) = 0.35 + 1.5V˙o ₂max (l/min),r = 0.87, p < 0.001. It also correlated well with exercise duration (r = 0.62, p < 0.001), suggesting that cardiac reserve is a major determinant of exercise capacity. In the study, cardiac reserve ranged from 0.27 to 5.65 W, indicating a 20-fold difference between the most impaired cardiac function and that of the fittest subject.

Conclusions A non-invasive method of estimating physiological cardiac reserve was developed. The reserve was found to be a major determinant of exercise capacity in a population of normal subjects and patients with heart disease. This method may thus be used to provide a clearer definition of the extent of cardiac impairment in patients with heart failure.