Physiological cardiac reserve: development of a non-invasive method and first estimates in man

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.
Methods—Subjects underwent treadmill, symptom limited cardiopulmonary exercise tests to measure aerobic exercise capacity (represented by O₂max) and cardiac reserve. Cardiac output was measured non-invasively using the CO₂ rebreathing technique.
Results—Cardiac power output (CPO_max) at peak exercise was found to be significantly related to aerobic capacity: CPO_max (W) = 0.35 + 1.5O₂max (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.

Keywords: cardiac reserve;  cardiac power output;  oxygen consumption;  congestive heart failure