Cardiac reserve refers to the difference between the rate at which the heart pumps blood and its maximum capacity for pumping blood at any given time. A measurement of the cardiac reserve may be a health indicator for some medical condition.
Cardiac reserve Wikipedia
Cardiac reserve has been measured in different ways over the history of the test.
It is possible to make a non-invasive measurement of cardiac reserve.
A measure of cardiac reserve can help predict the likelihood of heart failure when indicated.