Cardiac muscle already extracts about 75 percent of the oxygen delivered to it at rest, so the only way to give the myocardium more is to increase coronary flow — and the coronaries fill during diastole. Move the four dials and watch supply collapse against demand. Your job is to find the dial the nurse can actually turn.
The four determinants
Raises demand and cuts supply — tachycardia eats diastole first.
Afterload and wall tension. Rate × pressure is the demand side.
About 70 percent limits flow on exertion; about 90 percent limits flow at rest.
Normal roughly 12–16 g/dL in women, 14–18 g/dL in men.