Expansion
Heart rate, contractility and wall stress
Expansion
Three main determinants:
- Heart rate - the most important, and the easiest to manipulate
- Contractility - more forceful contraction consumes more oxygen
- Wall stress - by Laplace, this rises with pressure and chamber radius
Smaller contributions come from basal metabolism and the energy of electrical activation.
This framework explains the treatment of angina. Beta blockers reduce rate and contractility; nitrates reduce preload and therefore radius and wall stress; calcium channel blockers reduce afterload and contractility.
It also explains why tachycardia is doubly damaging in ischaemia: it raises demand while shortening diastole, which is when the left ventricle receives almost all of its coronary supply.