The myocardium extracts 70–80% of delivered oxygen at rest, so it has almost no extraction reserve. Increased demand is met by coronary vasodilation and increased flow, not by extracting more oxygen from each unit of blood.
Coronary flow reserve
Coronary flow reserve is the ratio of maximum hyperemic flow to resting flow, normally about 4 to 5. It represents the capacity to increase delivery when demand rises. A fixed stenosis that prevents this dilation reduces flow reserve before it reduces resting flow, which is why symptoms appear first during exertion.
Two ways to match supply and demand
Skeletal muscle at rest
- Extracts about 25% of delivered oxygen
- Large extraction reserve available
- Can meet rising demand partly by extracting more
Cardiac muscle at rest
- Extracts about 70–80% of delivered oxygen
- Almost no extraction reserve
- Must increase flow to meet rising demand
Exercise in a healthy person versus a narrowed artery
During vigorous exercise, heart rate and contractility rise, and myocardial oxygen demand may double or triple. In a healthy coronary tree, arterioles dilate and flow increases several-fold, so the tissue stays supplied. If a coronary artery has a fixed atherosclerotic narrowing, the downstream vessels are already dilated at rest to compensate, and they cannot dilate much further. Flow cannot rise to match demand, and the tissue downstream becomes ischemic — typically producing chest pain at a predictable level of exertion.