Expansion
Pressure work is far more costly in oxygen than volume work
Expansion
Stroke work = stroke volume x mean arterial pressure, corresponding to the area inside the pressure-volume loop.
It has two components:
- Volume (external) work - moving blood, which is relatively cheap in oxygen
- Pressure work - generating tension, which is expensive
Cardiac efficiency, the ratio of external work to oxygen consumed, is only about 20 to 25 per cent; the rest is heat.
The practical consequence is that the heart tolerates volume loading far better than pressure loading. A patient with severe aortic stenosis can develop angina despite normal coronary arteries, because pressure work has raised oxygen demand while hypertrophy has increased diffusion distance and compressed subendocardial vessels.