Atrial systole, isovolumetric contraction, ejection, isovolumetric relaxation, then filling
Mnemonic
Seven phases, but the useful skeleton is “two contractions, two relaxations, and the valves in between”:
- Atrial systole - the final 20 per cent of filling, the atrial kick, giving S4 if the ventricle is stiff
- Isovolumetric contraction - all valves closed, pressure rises, no volume change
- Rapid ejection
- Reduced ejection
- Isovolumetric relaxation - all valves closed again
- Rapid filling - giving S3 if the ventricle is compliant or overfilled
- Reduced filling (diastasis)
The two isovolumetric phases are the ones to remember: they are the only times all four valves are shut, and they define the boundaries of the pressure-volume loop.
S1 is mitral and tricuspid closing at the start of systole; S2 is aortic and pulmonary closing at its end. Diastole normally occupies about two thirds of the cycle, which is why tachycardia shortens diastole preferentially and compromises both ventricular filling and coronary perfusion.
Expansion
Systole
- Atrial systole - the atrial kick, contributing the last 10 to 20 per cent of filling
- Isovolumetric contraction - all valves closed, pressure rises, volume unchanged
- Rapid ejection - aortic and pulmonary valves open
- Reduced ejection - flow slows as the ventricle begins to relax
Diastole
- Isovolumetric relaxation - all valves closed again, pressure falls
- Rapid filling - atrioventricular valves open
- Reduced filling (diastasis) - the passive plateau before the next atrial systole
Diastole occupies about two-thirds of the cycle at rest. It shortens disproportionately with tachycardia, which is why a fast rate reduces both ventricular filling and coronary perfusion.