Expansion
About one-third systole and two-thirds diastole at rest
Expansion
At a resting rate of about 75 beats per minute the cycle lasts 0.8 seconds:
- Systole about 0.3 seconds, roughly one-third
- Diastole about 0.5 seconds, roughly two-thirds
As rate rises, systole shortens only slightly because it is dominated by the fixed duration of contraction and repolarisation. Diastole absorbs almost all of the shortening.
Two consequences, both of which make tachycardia dangerous in ischaemic and stenotic heart disease:
- Filling time falls, so stroke volume falls once rate exceeds roughly 160 to 180 beats per minute
- Coronary perfusion time falls, since the left ventricle is perfused almost entirely in diastole
This is the physiological argument for rate control in atrial fibrillation and for beta blockade in angina and aortic stenosis.