Absolute refractory period prevents tetany; the relative period allows vulnerable re-excitation
Mnemonic
“Absolute means no stimulus works; relative means a strong one might”:
- Absolute refractory period - sodium channels are inactivated and no stimulus of any strength can trigger an action potential
- Relative refractory period - some channels have recovered, so a stronger than normal stimulus can trigger a response, but it propagates slowly
The long refractory period of cardiac muscle is what prevents tetany, and therefore what makes the heart a pump rather than a sustained contraction. It lasts almost the whole of systole, roughly 250 ms, against 2 to 3 ms in skeletal muscle.
The vulnerable period sits at the end of the relative refractory period, corresponding to the peak of the T wave on the ECG. A stimulus landing there can precipitate ventricular fibrillation, which is the basis of the R on T phenomenon and the reason cardioversion must be synchronised to the R wave.
A long QT lengthens the vulnerable period, which is why QT prolonging drugs cause torsades de pointes.
Expansion
- Absolute refractory period - no stimulus of any strength produces another action potential, because the fast sodium channels are inactivated. It spans phases 0 to early 3
- Relative refractory period - a stronger than normal stimulus can produce an action potential, but it propagates slowly and abnormally. It corresponds to late phase 3
- Supranormal period - a brief window at the end of phase 3 where a weaker than normal stimulus can excite the cell
The refractory period lasts about 250 milliseconds, almost the whole of systole. This has two vital consequences: the heart cannot tetanise, so it must relax and refill between beats, and re-entry is normally prevented because tissue behind an advancing wavefront is inexcitable.
The relative refractory period corresponds to the T wave on the electrocardiogram, which is the vulnerable period. A stimulus landing there, the R on T phenomenon, can precipitate ventricular fibrillation.