Understanding cardiac alternans: the answer lies in the Ca2+ store.

Pulsus alternans is a condition wherein the heartbeat cyclically varies between weak and strong despite a constant heart rate. It was first described many years ago in experimental conditions, but it can be observed clinically, in particular, in patients with heart failure, where the presence of alternans portends a bad outcome (reviewed in1). Typically, alternans occurs at high heart rates, but the threshold frequency can be lowered by various conditions such as ionic disturbances or ischemia. The alternation of the strength of the pulse, or mechanical alternans, is accompanied by alternation of the electrical activity, the so-called “electrical alternans.” This is particularly evident on the repolarization phase: action potential recordings in vitro show alternation of the duration of the action potential (AP), either concordant with the mechanical alternans (long AP with a strong beat) or discordant (short AP with strong beat) (reviewed in1). Electrical alternans can be observed at the ECG level as alternation of the ST-segment and T-wave amplitude and form; T-wave alternans is commonly referred to as TWA.