- Results from partial intraventricular block every other heartbeat.
- The electrocardiogram (ECG) for this shows tachycardia (rapid heart rate), which is probably the reason the block has occurred, b/c when the rate of the heart is rapid, it may be impossible for some portions of the Purkinje system to recover from the previous refactory period quickly enough to respond during every succeeding heartbeat.  Also, many conditions that depress the heart, such as ischemia, myocarditis, or digitalis toxicity, can cause incomplete intraventricular block, resulting in electrical alternans.