What term describes the phenomenon of a cardiac cell firing an impulse before the next normal impulse occurs?

The term that describes the phenomenon of a cardiac cell firing an impulse before the next normal impulse occurs is "ectopic focus." This occurs when a site outside of the primary pacemaker (the sinoatrial node) initiates an electrical impulse. The ectopic focus can lead to an early depolarization of the cardiac tissue, resulting in an arrhythmia. This is significant in understanding dysrhythmias because it highlights how different areas of the heart can become irritable and fire prematurely, which can disrupt the normal cardiac cycle.

In contrast, sinus node escape refers to a situation where a slower pacemaker, often the SA node, fails to fire, and a lower pacemaker takes over temporarily until the SA node resumes its function. A pacemaker shift implies that the main site of impulse generation has changed, usually to a lower pacemaker in response to certain conditions. Reentrant arrhythmia involves a circuit of continuous re-excitation of a specific area of the heart, which is a different mechanism from that of an ectopic focus. Understanding the distinction between these terms is crucial for recognizing the underlying mechanisms of various cardiac dysrhythmias.