What defines a normal sinus rhythm?

A normal sinus rhythm is characterized by a regular heart rhythm that originates from the sinoatrial (SA) node, which serves as the heart's natural pacemaker. In a normal sinus rhythm, the electrical impulses that trigger heartbeats are generated in a consistent manner by the SA node, leading to a steady and predictable heart rate typically between 60 and 100 beats per minute. This regularity is essential for effective pumping of blood throughout the body.

The presence of a regular rhythm signifies that the heart is functioning properly and coordinating effectively, with each heartbeat occurring in a well-timed sequence. This contrasts with irregular heartbeats, which can indicate underlying arrhythmias or other cardiac issues.

Other options describe scenarios that do not align with a normal sinus rhythm. For example, an irregular heart rhythm (as in the first option) or variations that involve fast or slowed heart rates coupled with pauses or misses (as in the third and fourth options) suggest abnormalities in cardiac function that would not qualify as a normal sinus rhythm. Hence, the key characteristic of a normal sinus rhythm is its regularity and origination from the SA node.