Usually the human heartbeat is as uniform and regular as the beat of music, and the time interval between each heartbeat is approximately the same. The heartbeat may also vary in speed with breathing, emotion, activity and other different situations, but the difference between each heartbeat is very subtle and usually not easy to detect without special instruments. In some special cases, the regularity of the heartbeat changes, and some heartbeats occur earlier, just like a beat rush, which is medically called premature beat, or premature contraction. Premature beats can occur in both normal and diseased hearts.