What is a normal cardiac rhythm called?

Sinus rhythm is the name given to the normal cardiac rhythm of the heart where electrical stimuli are initiated in the SA node (the heart's natural pacemaker). Stimuli are then conducted through the AV node, bundle branches, and Purkinje fibres. A unique labelling system is used to identify each of the three waves.

Depolarisation and repolarisation of the atria and ventricles show up as 3 distinct waves on an electrocardiogram (ECG). To note the normal resting heart rate for an adult, it ranges from 60 to 100 beats a minute. Generally, a lower heart rate at rest implies more efficient heart function and better cardiovascular fitness. For example, a well-trained athlete might have a normal resting heart rate closer to 40 beats a minute.

To calculate heart rate (HR) from an ECG, it is done as follows: HR = 1,500/RR interval in millimeters, HR = 60/RR interval (R wave-to-R wave interval) in seconds, or HR = 300/number of large squares between successive R waves. In each case, the authors are referring to instantaneous HR, which is the number of times the heart would beat if successive RR intervals were constant.