Heart rate is calculated as the number of times the heart beats per minute (bpm). On an ECG tracing, the bpm is usually calculated as the number of QRS complexes (the ventricular rate). The rate is measured from the R-R interval, the distance between one R wave and the next consecutive R wave. If the atrial rate (the number of P waves) and the ventricular rate (the number of QRS complexes) vary, the analysis will show them as different rates, one atrial and one ventricular. In this case, both rates should be recorded.
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