Sinus rhythm: the normal rhythm of the heart

A rhythm is defined as three consecutive heartbeats with identical waveforms on the ECG. The similarity of the waveforms indicates that the origin of the impulse is the same. The sinoatrial (SA) node is the heart’s pacemaker under normal circumstances and the rhythm is referred to as sinus rhythm. Hence, sinus rhythm is the normal rhythm of the heart. The physiology of the SA node and pacemaker cells in the heart have been discussed previously.

Definition (criteria) for sinus rhythm

• Regular rhythm with a ventricular rate between 50 and 100 beats/min.
• P-wave with constant morphology preceding every QRS complex.
• The P-wave is positive in lead II.

Figure 1 (below) shows normal sinus rhythm at a paper speed of 25 mm/s.

Figure 2 (below) shows the same ECG at 50 mm/s.

Manual calculation of heart rate

At 25 mm/s paper speed, the heart rate is equal to 300 divided by the number of large boxes between two beats (for simplicity, use the distance between two R waves). As seen in Figure 2, there are 5 large boxes between two R waves, hence the heart rate is:

300/5 = 60 beats/min

At 50 mm/s paper speed, the heart rate is equal to 600 divided by the number of large boxes between two beats. As seen in Figure 2, there are 10 large boxes between two R waves:

600/10 = 60 beats/min.

Refer to Figure 3 for clarification.