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Sinus rhythm is the heart’s normal rhythm. It has been discussed extensively in this chapter. This article deals mainly with ECG features of sinus rhythm.

# ECG criteria for sinus rhythm

• Regular rhythm with 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 shows normal sinus rhythm at paper speed 25 mm/s. Figure 2 shows the exact same ECG at 50 mm/s.

Figure 1. Sinus rhythm. Paper speed 25 mm/s.

Figure 2. Sinus rhythm. Paper speed 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 further clarification.

Figure 3. Manual calculation of heart rate.

## Treatment

Sinus rhythm is the normal rhythm of the heart and no treatment is necessary.

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