On an ECG recorded at standard paper speed, the R-R interval spans exactly four large squares. The heart rate is:

• A 50 bpm
• B 60 bpm
• C 75 bpm ✓
• D 100 bpm

Explanation

Standard paper speed is 25 mm/s, so one large square (5 mm) equals 0.2 seconds and one small square equals 0.04 seconds. Dividing 300 by the number of large squares between successive R waves gives the rate: 300 divided by 4 equals 75 bpm. Alternatively, 0.8 seconds per beat gives 75 beats per minute. Option D corresponds to three large squares and option B to five, both common miscounts.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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