On an ECG recorded at standard paper speed, the distance between two consecutive R waves is exactly three large squares. The heart rate is:
- A 75 bpm
- B 80 bpm
- C 100 bpm ✓
- D 150 bpm
Correct answer: C. 100 bpm
Explanation
At the standard speed of 25 mm/s, one large square (5 mm) equals 0.2 seconds, giving 300 large squares per minute. Dividing 300 by the number of large squares between R waves gives the rate: 300 divided by 3 equals 100 bpm. Option D would apply if the gap were two large squares, and option A if it were four, so miscounting squares is the main trap here.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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