Cardiac Electrophysiology and Heart Failure Practice Test 2026 – The Comprehensive All-in-One Guide to Exam Success!

The key idea is estimating heart rate from the time between two consecutive R waves, the R-R interval. On standard ECG paper (speed 25 mm/s), a large box equals 0.2 seconds. Inverting that interval gives beats per minute: heart rate = 60 seconds per minute divided by the interval in seconds, which works out to 300 divided by the number of large boxes between two R waves.

For example, if the R-R interval spans 2 large boxes, the heart rate is 300/2 = 150 bpm; if it spans 3 large boxes, it’s 300/3 = 100 bpm. This method assumes a regular rhythm so the distance between successive R waves is consistent.

If the rhythm is irregular, other methods are preferable, such as counting the QRS complexes in 6 seconds and multiplying by 10 to estimate bpm, because the R-R method relies on a stable interval. The other options describe different approaches that do not use the R-R interval in the same way.