In the large-box method, how is heart rate estimated?

• A. Count the number of large boxes between two consecutive R waves and divide into 300
• B. Count the number of small boxes between two consecutive R waves and divide into 1500
• C. Count the number of QRS complexes in 6 seconds and multiply by 10
• D. Count the number of P waves between two consecutive R waves and divide into 600

Answer: (A)

In the large-box method, heart rate is estimated from the interval between two consecutive R waves on the ECG. Each large box represents 0.2 seconds, and there are 60 seconds in a minute. By counting how many large boxes separate the two R waves and dividing 300 by that number, you get the heart rate in beats per minute. For example, five large boxes between R waves gives 300/5 = 60 bpm; three large boxes give 100 bpm.

Other approaches aren’t the large-box method. Counting small boxes between R waves and dividing into 1500 uses the small-box scale, which arrives at the same idea but with a different divisor. Counting QRS complexes in six seconds and multiplying by 10 is the six-second method, a different technique. Counting P waves between R waves doesn’t directly reflect the ventricular rate, since P waves relate to atrial activity.