How is the heart rate determined on electrocardiography (ECG)?

Updated: Mar 11, 2019
  • Author: Tarek Ajam, MD, MS; Chief Editor: Terrence X O'Brien, MD, MS, FACC  more...
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Answer

Answer

When the cardiac rhythm is regular, the heart rate can be determined by the interval between two successive QRS complexes. On standard paper with the most common tracing settings, the heart rate is calculated by dividing the number of large boxes (5 mm or 0.2 seconds) between two successive QRS complexes into 300. For example, if the interval between two QRS complexes is two large boxes, then the rate is 150 beats per minute (bpm) (300 ÷ 2 = 150 bpm). See the following images.

Heart rates associated with each of the large boxe Heart rates associated with each of the large boxes in the following order are 300, 150, 100, 75, 60, 50, 43, 37, 33 beats per minute (bpm).
Heart rate boxes. Heart rate boxes.

If the heart rate is irregular, count the number of QRS complexes on the ECG and multiply by 6 to obtain the average heart rate in bpm (the ECG displays a period of 10 seconds; thus, 6 × 10 seconds = 60 seconds [1 minute]).


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