Heart rate is calculated as the number of times the heart beats per minute. It usually measures ventricular rate (the number of QRS complexes) but can refer to atrial rate (the number of P waves). The method chosen to calculate HR varies according to rate and regularity on the ECG tracing.
Method 1: Count Large Boxes
Regular rhythms can be quickly determined by counting the number of large graph boxes between two R waves. That number is divided into 300 to calculate bpm. The rates for the first one to six large boxes can be easily memorized. Remember: 60 sec/min divided by 0.20 sec/large box = 300 large boxes/min.
Method 2: Count Small Boxes
Sometimes it is necessary to count the number of small boxes between two R waves for fast heart rates. That number is divided into 1500 to calculate bpm. Remember: 60 sec/min divided by 0.04 sec/small box =1500 small boxes/min.
Examples: If there are six small boxes between two R waves:
1500/6 = 250 bpm.
If there are ten small boxes between two R waves:
1500/10 =150 bpm.
Method 3: Six-Second ECG Rhythm Strip
The best method for measuring irregular rates with varying R-R intervals is to count the
number of R waves in a 6-sec strip and multiply by 10. This gives the average number of bpm.
per 60 sec (1 min).