Calculation of Rate of ECG

Rate is cycles or beats per minute. Sinoatrial rates are normally between 60-100/minute. Rates less than 60 constitute bradycardia while the rates above 100 is called tachycardia in a normal adult. SA node is the usual pacemaker, other potential pacemakers (if SA node fails) are atrial pacemakers with inherent rates of 60-80, AV node (rate 40-60), or ventricular pacer (rate 20-40).

In certain pathologic conditions ectopic pacemakers can go much faster at rates 150-250 cycles/minute and usurp the control of the SA node. There are three methods of calculating rate:Most Common Method (Most Rates can be Calculated this Way) Find an R-wave on a heavy line (large box) count off “300, 150, 100, 75, 60, 50” for each large box you land on until you reach the next R-wave. Estimate the rate if the second R-wave doesn’t fall on a heavy black line.

Common Methods of Calculation of Heart Rate of ECG

Mathematical Method

This method is recommended if there is regular bradycardia, i.e. - rate < 50. Divide 300 with the total number of large boxes between two R-waves. This gives the ventricular rate.

Six-second Method 

Count off 30 large boxes = 6 seconds (remember 1 large box = 0.2 seconds, so 30 large boxes = 6 seconds). Then, count the number of R-R intervals in six seconds and multiply by 10. This is the

number of ventricular beats per minute. This is most useful if there is an irregular rhythm (like atrial fibrillation) when one wants to know an average rate. All these methods assume the speed of ECG recording at 25mm/second.

Six second method of calculation of heart rate