Heart rate of a normal adult patient at rest is between 60 and 100 beats/min. A heart rate slower than 60 beats/min is called bradycardia; a heart rate faster than 100 beats/min is called tachycardia. To determine the heart rate from a recording made by modern ECG machines is relatively simple. These machines make a 12-lead ECG tracing over a 10-second period. One row starts with lead I, switches to aVR to be followed by V1, and ends with V4. A second row starts with lead II and records aVL, V2, and V5 in sequence while a third row records lead III, aVF, V3, and V6 in sequence. Nearly all machines offer continuous recording of lead II in a fourth row and some others offer even more. Despite lead-switching in mid-course, recording is continuous without interruption over the 10-second period. Therefore heart rate per minute can be determined by counting the number of beats on any one row and multiplying this number by 6.
A second method of determining heart rate is shown below:
The heart rate is 300 beats/min when 2 consecutive QRS complexes are one 5-mm-division (200 ms) apart.
By the same token, the heart rates are 150, 100, 75, 60, and 50 beats/min when 2 consecutive QRS complexes are two, three, four, five, and six 5-mm-divisions (400, 600, 800, 1000, 1200 ms) apart respectively.
If the distance between 2 consecutive QRS complexes does not equal to a whole number of 5-mm-divisions, a rough estimate of the heart rate will have to be made as shown in the tracing.
This method is particularly useful in determining heart rates at different times in a tracing in which the rhythm is irregular:
A third method applies to single-lead rhythm strips printed from ECG monitors in critical care areas:
These rhythm strips have 3-second marks and heart rate is a matter of multiplying the number of QRS complexes in a 6-second period by 10. For very slow heart rates in which there are few QRS complexes in a 6-second interval, accuracy can be improved by multiplying the number of QRS complexes in a 12-second period by 5.