There are two different rates that can be determined on an ECG. The atrial rate is indicated by the frequency of the P waves. The ventricular rate is indicated by the frequency of the QRS complexes.
In the absence of disease, the atrial rate should be the same as the ventricular rate. However, certain conditions including third-degree atrioventricular nodal block or ventricular tachycardia can alter this normal relationship, causing “AV dissociation.” In this setting, the atrial rate (P waves) and ventricular rate (QRS complexes) are at different heart rates.
One quick and easy way to measure the ventricular rate is to examine the RR interval — that is, the distance between two consecutive R waves — and use a standard scale to find the rate. If two consecutive R waves are separated by only one large box, then the rate is 300 beats per minute. If the R waves are separated by two large blocks, then the ventricular rate is 150 bpm. Continuing down the scale, if two consecutive R waves are separated by eight large boxes, then the rate is 37 bpm. The pictorial explanation of this method is shown here.
Another quick way to calculate the rate is based on the entire ECG being 10 seconds. By counting the number of QRS complexes and multiplying by six, the number per minute can be calculated — because 10 seconds times six equals 60 seconds, or 1 minute. This is a better method when the QRS complexes are irregular, as during atrial fibrillation, in which case the RR intervals may vary from beat to beat.
Below are examples using each method.
Note that the QRS complexes are about 5 1/2 large boxes apart. Referencing the above image, it can be determined that the ventricular heart rate is between 50 and 60 bpm. This is a full 10-second rhythm strip, and there are nine QRS complexes total. Multiply the number of QRS complexes by six, and the exact heart rate is 54 bpm. There is one P wave for each QRS complex, thus the atrial rate is the same.
These QRS complexes are exactly three large boxes apart; therefore, the ventricular heart rate is 100 bpm. Now, multiply the number of QRS complexes on this strip by six. This would be 17 x 6 = 102. There is one P wave for each QRS complex, thus the atrial rate is the same.
These QRS complexes are less than two large boxes apart, thus the heart rate is between 150 and 300 bpm. Multiply the number of QRS complexes by six for the ventricular rate — that is, 29 x 6 = 174 bpm. There is likely one P wave for each QRS complex (difficult to see on this strip), thus the atrial rate is likely the same.
The below ECG strip shows the irregularly irregular QRS complexes present during atrial fibrillation. Using the first method to determine heart rate would not be accurate because the RR intervals vary significantly. The best way to determine the ventricular heart rate would be to simply count the QRS complexes and multiply by 6, which would be 15 x 6 = 90 bpm. The P waves are not able to be identified in atrial fibrillation, and it is assumed that the atrial rate is between 400 and 600 bpm.
This ECG strip shows AV dissociation, meaning the P waves (indicating atrial activity) are at a different rate than the QRS complexes (indicating ventricular activity), as explained earlier. This rhythm is actually an accelerated idioventricular rhythm, or slow ventricular tachycardia. The atrial rate is indicated by the P waves. There are almost exactly five large boxes between P waves, indicating an atrial rate of 60 bpm. There are a total of ten P waves on this strip (difficult to see some of them, as they are intermittently buried in the QRS complexes) and 10 x 6 = 60. This confirms the first method. There are just more than four big boxes between each QRS complexes, thus the ventricular rate is between 60 and 75. Because there is a total of eleven QRS complexes in this full 10-second strip, the calculation for the actual ventricular rate is 11 x 6 = 66 bpm.