There are two different rates that can be determined on ECGs. The atrial rate is indicated by the frequency of the P waves and the ventricular rate is indicated by the frequency of the QRS complexes.
Normally, the atrial rate should be the same as the ventricular rate in the absence of disease, however certain conditions, such as third degree AV 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 (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 beats per minute. The scale continues down to show that if two consecutive R waves are separated by 8 large boxes, then the rate is 37 beats per minute. The pictorial explanation of this method is to the right.
Another quick way to calculate the rate is based on the fact that the entire ECG is 10 seconds. So by counting the number of QRS complexes and multiplying by 6, the number per minute can be calculated (since 10 seconds times 6 is 60 seconds or 1 minute). This is a better method when the QRS complexes are irregular (such as during atrial fibrillation) which makes the first method less accurate, since the RR intervals may vary from beat to beat in this setting. Below are some examples using each method.
Example 1: Note that the QRS complexes are about five and a half large boxes apart. Referencing the above image it can be determined that the ventricular heart rate is between 50 and 60 beats per minute. This is a full 10 second rhythm strip. There are 9 QRS complexes total. Multiply the number of QRS complexes by 6 and the exact heart rate is 54 beats per minute. There is 1 P wave for each QRS complex and thus the atrial rate is the same.
Example 2: These QRS complexes are exactly three large boxes apart and thus the ventricular heart rate is 100 beats per minute. Now multiple the number of QRS complexes on this strip by 6. This would be 17 x 6 = 102. There is 1 P wave for each QRS complex and thus the atrial rate is the same.
Example 3: These QRS complexes are less than 2 large boxes apart and thus the heart rate is between 150 and 300. Multiplying the number of QRS complexes by 6 would give 29 x 6 = 174 beats per minute. There is likely 1 P wave for each QRS complex (difficult to see on this strop) and thus the atrial rate is likely the same.
Example 4: 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 since the R-R intervals vary significantly. The best way to determine the ventricular heart rate would be to simply count the QRS complexes and multiple by 6 which would be 15 x 6 = 90 beats per minute. The P waves are not able to be identified in atrial fibrillation and it is assumed that the atrial rate is between 400-600 beats per minute.
Example 5: 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). This rhythm is actually an accelerated idioventricular rhythm (slow ventricular tachycardia). The atrial rate is indicated by the P waves. There are almost exactly 5 large boxes between P waves indicating an atrial rate of 60 beats per minute. There are a total of 10 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 confirming the first method. There are just more than 4 big boxes between each QRS complexes and thus the ventricular rate is between 60 and 75. Since there is a total of 11 QRS complexes in this full 10 second strip, the actual ventricular rate is 11 x 6 = 66 beats per minute.
By Steven Lome