Determining Heart Rate
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 determine 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 below:
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 making the first method less accurate, since the RR intervals may vary from beat to beat in this setting.
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