PP interval: interval between the P waves due to atrial depolarization. PP interval is used to calculate the atrial rate. In sinus rhythm, PP interval and RR interval are the same. Hence atrial rates and ventricular rates are not calculated separately. But when there is AV (atrioventricular) dissociation as in complete heart block, atrial rate is different from the ventricular rate. In complete heart block, PP interval is shorter than the RR interval, meaning that atrial rate is higher than the ventricular rate. In ventricular tachycardia with interference AV dissociation, PP interval is longer than the RR interval, indicating that atrial rate is lower than ventricular rate. PP interval and RR intervals are nearly identical in isorhythmic AV dissociation.
Atrial rate is obtained by 60000 by PP interval in milliseconds (60000 milliseconds corresponding to one minute, giving the atrial rate in beats per minute). If the PP interval is measured in millimeters as in a standard ECG with paper speed of 25 millimeters per second, atrial rate is obtained by dividing 1500 by the PP interval in millimeters. This approach is more commonly adopted while calculating the atrial rate from the ECG as it avoids conversion of intervals initially into milliseconds. In some of the modern devices, there is provision for automatic calculation of heart rates, but it seldom calculates the atrial rate separately.
PP interval can vary slightly in normal individuals from cycle to cycle due to respiratory sinus arrhythmia. The atrial rate increases in inspiration and decreases in expiration. This is because of cyclical changes in left ventricular output with respiration and reflex changes in heart rate.