Electrodes placed directly over the sinus node in experimental animals and in patients undergoing cardiac surgery have been used to record the electrical signals associated with sinus node depolarization. These studies have shown that the normal interval from sinus node depolarization to the onset of the P wave is from 25 to 50 ms. (Hariman el al, Circulation 61:1024,1988).
In the tracing from the 23 year old female, shown again here, the atrial rate is 44 and there is 1:1 AV conduction with a normal PR interval (0.18 seconds). The relationship of the sinus node impulse (arrows) to the onset of the P wave, the SA interval, may be normal, as depicted in the upper ladder diagram. However, it is impossible to exclude the possibility that the SA interval is prolonged (i.e.1st degree SA block), as depicted in the lower diagram. A third possibility is shown on the next page.