A method using reverse playback of ECG recordings demonstrates the presence of small notches on the normal P wave and a constant PQ interval.
A method to observe the average pattern of the amplified P wave is described. ECG was recorded on a magnetic analogue tape and then played back in reverse so that the falling phase of the R wave was capable of furnishing the required triggering voltage of the oscilloscope sweep. The reversed record was then amplified. The photographically superimposed curves represented the average P wave pattern. It was found that the PQ interval in human ECG was remarkably constant in normal condition and that notches of about the same size as noises were seen on the P wave. The significance of these notches was studied experimentally in the canine heart by placing the surface electrodes at various sites of the heart.
Irisawa et al. (Fri,) studied this question.