Determination of the spatial and intensity properties of atrial repolarization potentials in the dog

Body surface electrical potentials generated by atrial repolarization (Ta wave) normally extend from the P wave into or through the QRS complex. Thus, the Ta wave is partially obscured by the QRS complex and, conversely, the QRS complex is composed of atrial recovery as well as ventricular depolarization forces. To better study the spatial patterns and magnitudes of the Ta wave, transient atrioventricular (AV) block was induced in 15 dogs by atrial pacing using surgically implanted left or right atrial electrodes. ECG potentials were registered from 84 torso electrodes, and cycles with normal and with blocked AV conduction were segregated. In blocked cycles, the duration of the Ta wave measured 248.0 +/- 25.3 msec with right and 256.0 +/- 38.4 msec with left atrial stimulation. In all cases, the Ta wave extended into the QRS and, in 75% of cases, it extended into the S-T segment. Peak Ta magnitudes, measuring 50.7 +/- 17.9 and 51.8 +/- 21.7 uV RMS with right and left atrial stimulation, respectively, occurred during the P-R segment in all cases. The effect of Ta wave superposition on the QRS complex was assessed by subtraction of patterns in cycles with blocked AV conduction from those with intact AV conduction. Differences between directly recorded QRS waveforms and those computed by subtraction of the Ta wave were small; correlation coefficients exceeded 0.96 and differences in instantaneous RMS potential were less than 6%. Thus, the atrial recovery waveform does extend into the QRS and into the S-T segment in most cases, but the effect of this superposition of Ta on QRS waveforms is quantitatively small.