The sequence of retrograde atrial activation in the canine heart. Correlation with positive and negative retrograde P waves.

The relationship of P-wave polarity and morphology in leads II, III, and aVF to the sequence of atrial activation was studied in the canine heart when the atria were paced from the region of the sinus node or the posterior-inferior left atrium and when retrograde activation of the atria occurred with right ventricular epicardial pacing. Deeply negative P waves in leads II, III, and aVF which occurred when the posterior-inferior left atrium was paced were associated with true retrograde activation of the atria. Positive P waves recorded in leads II, III, and aVF during retrograde atrial capture with right ventricular pacing were associated with rapid retrograde spread of the impulse in the interatrial septum to the region of Bachmann's bundle from which site the impulse spread to depolarize significant portions of both atria in a manner similar to that demonstrated during pacing from the region of the sinus node. When the atria were paced from a site just anterior to the coronary sinus ostium, positive P waves recorded in leads II, III, and aVF were associated with early activation in the vicinity of Bachmann's bundle and later activation of the posterior-inferior left atrium. When the atria were paced from a site just posterior to the coronary sinus ostium, negative P waves in leads II, III, and aVF were associated with early activation of the posterior-inferior left atrium and later activation in the vicinity of Bachmann's bundle. It was concluded that the time of arrival of the impulse at Bachmann's bundle relative to that at the posterior left atrium and the direction of spread of the impulse from and within Bachmann's bundle are critical in determining P-wave polarity and morphology.