Studies were performed in 6 patients during open heart surgery to correlate the relative sequence of atrial activation with the polarity of the retrograde P wave in electrocardiographic leads II, III, and aVF. Bipolar atrial electrograms were recorded from selected sites during threshold pacing from sites low on the right side of the atrial septum which when paced resulted in the inscription of either negative or positive P waves in electrocardiograph leads II, III, and aVF. When the atria were paced from a site just anterior to the coronary sinus ostium (negative P waves), the posteroinferior left atrium was activated early and Bachmann's bundle late during the inscription of the P wave. When the atria were paced from a site 2 cm anterior to the coronary sinus ostial site (positive P waves), Bachmann's bundle was activated early during the P wave and the posteroinferior left atrium was activated relatively later. These data from man are consistent with previous studies in the canine heart. Therefore, we deduced that in man during the inscription of negative retrograde P waves in leads II, III, and aVF, atrial activation occurs primarily in a retrograde fashion. However, during the inscription of positive retrograde P waves in man, activation occurs rapidly up the interatrial septum (we believe via the anterior internodal pathway) to Bachmann's bundle, from where it then spreads in a manner similar to that which occurs during normal sinus rhythm.
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