Introduction: Left atrial (LA) dyssynchrony, defined as LA septal-lateral delay, is felt to improve with septal pacing.
Methods: Eight canines underwent sternotomy during general anesthesia. Bipolar plunge electrodes delivered transmyocardially over the septal, anterior, posterior, and lateral LA recorded local activation during pacing just faster than sinus from sinus node, right atrial appendage (RAA), Bachmann’s bundle (BBR) and coronary sinus ostium (CSO).
Results: All canines presented in sinus rhythm. Pacing from sinus node synchronously activated the septum at BBR and CSO. The LA was activated from septal to lateral by simultaneously propagating anterior and posterior wavefronts. RAA pacing activated the septum earlier at BBR than CSO, and the anterior LA was activated before the posterior LA. Septal pacing from BBR preexcited the anterior but not posterior LA. Septal pacing from CSO preexcited the posterior but not anterior LA.
Conclusions: RAA, BBR and CSO pacing create dispersion of septal activation, with intra-LA activation delay between the anterior and posterior walls, concordant with pattern of septal activation. This dyssynchronous LA activation would not be detected by measuring LA septal to lateral delays.