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1 Laboratory of Cardiac Energetics, National Heart, Lung, and Blood Institute, NIH, DHHS, Bethesda, MD, USA; Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD, USA
2 Laboratory of Cardiac Energetics, National Heart, Lung, and Blood Institute, NIH, DHHS, Bethesda, MD, USA
3 Department of Cardiology, Johns Hopkins University, Baltimore, MD, USA
* To whom correspondence should be addressed. E-mail: fariso{at}nih.gov.
Cardiac resynchronization therapy (CRT) has been most typically achieved by bi-ventricular (RV-LV) stimulation. However, left ventricular (LV) free-wall pacing appears equally effective in acute and chronic clinical studies. Recent data suggest electrical synchrony measured epicardially is not required to yield effective mechanical synchronization, whereas endocardial mapping data suggest synchrony (fusion with intrinsic conduction) is important. To better understand this disparity, we simultaneously mapped both endocardial and epicardial electrical activation during LV free-wall pacing at varying atrioventricular delays (AV-delay 0-150ms) in six normal dogs using a 64-electrode LV endocardial basket and a 128-electrode epicardial sock. The transition from dyssynchronous LV-paced activation to synchronous RA-paced activation was studied by constructing activation time maps for both endo- and epicardial surfaces as a function of increasing AV-delay. The AV-delay at the transition from dyssynchronous to synchronous activation was defined as the transition delay (AVt). AVt was variable among experiments, ranging 44-93 ms on the epicardium and 47-105 ms on the endocardium. Differences in endo- and epicardial AVt were smaller (-17 - +12ms) and not significant on average (-5.0±5.2ms). In no instance was the transition to synchrony complete on one surface without substantial concurrent transition on the other surface. We conclude that both epicardial and endocardial synchrony due to fusion of native with ventricular stimulation occur nearly concurrently. Assessment of electrical epicardial delay, as often used clinically during CRT lead placement, should provide adequate assessment of stimulation delay for inner wall layers as well.
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