Pacing polarity and left ventricular mechanical activation sequence in cardiac resynchronization therapy

The aim of this study is to evaluate the relationship between polarity of left ventricular (LV) pacing and the resultant regional, global, and transmural mechanical sequence of contraction. Cardiac resynchronization therapy (CRT) is widely utilized in patients with drug refractory congestive heart failure with systolic dysfunction (EF < 35 %) and intraventricular conduction delay (QRS duration > 120 ms). However, little is known about polarity of pacing stimulation and the resultant differences in LV mechanics. The polarity of pacing was altered sequentially in 20 patients (73 +/- 13, 16 males) with preexisting biventricular devices with potential choice of multiple vectors for pacing stimulation. Initial unipolar or extended bipolar configurations were switched to bipolar configuration or vice versa, and echocardiographic images were acquired for off-line analysis. Regional and global LV longitudinal and radial mechanics were assessed selectively from the subendocardial and subepicardial regions with 2D speckle-tracking echocardiography. Left ventricular capture by each vector configuration was confirmed by local lead capture and appropriate QRS alteration. Unipolar pacing resulted in increased dispersion of LV regional endocardial strains with a higher base-to-apex gradients of longitudinal shortening strains (P < 0.05). LV longitudinal shortening strain magnitude was higher at LV base with bipolar stimulation in comparison with unipolar stimulation (-10.5 +/- 10.5 vs. -4.2 +/- 6.3, P = 0.02). There is a difference in the mechanical activation sequence of the LV between unipolar vs. bipolar pacing stimulation. This may have important implications for CRT.