Site-Specific Differences in Latency Intervals during Biventricular Pacing: Impact on Paced QRS Morphology and Echo-Optimized V-V Interval

OBJECTIVE: To investigate differences in latency intervals during right ventricular (RV) pacing and left ventricular (LV) pacing from the (postero-)lateral cardiac vein in cardiac resynchronization therapy (CRT) patients and their relationship to echo-optimized interventricular (V-V) intervals and paced QRS morphology. METHODS: We recorded digital 12-lead electrocardiograms in 40 CRT patients during RV, LV, and biventricular pacing at three output settings. Stimulus-to-earliest QRS deflection (latency) intervals were measured in all leads. Echocardiographic atrioventricular (AV) and V-V optimization was performed using aortic velocity time integrals. RESULTS: Latency intervals were longer during LV (34 ± 17, 29 ± 15, 28 ± 15 ms) versus RV apical pacing (17 ± 8, 15 ± 8, 13 ± 7 ms) for threshold, threshold ×3, and maximal output, respectively (P < 0.001), and shortened with increased stimulus strength (P < 0.05). The echo-optimized V-V interval was 58 ± 31 ms in five of 40 (12%) patients with LV latency ≥ 40 ms compared to 29 ± 20 ms in 35 patients with LV latency < 40 ms (P < 0.01). During simultaneous biventricular pacing, four of five (80%) patients with LV latency ≥ 40 ms exhibited a left bundle branch block (LBBB) pattern in lead V(1) compared to three of 35 (9%) patients with LV latency < 40 ms (P < 0.01). After optimization, all five patients with LV latency ≥ 40 ms registered a dominant R wave in lead V(1) . CONCLUSIONS: LV pacing from the lateral cardiac vein is associated with longer latency intervals than endocardial RV pacing. LV latency causes delayed LV activation and requires V-V interval adjustment to improve hemodynamic response to CRT. Patients with LV latency ≥ 40 ms most often display an LBBB pattern in lead V(1) during simultaneous biventricular pacing, but a right bundle branch block after V-V interval optimization.