The effect of ventricular sequential contraction on helical heart during pacing: high septal pacing versus biventricular pacing

OBJECTIVE: To investigate the effect of biventricular and high septal pacing on the normal contraction sequence of the helical ventricular myocardial band, and its impact on left ventricular function. METHODS: Ten pigs (25-68 kg) underwent analysis of percent segmental shortening (SS%) by sonomicrometry, with crystals placed along the fiber orientation of the ascending, descending segments, and posterior LV wall within the spatial geometry of the helical heart. Unipolar pacing electrodes stimulated the right atrium (RA) and either the right ventricular apex and left ventricular posterior wall (atrio-biventricular), or the proximal high septum (atrio-high septal). Systemic hemodynamics, QRS-interval, cardiac index (CI), systolic and diastolic LV functions and pressure-dimension loops (P-D) were analyzed and cardiac motion was monitored by video analysis. RESULTS: Pacing increased normal sinus heart rate (NSR) from 77+/-9 beats/min to 98+/-5 beats/min. Atrial pacing did not change the NSR hemodynamic variables. Conversely, atrio-biventricular pacing prolonged the QRS-interval (91+/-14 ms vs 56+/-11 ms at baseline, p<0.05) and decreased mean arterial pressure (50+/-4 mmHg vs 58+/-12 mmHg), CI (3.4+/-0.3 L/(min m2) vs 4.0+/-0.8 L/(min m2)) and PRSW (71+/-25%) compared to NSR (p<0.05). Furthermore, atrio-biventricular pacing decreased SS% in all segments, especially at the LV posterior wall (71% of baseline, p<0.05), and disrupted the NSR shortening sequence (progression from descending to posterior to ascending regions). Changes were characterized by premature stimulation of the posterior wall segment adjacent to the pacer stimulus, with associated (1) decrease of pressure-dimension loop area, (2) desynchronization of P-D loops and (3) consistent loss of the twisting pattern of visible cardiac motion. In contrast, atrio-high septal pacing restored systemic hemodynamics, LV systolic and diastolic functions to baseline values and preserved the normal sequence of shortening of the ventricular myocardial band. CONCLUSIONS: (1) Biventricular pacing disrupts of the natural sequence of shortening of the myocardial band and results in impaired LV function. (2) High septal pacing preserves the sequential shortening pattern of the myocardial band and LV function.