Mechanism of cardiac functional improvement by biventricular pacing

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Left ventricular (LV) dysfunction was induced in an anesthetized and open chest pig by infusion of esmolol (400 - 500 μg/kg/min) and phenylephrine (200 - 300 μg/kg/min), and mild but significant functional mitral regurgitation (MR) was created. The heart rate was kept constant (100 beats/min) by atrial pacing. LV function was evaluated by LV peak + and -dP/dt. MR was quantified by proximal isovelocity surface area (PISA) method. Addition of right ventricular pacing (atrio-ventricular sequential pacing) resulted in significant decrease in LV peak + dP/dt (1192±30 to 874±178, p<0.01) and LV peak - dP/dt (- 829±78 to - 744±148, p<0.01), which was accompanied by significant increase in MR volume (4.2±2.4 to 12.0±2.7 ml/beat, p<0.01). Addition of LV pacing (biventricular pacing) failed to improve LV function (LV + dP/dt: 874±178 to 798±64, n.s., LV-dP/dt: -744±148 to - 681±91, n.s.) and MR volume did not significantly changed either (12.0±2.7 to 13.0±5.1 ml/beat, n.s.). These results suggest that LV dysfunction by asynchrony with reduced peak LV + and - dP/dt results in worsening of functional MR. Cardiac re-synchronization therapy with biventricular pacing is expected to improve LV dysfunction and worsened functional MR by asynchrony, however, it was not effective in this animal model, requiring further investigations.