Mechanisms whereby rapid RV pacing causes LV dysfunction: perfusion-contraction matching and NO

Abstract

Incessant tachycardia induces dilated cardiomyopathy in humans and experimental models; mechanisms are incompletely understood. We hypothesized that excessive chronotropic demands require compensatory contractility reductions to balance metabolic requirements. We studied 24 conscious dogs during rapid right ventricular (RV) pacing over 4 wk. We measured hemodynamic, coronary blood flow (CBF), myocardial O2 consumption (MV˙o 2) responses, myocardial nitric oxide (NO) production, and substrate utilization. Early pacing (6 h) resulted in decreased heart rate (HR)-adjusted coronary blood flow (CBF), MV˙o 2 (CBF/beat: 0.33 ± 0.02 to 0.19 ± 0.01 ml, P < 0.001, MV˙o 2/beat: 0.031 ± 0.002 to 0.016 ± 0.001 ml O2, P < 0.001), and contractility [left ventricular (LV) first derivative pressure (dP/d t)/LV end-diastolic diameter (EDD): 65 ± 4 to 44 ± 3 mmHg · s−1 · mm−1, P < 0.01], consistent with flow-metabolism-function coupling, which persisted over the first 72 h of pacing (CBF/beat: 0.15 ± 0.01 ml, MV˙o 2/beat: 0.013 ± 0.001 ml O2, P < 0.001). Thereafter, CBF per beat and MV˙o 2 per beat increased (CBF/beat: 0.25 ± 0.01 ml, MV˙o 2/beat: 0.021 ± 0.001 ml O2 at 28 days, P < 0.01 vs. 72 h). Contractility declined [(LV dP/d t)/LVEDD: 19 ± 2 mmHg · s−1 · mm−1, P < 0.0001], signifying flow-function mismatch. Cardiac NO production, endothelial NO synthase expression, and fatty acid utilization decreased in late phase, whereas glycogen content and lactate uptake increased. Incessant tachycardia induces contractile, metabolic, and flow abnormalities reflecting flow-function matching early, but progresses to LV dysfunction late, despite restoration of flow and metabolism. The shift to flow-function mismatch is associated with impaired myocardial NO production.