Abstract
Background: Biventricular epicardial (Epi) pacing can augment left ventricular (LV) function in heart failure. We postulated that these beneficial effects could involve catecholamine release from local autonomic nerve activation. This hypothesis was tested by applying Epi pacing stimuli during the muscle’s absolute refractory period (ARP) thereby activating intrinsic nerves without altering activation sequence. Methods: Anesthetized farm pigs (n=5) were instrumented with a LV high-fidelity pressure transducer, left atrial and LV Epi pacing electrodes, and sonomicrometer segment length (SL) gauges placed proximal and distal to the LV stimulation site. A small catheter was placed into the great cardiac vein adjacent to the LV pacing site for blood sampling. During atrial pacing at constant rate, 3 electrical pulses (0.8ms, 2-3x threshold volts) were applied to the LV Epi electrodes during the ARP. An experimental run consisted of baseline measurements, with 10 minutes of stimulation followed by 5 minutes of recovery repeated 3 times before and with ß- receptor blockade (BB, metoprolol, 5 mg). Coronary venous blood was sampled at baseline and at 5 and 10 minutes of stimulation for norepinephrine analysis (NE by ELISA). Results: ARP stimulation produced a significant increase in coronary venous NE levels (p< 0.01) accompanied by a significant 10-15% increase in LV dP/dt max (p< 0.01). Segment length vs. LV pressure loop area proximal to the stimulus site was significantly decreased (29.8 ± 10.9 mmHg*mm) compared to baseline (92.6 ± 8.2 mmHg*mm) but was unchanged at the distal site. BB abolished the changes in segment function and blocked the increase in LV dP/dt despite continued NE release. Conclusion: These results demonstrate that ARP EPI pacing induces NE release and augments global LV function. This effect may contribute to the beneficial effect of biventricular Epi pacing in some patients with heart failure.
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