Ivabradine induces an increase in ventricular fibrillation threshold during acute myocardial ischemia: an experimental study

BACKGROUND

Tachycardia often facilitates ischemic ventricular fibrillation (VF).

OBJECTIVE

This study assessed the impact of ivabradine (IVA), a selective inhibitor of the cardiac pacemaker If current, on ventricular fibrillation threshold (VFT) during acute myocardial ischemia.

METHODS

The experiments were conducted on a total of 54 domestic pigs. Myocardial ischemia was induced in anesthetized pigs by total 1-minute coronary occlusion at baseline and then on 2 occasions after intravenous administration of saline or 0.5 mg/kg of IVA. VF was triggered by electrical stimuli of increasing intensity at a fixed rate. Heart rate (HR), VFT, monophasic action potential duration, and peak of the time derivative of left ventricular pressure (LV dP/dt max) were monitored on each occasion. The activity of mitochondrial succinodehydrogenase was measured on heart sections.

RESULTS

Compared with controls, IVA induced a 31% reduction in HR, a 2.9-fold increase in VFT, and prevented ischemia-induced monophasic action potential duration shortening (+1 +/- 12 vs. -14 +/- 11 milliseconds) without affecting peak LV dP/dt. This beneficial effect on VFT was mainly due to HR reduction and was accompanied by a significant reduction in the hypoxic area (26% +/- 1% vs. 38% +/- 1%, P < 0.0001).

CONCLUSION

HR reduction and the decrease in myocardial damage induced by IVA protected against primary ischemic VF without altering myocardial contractility.

The heart rate-lowering agent ivabradine inhibits the pacemaker current I(f) in human atrial myocytes

INTRODUCTION

It has been speculated that pacemaker current (I(f)) in human atria could play a role in causing ectopic atrial automaticity. Ivabradine is a novel selective and specific I(f) inhibitor in the sinus node that reduces heart rate without any negative inotropic effect. The aim of the study was to explore possible effects of ivabradine on I(f) in atrial myocytes.

METHODS AND RESULTS

Using patch-clamp technique, we studied effects of ivabradine on I(f) present in atrial myocytes isolated from human right appendages of patients undergoing cardiac surgery. The identification of HCN isoforms was obtained by means of multiplex single-cell RT-PCR. Ivabradine induced a marked concentration and use-dependent I(f) inhibition with an IC50 at steady state of 2.9 microM. Time constant of block development (Tau(on)) decreases with the increase in the ivabradine concentration. Use-dependent inhibition induced by ivabradine (3 microM) was not modified in the presence of cAMP (10 microM) in the pipette solution. Multiplex single-cell RT-PCR indicates that the major HCN gene subtype detected in atria was HCN2. HCN4 is detected weakly and HCN1 is not significantly detected.

CONCLUSIONS

Ivabradine inhibits I(f) current in the nonpacemaker cell with characteristics similar to those described previously in rabbit sinus node cells, but revealed a lesser sensitivity for I(f) recorded in human atrial cell than hHCN4 subunits considered as the major contributors to native f-channels in human sinoatrial node. A potential protection of atrial arrhythmias by ivabradine is discussed.