Abstract

We investigated the effects of the selective bradycardic agent ivabradine, an I_f channel inhibitor, on exercise-induced ischemia and resulting myocardial stunning. Seven dogs were chronically instrumented to measure left ventricular (LV) wall thickening (Wth), aortic pressure and coronary blood flow (CBFv) (Doppler). Circumflex coronary artery stenosis was set up to suppress the increase in CBFv during a 10 min treadmill exercise. During exercise under saline, LVWth in the ischemic zone was depressed (−70 ± 4%) and a prolonged myocardial stunning was subsequently observed. Infusion of ivabradine started before exercise significantly reduced heart rate (HR) at rest (−22 ± 7%), during exercise (−33 ± 4%) and throughout the recovery period (−21 ± 2%). By reducing HR during exercise, ivabradine simultaneously improved LVWth compared with saline (14 ± 1% versus 7 ± 1%, respectively) and subendocardial perfusion (microspheres). This anti-ischemic effect was subsequently responsible for a strong decrease in the intensity and severity of myocardial stunning. All these beneficial effects were abolished when HR reduction during exercise was suppressed by atrial pacing. Interestingly, when ivabradine infusion was started after exercise, LVWth was still significantly enhanced and myocardial stunning strongly attenuated. This direct effect of ivabradine on the stunned myocardium disappeared when HR reduction was suppressed by atrial pacing at rest. In conclusion, this study demonstrates that ivabradine exerts an anti-ischemic effect that is responsible for subsequent protection against myocardial stunning. Furthermore, administration of ivabradine after the ischemic insult still improves LVWth of the stunned myocardium.