BACKGROUND: Heart rates during ischaemia and reperfusion are possible determinants of reperfusion arrhythmias. We used ivabradine, a selective I(f) current inhibitor, to assess the effects of heart rate reduction (HRR) during ischaemia-reperfusion on reperfusion ventricular arrhythmias and assessed potential anti-arrhythmic mechanisms by optical mapping. METHODS: Five groups of rat hearts were subjected to regional ischaemia by left anterior descending artery occlusion for 8minutes followed by 10minutes reperfusion: (1) Control n=10; (2) 1μM ivabradine perfusion n=10; (3) 1μM ivabradine+5Hz atrial pacing throughout ischaemia-reperfusion n=5; (4) 1μM ivabradine+5Hz pacing only at reperfusion; (5) 100μM ivabradine was used as a 1mL bolus upon reperfusion. For optical mapping, 10 hearts (ivabradine n=5; 5Hz pacing n=5) were subjected to global ischaemia whilst transmembrane voltage transients were recorded. Epicardial activation was mapped, and the rate of development of ischaemia-induced electrophysiological changes was assessed. RESULTS: HRR observed in the ivabradine group during both ischaemia (195±11bpm vs. control 272±14bpm, p<0.05) and at reperfusion (168±13bpm vs. 276±14bpm, p<0.05) was associated with reduced reperfusion ventricular fibrillation (VF) incidence (20% vs. 90%, p<0.05). Pacing throughout ischaemia-reperfusion abolished the protective effects of ivabradine (100% VF), whereas pacing at reperfusion only partially attenuated this effect (40% VF). Ivabradine, given as a bolus at reperfusion, did not significantly affect VF incidence (80% VF). Optical mapping experiments showed a delay to ischaemia-induced conduction slowing (Time to 50% conduction slowing: 10.2±1.3 mins vs. 5.1±0.7 mins, p<0.05) and to loss of electrical excitability in ivabradine-perfused hearts (27.7±4.3 mins vs. 14.5±0.6 mins, p<0.05). CONCLUSIONS: Ivabradine administered throughout ischaemia and reperfusion reduced reperfusion VF incidence through HRR. Heart rate during ischaemia is a major determinant of reperfusion arrhythmias. Heart rate at reperfusion alone was not a determinant of reperfusion VF, as neither a bolus of ivabradine nor pacing immediately prior to reperfusion significantly altered reperfusion VF incidence. This anti-arrhythmic effect of heart rate reduction during ischaemia may reflect slower development of ischaemia-induced electrophysiological changes.