Isoprenaline increases the slopes of restitution trajectory in the conscious rabbit with ischemic heart failure

Roughly speaking, restitution is the dependence of recovery time of cardiac electrical activity on heart rate. Increased restitution slope is theorized to be predictive of sudden death after heart injury such as from coronary artery occlusion (ischemia). Adrenaline analogs are known to increase restitution slope in normal hearts, but their effects in failing hearts are unknown. Twenty-six rabbits underwent coronary ligation (n = 15) or sham surgery (n = 11) and implantation of a lead in the heart for recording electrocardiograms. Eight weeks later, unanesthetized rabbits were given 0.25-2.0 ml of 1 mu mol/L isoprenaline intravenously, which increased heart rate. Heart rate was quantified by time between QRS peaks (RR) and heart activity duration by R to T peak time (QTp). Ligated rabbits (n = 6) had lower ejection fraction than sham rabbits (n = 7, p < 0.0001) indicative of heart failure, but similar baseline RR (269 +/- 15 vs 292 +/- 23 ms, p = 0.07), QTp (104 +/- 17 vs 91 +/- 9 ms, p = 0.1), and isoprenaline-induced minimum RR (204 +/- 11 vs 208 +/- 6 ms, p = 0.4). The trajectory of QTp vs TQ plots displayed hysteresis and regions of negative slope. The slope of the positive slope region was > 1 in ligated rabbits (1.27 +/- 0.66) and < 1 in sham rabbits (0.35 +/- 0.14, p = 0.004). The absolute value of the negative slope was greater in ligated rabbits (-aEuro parts per thousand 0.81 +/- 0.52 vs -aEuro parts per thousand 0.35 +/- 0.14, p = 0.04). Isoprenaline increased heart rate and slopes of restitution trajectory in failing hearts. The dynamics of restitution trajectory may hold clues for sudden death in heart failure patients.