  Background: An increased QT/RR slope is hypothesized to be predictive of
sudden cardiac death after myocardial infarction. Previous studies have shown
that beta-adrenergic stimulation increases QT/RR slope, but the effects of
beta-adrenergic stimulation on QT/RR slope in heart failure are unknown.
Methods: New Zealand White rabbits underwent coronary ligation (n=15) or sham
surgery (n=11), and implantation of a pediatric pacemaker lead in the right
ventricle for chronic ECG recording. Eight weeks after surgery, unsedated
rabbits were given intravenous administrations of 0.25 to 2.0 ml of 1
micromol/l isoprenaline, while peak QRS to QRS (RR) and Q to T peak (QT)
intervals were measured. Results: Ligated rabbits (n=6) had lower LVEF than
sham rabbits (n=7, p<.0001), but similar baseline RR (269 +/- 15 vs 292 +/- 23
ms, p=.07), QT (104 +/- 17 vs 91 +/- 9 ms, p=.1) and minimum RR (204 +/- 11 vs
208 +/- 6 ms, p=.4) intervals induced by isoprenaline (0.79 +/- 0.18 vs 0.73
+/- 0.14 ml, p=.6). Hysteresis in QT vs TQ interval plots displayed biphasic
restitution 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=.004). Absolute value of the negative slope was greater in ligated
rabbits (-0.81 +/- 0.52 vs -0.35 +/- 0.14, p=.04). Conclusion: Ischaemic heart
failure produces steeper restitution slopes during beta-adrenergically induced
QT/TQ hysteresis. This could underlie the propensity of failing hearts to
arrhythmias.
