Ventricular repolarisation and holter monitoring: role of sympathetic blockade on the physiology of the QT/RR ratio

Circadian variations of the QT interval and its heart rate dependency have been established. However, the respective roles of the sympathetic and parasympathetic nervous systems in their regulation are still undetermined. Eighteen healthy volunteers (average age 39 +/- 7 years, 10 men) were recruited and selected randomly to receive either placebo or atenolol (100 mg/day). The treatments were crossed after 7 days. The rate dependency of the QT was assessed by day and by night by 24 hour Holter ECG monitoring. The effects of atenolol on the rate dependency of the QT interval depend on the time of day. During the daytime, the QT rate dependency was reduced by atenolol (0.180 (0.162: 0.198) versus 0.216 (0.195: 0.236) with placebo, p< 0.01) whereas during the night, the QT rate dependency was the same in both groups. Therefore, the betablocker is associated with an inversion of the daily modulation of the QT fate dependency. The daytime rate-dependency of the QT interval in decreased with betablocker therapy. This result suggests a direct or indirect influence of the sympathetic nervous system on the rate dependency of ventricular repolarisation