Accelerated QT adaptation following atropine‐induced heart rate increase in LQT1 patients versus healthy controls: A sign of disturbed hysteresis

Hysteresis, a ubiquitous regulatory phenomenon, is a salient feature of the adaptation of ventricular repolarization duration to heart rate (HR) change. We therefore compared the QT interval adaptation to rapid HR increase in patients with the long QT syndrome type 1 (LQT1) versus healthy controls because LQT1 is caused by loss‐of‐function mutations affecting the repolarizing potassium channel current I(Ks), presumably an important player in QT hysteresis. The study was performed in an outpatient hospital setting. HR was increased in LQT1 patients and controls by administering an intravenous bolus of atropine (0.04 mg/kg body weight) for 30 s. RR and QT intervals were recorded by continuous Frank vectorcardiography. Atropine induced transient expected side effects but no adverse arrhythmias. There was no difference in HR response (RR intervals) to atropine between the groups. Although atropine‐induced ΔQT was 48% greater in 18 LQT1 patients than in 28 controls (p < 0.001), QT adaptation was on average 25% faster in LQT1 patients (measured as the time constant τ for the mono‐exponential function and the time for 90% of ΔQT; p < 0.01); however, there was some overlap between the groups, possibly a beta‐blocker effect. The shorter QT adaptation time to atropine‐induced HR increase in LQT1 patients on the group level corroborates the importance of I(Ks) in QT adaptation hysteresis in humans and shows that LQT1 patients have a disturbed ultra‐rapid cardiac memory. On the individual level, the QT adaptation time possibly reflects the effect‐size of the loss‐of‐function mutation, but its clinical implications need to be shown.