Delayed adaptation of ventricular repolarization after sudden changes in heart rate due to conversion of atrial fibrillation. A potential risk factor for proarrhythmia?

AIMS: Onset and termination of atrial fibrillation are often associated with abrupt changes in heart rate. Presence and time-course of delayed adaptation of the QT/QTc interval are unknown, but a temporary "mismatch" between rate and the QT interval may enhance the risk of proarrhythmia. METHODS: In a prospective two-part study, time-course of adaptation of ventricular repolarization after abrupt changes in heart rate was assessed during termination of Holter ECG-documented atrial fibrillation episodes (Group 1, 32 patients) and subsequently in 20 patients with sick sinus syndrome and cardiac pacing initiating abrupt bi-directional changes in paced heart rate (Group 2). RESULTS: Conversion of atrial fibrillation showed a 32+/-21 bpm fall in heart rate (P<0.05). Restoration of the QTc interval afterwards was delayed by < or =1 min in 27%, by 1-2 min in 21%, by 2-5 min in 11% and by >5 min in 41% of the cases. Atrial pacing simulating a 30 bpm fall/increase in atrial rate demonstrated that a subsequent transient rate-QT mismatch is a physiological phenomenon (fall of 100 to 70 bpm: initially 90% of the proper QTc interval, compared with 94% after conversion of atrial fibrillation). The restoration curve of QTc adaptation showed an initially fast and subsequently slower time component, with interindividual variation. Clinical parameters, baseline heart rate or the direction of rate changes were not predictive. CONCLUSION: Delayed adaptation of ventricular repolarization following atrial fibrillation onset and termination is common, requiring minutes for restoring the QT/QTc steady state. Clinical parameters fail to predict patients with a long-lasting rate-QT mismatch. It may carry a significant arrhythmogenic risk particularly in patients on QT altering medication.