Predictors associated with significant QTc prolongation
We found that chronic Beta blocker usage was associated with clinically significant QTc prolongation. Surprisingly, no other independent clinical or demographic characteristic was found to be associated with QTc prolongation. Previous studies have demonstrated controversy regarding the effect of beta blocker on the QT interval. Studies in patients with long QT demonstrated that at faster heart rates, beta-blockers shortened the maximum QT interval and resulted in shorter QTc, whereas at slower heart rates beta-blockers lengthened the maximum QT interval and resulted in longer QTc.19 We believe that these results are consistent with our findings as the prolongation of the QT interval was most likely to occur during relative bradycardia post CV. Furthermore, in patients with AF, Beta blocker medication may be a marker of a more resistant or more progressive AF disease with rapid ventricular response, resulting in the need for treatment with Beta blockers for rate management. In our study, we failed to show association between previously reported risk factors (female gender, age, potassium level, and magnesium level20) and the subsequent risk for QTc prolongation. We believe this is mainly due to the small number of our cohort.
Our study has several limitations. The lack of control group plays an important role, yet we aimed to assess the benefit of further monitoring per patient, using the conventional monitoring period as our control measurement. Some of the patients had AF recurrence (whether transient or persistent) which may affect the accuracy of the QTc calculation made by the software (using the Bazzet formula which may overestimate the QTc during tachycardia), however, we educated all the results with delta of more than 10%, and we used a 4 hour median to correct for short episodes of tachyarrhythmias or bradyarrhythmia’s.