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.