Introduction: Measurement of QTc intervals during atrial arrhythmias is relevant to the safety of antiarrhythmic drug delivery. Atrial fibrillation (AF) waves may affect the T wave and hinder the identification of fiducial points, possibly resulting in inaccurate QT measurements. In addition, constant fluctuation in heart rate complicates the analysis
AIM: Compare QT and QTc intervals in AF patients before and after cardioversion.
Methods: 21 patients suffering from AF underwent electrical cardioversion. All were in AF at the time of the procedure. Cardioversion restored sinus rhythm in all patients. Pseudo-orthogonal Holter ECGs were continuously recorded during at least 1 hour before and 1 hour after the procedure. RR and QT time series were extracted and semi-automatically validated. For Q onset and T end identification, the lead with the most identifiable T wave was used. QTc intervals were computed using Bazett, Fridericia and patient-specific correction formulae, both with and without QT hysteresis correction with a time constant of 2 minutes (moving average of past RR values).
Results: Both RR and QT intervals were prolonged after cardioversion (RR = 1048±172 ms in sinus rhythm vs 691±127 ms in AF; QT = 412±23 ms vs 361±26 ms). After correction for heart rate, the difference QTc(sinus rhythm)-QTc(AF) was -28±20 ms (p< 0.001) with Bazett’s formula, thus indicating overcorrection, 0.2±10.2 ms (p=0.9) with Fridericia’s and -0.9±5.1 ms (p=0.4) with patient-specific correction. The root-mean-square difference between QTc(sinus rhythm) and QTc(AF) was 34 ms with Bazett’s formula, 10 ms with Fridericia’s and 5 ms with patient-specific correction. The variability of QTc over one hour during AF was smaller with patient-specific correction (Bazett: 51±9 ms; Fridericia: 34±8 ms; patient-specific: 20±7 ms). QTc variability was significantly further reduced when hysteresis correction was applied (Bazett: 22±10 ms; Fridericia: 20±10 ms; patient-specific: 17±7 ms).
Conclusions: QTc measurements during AF were consistent with values obtained after sinus rhythm was restored. Although patient-specific formula performance was better, Fridericia's correction combined with hysteresis reduction was found to be sufficiently reliable for the assessment of the QTc in AF.