QTc interval evaluation in patients with right bundle branch block or bifascicular blocks

Abstract Background The right bundle branch block (RBBB) and the bifascicular blocks affect QRS duration in the right precordial leads, which are usually used for QT interval determination. Up to now, there is no clear recommendation how to determine QT interval in patients with RBBB or bifascicular block. Hypothesis The hypothesis of the present study was to evaluate the feasibility of a simple formula for RBBB and bifascicular block correction, thereby making it easier to determine the QTc interval. Methods In patients with intrinsic QRS duration <120 ms, artificial RBBB with either left posterior (LPFB) or left anterior fascicular block (LAFB), created by left ventricular pacing maneuvers, were corrected using the Bogossian formula (QTm) and afterward were heart rate corrected (QTmc). Heart rate correction was performed using different heart rate formulas in comparison to each other. The QTmc intervals were compared in each patient with the QTc interval during intrinsic rhythm. Results A total of scheduled 71 patients were included in this prospective multicenter observational comparative study. Compared to intrinsic QTc interval, the mean ΔQTmc interval by combination of the Bogossian and the Hodge formulas was −3 ± 24 ms in RBBB + LPFB (P = .44) and −6 ± 25 ms in RBBB + LAFB (P = .15). The Bogossian formula showed a significant deviation from the actual QTc interval with both the Bazett and the Fridericia formulas. Conclusion In combination with the Hodge formula, the Boggosian formula delivered the best results in comparing the true QTc interval in narrow QRS with the QTmc interval in the presence of a bifascicular block.


| INTRODUCTION
Evaluation of the QTc interval is an important diagnostic tool in clinical practice to identify patients at high risk for ventricular tachycardia and sudden cardiac death. 1 The presence of a bundle branch block (BBB) represents a particular challenge in properly measuring the QTc interval. 2 Following international recommendations, QT interval should be measured in leads showing the longest QT interval, which is usually in right precordial leads. 3 In presence of a right bundle branch block (RBBB) or a bifascicular block, these leads are strongest affected by conduction delay and therefore hamper adequate measurement. In 2014, a new formula for evaluation of the QT interval in patients with left bundle branch block (LBBB) was reported ( Figure 1). 4 Application of this so-called "Bogossian formula," in combination with the Bazett formula for heart rate correction showed to be a simple and reliable tool in clinical practice for QTc interval evaluation in patients with preserved or reduced left ventricular ejection fraction and LBBB. [4][5][6][7] However, the importance of the Bogossian formula has never been evaluated in patients with RBBB or bifascicular block. Moreover, for QTc interval evaluation, the Bogossian formula has never been combined with Fridericia's or Hodge's formulas.

| METHODS
Patients who underwent left sided electrophysiologic study, respectively, ablation therapy, were eligible to participate in this prospective, multicenter observational study. Patients were included who had an intrinsic QRS interval of <120 ms, were between 18 11 In presence of atrial fibrillation, 10 consecutive RR intervals were averaged for the heart rate corrected QT interval. All RBBB corrections and QTc measurements were performed by two electrophysiologists (young electrophysiologist and experienced electrophysiologist) who worked independently from each other.

| Statistical analysis
Continuous data are displayed as mean ± SD. Data were analyzed by employing a computerized database (Microsoft Excel 2010, Microsoft, Redmond, Washington) and were statistically evaluated using SPSS Software Release 23.0.0. The Shapiro-Wilk test was employed to assess normal distribution. Differences between groups were determined by Student's unpaired t-test. Differences were regarded significant when P < .05 (two-sided).

| RESULTS
A total of 71 patients who met inclusion criteria were enrolled in this prospective study. Patient characteristics were displayed in Table 1

| DISCUSSION
To best of our knowledge, this is the first prospective multicenter study to assess QTc interval evaluation in patients with bifascicular blocks. This trial shows that bifascicular blocks can be simple and reliable corrected by using the Bogossian Formula, followed by an acceptable heart rate-adjusted QT interval determination using the Hodge formula.
Bundle branch blocks are frequent ECG findings, especially in the presence of a structural heart disease. Right bundle branch and/or bifascicular blocks increase with age and affect approximately 1% of the general population. 12,13 Furthermore, in patients hospitalized due to syncopes, bifascicular blocks can be detected with a frequency of up to 8%. [14][15][16] In BBB, the cardiac conduction disorder is mainly affected by delay in depolarization, visible on ECG as extension of the QRS duration.
Therefore, some authors recommend to use the JT interval for QT determination, due to its independence of QRS wideness. 17,18 However, the JT interval is rarely used in clinical practice, perhaps due to well-established automated ECG measurements with rate correction of the QT interval. Nevertheless, these automated ECG measurements are not suitable in the presence of a BBB. In our study, the JT interval during left ventricular pacing was significantly shorter compared to the JT interval during intrinsic rhythm. This is explained by the required high left ventricular stimulation rate due to observed heightened mechanoelectrical feedback compared to right ventricular pacing in our initial trial. 4 It is known that a higher heart rate results in an increasing difference between intrinsic and heart rate corrected QT interval. 6,7,19 While the Bazett formula leaves a strong positive residual correlation with heart rate, the Fridericia formula leaves a negative correlation. 3 For heart rate-dependent QT correction using Bazett's or Fridericia's formulas deviations of more than 20 ms are considered to be possible. 3 Although this study involves artificially created bifascicular blocks, similar to those initially shown with the LBBB, the experimentally validated Bogossian formula has already shown that it holds true in clinical setting. 5 The present study examined the feasibility of correcting RBBB and bifascicular blocks. Although we did not create an isolated RBBB ECG pattern with our stimulation maneuvers, it could be shown, that the longest QT interval could be measured in the leads V1 or V2.
Since these leads are also most affected in isolated RBBB, it can be assumed that the Bogossian formula can also be applied to this form of bundle branch block.
An adequate evaluation of the QT interval is anything but trivial.
Even in experienced colleagues, a correct hit rate of only 60% has been described in the past. 21 In case of a BBB, the evaluation is made even more difficult. Therefore, standardized, simple and investigator-

| CONCLUSION
In the present study, the Bogossian formula in combination with the Hodge formula delivered the best results in comparing the "true" QTc interval in narrow QRS with the corrected QT-interval in the presence of bifascicular blocks, which were created by left ventricular pacing.
The utilization of these formulas and the reproducibility of the results were independent of the operator. Therefore, the Bogossian formula seems to be a simple and investigator-friendly method to correct BBB and to facilitate the determination of QTc interval, not only in patients with LBBB as previously reported, but also in RBBB with or without additional block of one of the left ventricular fascicles.