Right bundle branch block (RBBB) configuration is an unexpected finding during right ventricular (RV) pacing that raises the suspicion of inadvertent left ventricular lead positioning. The aim of this study was to evaluate the prevalence of paced RBBB pattern in relation to RV lead location. This is a secondary analysis of a prospective, multicenter study, which randomized implantable cardioverter defibrillator recipients to an apical versus midseptal defibrillator lead positioning. A 12-lead electrocardiogram was recorded during intrinsic rhythm and RV pacing. Paced RBBB–like pattern was defined as positive (>0.05 mV) net amplitude of QRS complex in leads V1 and/or V2. In total, 226 patients (65.6 ± 12.0 years, 20.8% women, 53.1% apical site) were included in the study. The prevalence of paced RBBB pattern in the total population was 15.5%. A significantly lower percentage of patients in the midseptal group demonstrated RBBB-type configuration during RV pacing compared with the apical group (1.9% vs 27.5%, p <0.001). Baseline RBBB, prolonged QRS duration during intrinsic rhythm, and reduced ejection fraction were not associated with increased likelihood of paced RBBB. In the subgroup of patients with RBBB type during pacing, 91.4% of patients had a paced QRS axis from −30° to −90°, whereas 100% of patients displayed a negative QRS vector at lead V3. In conclusion, RBBB configuration is encountered in a considerable percentage of device recipients during uncomplicated RV pacing. Midseptal lead positioning is associated with significantly lower likelihood of paced RBBB pattern compared with apical location.
Right ventricular (RV) pacing typically results in a left bundle branch block (LBBB) configuration. The documentation of a right bundle branch block (RBBB) during RV pacing raises the suspicion of inadvertent left ventricular (LV) lead positioning or myocardial perforation. Taking into consideration that this unexpected finding is not rare, with a reported prevalence ranging from 8% to 16.6%, several electrocardiographic criteria have been proposed to differentiate RV from LV lead location. Convincing data have documented the detrimental effect of traditional apical pacing, thus resulting in a shift of device implantation routine toward alternate target pacing locations, such as midseptum. In comparison to apical pacing, midseptal pacing penetrates earlier the intrinsic conduction system, alters the direction of impulse propagation, and modifies the mean frontal QRS axis and its relative position in respect to the precordial leads. These characteristics may also have an impact on the manifestation of RBBB pattern during midseptal pacing. The effect of RV lead positioning on the prevalence of RBBB configuration during RV pacing has not been investigated yet. The aim of this study was to evaluate the prevalence of RV paced RBBB pattern in relation to RV lead location in a cohort of implantable cardioverter defibrillator (ICD) recipients randomized to an apical versus a midseptal defibrillation lead location.
Methods
This is a secondary analysis of the randomized, prospective, multicenter Septal Positioning of ventricular ICD Electrodes study. A detailed description of the study method has been previously reported. In brief, patients with a standard indication for ICD implantation were randomized to an apical versus midseptal defibrillator lead positioning. Appropriate lead placement in the randomized site was documented intraoperatively using standard fluoroscopic criteria both in right anterior oblique and left anterior oblique projections and was validated post hoc by an independent adjudication committee blinded to the patient’s randomization arm. The study was approved by the institutional review boards of all participating institutions. All patients provided written informed consent for their participation in the study.
In all enrolled patients, surface 12-lead electrocardiograms were recorded (1) during intrinsic rhythm and (2) during RV pacing, using either a DDD pacing mode with the shortest programmable atrioventricular delay or VVI pacing mode at a rate faster than the spontaneous rate, aiming to avoid fusion or pseudofusion and to ensure recording of fully paced complexes.
The QRS width was defined as the maximal QRS duration in all leads. The QRS axis was calculated in degrees by the vector method using the net voltage of QRS complexes in leads I and aVF. In all leads, QRS morphology was determined and classified in one of the following patterns: qR, Qr, QR, QS, rS, Rs, RS, monophasic R, rsr, and qrs. In each lead, the vector of the QRS complex was ascertained based on the net amplitude of the QRS complex (maximal positive minus maximal negative deflection voltage of the QRS complex [R-Q-S]). The vector was classified as positive if the net amplitude was >0.05 mV, negative if <−0.05 mV and isoelectric if between −0.05 and 0.05 mV.
Case definition of a RBBB-like pattern during RV pacing was based on the presence of a positive QRS complex vector in leads V1 and/or V2. In the subgroup of patients with a RBBB-like pattern, the transition lead was defined as the earliest lead in the precordial sequence with a change of the QRS vector from positive to negative.
Continuous data are presented as mean ± SD, whereas categorical data as count and percentages. The Pearson’s chi-square test for categorical variables and Student’s t test for continuous variables were used to compare parameters between groups of interest. Logistic regression analysis was used to evaluate the association between a continuous or categorical explanatory variable and a single categorical response variable. Adjusted odds ratios (ORs) and CIs were calculated from the logistic regression variable estimates. All tests were considered to be significant if p <0.05. Statistical analyses were performed with SPSS statistical software, version 16.0, (SPSS, Chicago, Illinois).
Results
The study population included 226 patients (65.6 ± 12.0 years, 20.8% women, 53.1% apical lead positioning, ejection fraction 28.2% ± 10.8%). Ischemic cardiomyopathy was present in 57.1% of patients, whereas 47.8% were in New York Heart Association class I or II. A positive vector in the right precordial leads during RV pacing was found in 35 patients, resulting in a prevalence of 15.5% in the total patient population, Figure 1 . A significantly lower percentage of patients in the midseptal group demonstrated RBBB-type pattern during RV pacing compared with those randomized in the apical group (1.9% vs 27.5%, p <0.001). Baseline characteristics of patients with RBBB-type pattern during RV pacing compared to those without are presented in Table 1 .
| Variable | Right Bundle Branch Block Type | p value | |
|---|---|---|---|
| Yes (n=35) | No (n=191) | ||
| Men | 80% | 79% | 1.0 |
| Age (years) | 69 (13) | 65 (12) | 0.12 |
| NYHA class I-II | 51% | 47% | 0.72 |
| Diabetes mellitus | 23% | 30% | 0.42 |
| Hypertension | 71% | 65% | 0.56 |
| Ischemic cardiomyopathy | 66% | 56% | 0.35 |
| Biventricular defibrillator | 37% | 33% | 0.70 |
In our population of patients with paced RBBB, 37.1% had a precordial transition at lead V2 and 62.9% had a transition at lead V3. The type of QRS vector in precordial leads V1 to V4 in patients with a RBBB pattern during RV pacing is presented in Figure 2 . In the same subgroup, the morphology of the QRS complex in lead V1 was Rs in 14 patients (40%), monophasic R in 13 patients (37.1%), RS in 4 patients (11.4%), rSR in 3 patients (8.6%), and qR in 1 patient (2.9%). Among the 20 patients with a positive vector in lead V2, the morphology of the QRS complex was monophasic R in 7 patients (35%), Rs in 6 patients (30%), RS in 5 patients (25%), qR in 1 patient (5%), and qrs in 1 patient (5%).
The mean frontal paced QRS axis in patients with RBBB type during RV pacing was −75.6° ± 41.4°. The respective dot plot of paced frontal QRS axis is presented in Figure 3 . In total, 91.4% of patients (32 of 35) in this subgroup had an RV paced QRS axis between −30° and −90°. Two patients with paced RBBB had a northwest QRS axis (−102° with midseptal location and −167° with apical location) and one had a rightward QRS axis (137° with midseptal positioning).
The presence of RBBB during intrinsic rhythm was not a significant predictor of RBBB pattern during RV pacing (OR 1.62, 95% CI 0.56 to 4.70, p = 0.38). Furthermore, we evaluated whether conduction delay influences the occurrence of RBBB configuration during RV pacing using prolonged QRS duration and the presence of LBBB during intrinsic rhythm as surrogate end points. QRS duration during intrinsic rhythm, accounted as a continuous variable, was not shown to significantly predict RBBB type during RV pacing (OR 1.009, 95% CI 0.997 to 1.021, p = 0.25). When accounted as categorical variable, neither prolonged intrinsic QRS duration (>120 ms) nor very prolonged QRS duration (>150 ms) were associated with significantly higher likelihood of RBBB configuration during RV pacing (OR 1.97, 95% CI 0.94 to 4.14, p = 0.07 and OR 1.44, 95% CI 0.65 to 3.16, p = 0.37, respectively). The presence of LBBB during intrinsic rhythm was not a predictor of paced RBBB (OR 1.28, 95% CI 0.56 to 2.76, p = 0.43).
The presence of systolic dysfunction was not a significant predictor of RBBB pattern occurrence during RV pacing, when accounted either as a continuous variable (OR 1.01, 95% CI 0.97 to 1.04, p = 0.69), or as a categorical variable with a cutoff limit of 35% (OR 1.18, 95% CI 0.38 to 3.64, p = 0.77). The only significant predictor of the manifestation of RBBB configuration during RV pacing was the apical positioning of the RV lead (OR 19.6, 95% CI 4.61 to 83.33, p <0.001).
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