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pharmacological provocation (ajmaline, procainamide, or flecainide). Scheinman et al. studied the prognostic value of the HV: the progression rate to AVB at 4 years was 4% in patients with HV <70 ms, 12% in patients with HV between 70 and 100 ms, and 24% in patients with HV >100 ms.121 Development of intra- or infra-His block at incremental atrial pacing or by pharmacological stress test increases the sensitivity and positive predictive value of the EPS to identify patients who will develop AVB.116118,120,122,212 A positive EPS yielded a positive predictive value as high as 80% to identify patients who develop AVB. This finding has been indirectly confirmed by a study that showed a significant reduction in syncopal recurrences in patients with positive EPS treated with a pacemaker, compared with a control group of untreated patients with a negative EPS.119 In patients with unexplained syncope and bifascicular block, EPS is highly sensitive in identifying patients with intermittent or impending high-degree AVB. However, a negative EPS cannot rule out intermittent/paroxysmal AVB as the cause of syncope. Indeed, in patients with a negative EPS, intermittent or stable AVB was documented by ILR in 50% of cases. Therefore, elderly patients with bifascicular block and unexplained syncope might benefit from an empirical pacemaker, especially in unpredictable and recurrent syncope that exposes the patient to a high risk of traumatic recurrences. The decision to implant a pacemaker in these patients should be based on individual riskbenefit evaluation.213

5.3.1.3 Alternating bundle branch block This rare condition refers to situations in which there is clear ECG evidence for block in all three fascicles on successive ECGs; examples are LBBB and RBBB morphologies on successive ECGs, or RBBB with associated left anterior fascicular block on one ECG and left posterior fascicular block on another ECG.214 There is general consensus that this phenomenon is associated with significant infranodal disease and that patients will progress rapidly toward AVB. Therefore, a pacemaker should be implanted as soon as the alternating BBB is detected, even in the absence of symptoms.

5.3.1.4 Bundle branch block without symptoms Permanent pacemaker implantation is not indicated for BBB without symptoms, with the exception of alternating BBB, because only a minority of these patients will develop AVB (12% per year).115,121,215 The risks of pacemaker implantation and long-term transvenous lead complications are higher than the benefits of pacemaker implantation.216,217

5.3.1.5 Patients with neuromuscular diseases In patients with neuromuscular diseases, cardiac pacing should be considered, as any degree of fascicular block can progress unpredictably, even in the absence of symptoms (see section 8.5). 5.3.2 Pacing mode and algorithm selection In intermittent bradycardia, pacing may be required only for short periods. In this situation, the benefits of bradycardia and pause prevention must be weighed against the detrimental effects of permanent pacing, particularly pacing-induced HF. Low base-rate programming to achieve backup pacing, and manual adaptation of AV interval, programming AV hysteresis, or other specific algorithms preventing unnecessary RV pacing, play a particularly important role in this patient group.144,148

In patients in SR, the optimal pacing mode is DDD. The strong evidence of superiority of DDD vs. VVI pacing is limited to improvement in symptoms and quality of life. Conversely, there is strong evidence of non-superiority with regard to survival and morbidity.20 Therefore, in elderly or frail patients with intermittent bradycardia, the decision regarding the pacing mode should be made on an individual basis, taking into consideration the increased complication risk and costs of DDD (Figure 5). VDD may be a pacing mode alternative for patients with advanced AV conduction abnormalities and spared sinus node function. In comparison with DDD, VDD system implantation is associated with fewer complications, shorter procedure and fluoroscopy times, and a high incidence of atrial undersensing.218 Potential atrial undersensing is contributing to the low use of this system as most operators are aiming for AV synchrony.

Recommendations for pacing in patients with bundle branch block

Recommendations Classa Levelb

In patients with unexplained syncope and bifascicular block, a pacemaker is indicated in the presence of either a baseline HV of >_70 ms, second- or third-degree intra- or infra-Hisian block during incremental atrial pacing, or an abnormal response to pharmacological challenge.119,120 I B Pacing is indicated in patients with alternating BBB with or without symptoms. I C Pacing may be considered in selected patients with unexplained syncope and bifascicular block without EPS (elderly, frail patients, high-risk and/ or recurrent syncope).213 IIb B Pacing is not recommended for asymptomatic BBB or bifascicular block.115,121,215 III B

BBB = bundle branch block; EPS = electrophysiology study; HV = Hisventricular interval. aClass of recommendation. bLevel of evidence. ESC 2021

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