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Isolated right bundle branch block in asymptomatic patients: not inconsequential as previously thought?
  1. Arun R Sridhar1,
  2. Santosh K Padala2
  1. 1 Medicine, Section of Cardiac Electrophysiology, University of Washington Medical Center, Seattle, Washington, USA
  2. 2 Department of Medicine Section of Cardiac Electrophysiology, Virginia Commonwealth University, Richmond, Virginia, USA
  1. Correspondence to Dr Arun R Sridhar, Section of Cardiac Electrophysiology, University of Washington Medical Center, Seattle, WA 98195, USA; arun11{at}cardiology.washington.edu

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Bundle branch block (BBB) is noted commonly after myocardial infarctions, myocarditis and cardiomyopathy; however, isolated BBB (in the absence of structural heart disease) is not infrequent. Right bundle branch block (RBBB) is much more prevalent in population compared with left bundle branch block (LBBB). This is likely because the right bundle branch (BB) is a smaller, more discrete structure compared with the large multifasciculate left bundle; and as such is more prone to complete interruption of function in response to disease.1

The aetiology of isolated BBB in subjects without evidence of structural heart disease is usually an age-related degeneration of the conduction system, which may be either a focal (Lev’s disease) or a diffuse (Lenegre’s disease) process.2 3 Others may have undetected ischaemia, valvular heart disease or cardiomyopathy.

Several previous studies have looked at the natural history of asymptomatic BBB in patients with no known structural heart disease (online supplementary table). Multiple studies have shown that isolated LBBB portends a poor cardiovascular prognosis, and is associated with a future risk of heart failure, worsening conduction disease requiring pacemaker placement and higher mortality.4 Isolated RBBB, on the other hand, is widely considered to be a benign finding and of favourable prognosis.5 Studies on the prognostic significance of isolated RBBB have provided conflicting results perhaps due to relatively small sample size and short follow-up time duration (online supplementary table).

Supplemental material

In their Heart paper, Rasmussen et al 6 present the clinical implications of asymptomatic BBB detected on 12-lead ECG among middle-aged and elderly individuals without known cardiovascular disease. The study is a retrospective cohort analysis of the Copenhagen ECG study examining 202 268 patients (men 43%; women 57%) who received an ECG between 2001 and 2011 and were followed up over a median period of 7.8 years.

The prevalence of LBBB, RBBB, incomplete RBBB and intraventricular conduction delay (IVCD) among men was 0.4%, 2.6%, 1% and 2.3% and among women was 0.6%, 1.1%, 0.4% and 0.9%, respectively. There was a statistically significant association between incident heart failure, pacemaker implantation and any BBB (LBBB, RBBB and IVCD) in both men and women. RBBB and LBBB were associated with increased hazard risk of cardiovascular death in men, whereas IVCD was associated with increased hazard risk of cardiovascular death in both men and women. LBBB and IVCD were also associated with increased risk of myocardial infraction in both men and women. Furthermore, increasing QRS duration among patients with LBBB was associated with increased hazard of heart failure in both sexes, whereas among patients with RBBB, it was associated with increased hazard of pacemaker implantation; a finding that has not been reported before.

The authors are to be congratulated for their important contribution to our understanding of long-term outcomes of patients with asymptomatic BBB detected on ECG. Several aspects of this work deserve comment:

  • To our knowledge, this is the largest study of asymptomatic patients with BBB to date with a long follow-up using the robust Danish administrative healthcare registries.

  • The authors used standard definitions of BBB as recommended by the society guidelines. To verify the automated algorithm used by the software to identify BBB correctly, ECGs were randomly sampled and manually adjudicated to confirm the findings.

  • To exclude the possibility of manifest but as yet undiagnosed heart failure at the time of the initial ECG recording, the authors performed landmark analysis, in which only individuals with baseline LBBB and IVCD who were event free at 1-year follow-up were included. There was still a striking association between heart failure and these BBB types.

  • All three types of conduction delays in the ventricles—LBBB, RBBB and IVCD—were noted to be associated with adverse cardiovascular outcomes. In addition, increasing QRS width was shown to predict worse outcomes.

There are limited data on the long-term outcomes in patients with isolated IVCD.7 8 In the present study, patients with isolated IVCD were also noted to have worse cardiovascular outcomes including cardiac death in both men and women. As discussed above, IVCD could merely be a marker of underlying silent coronary artery disease or undiagnosed cardiomyopathy that progresses to adverse outcomes. IVCD reflects myocardial disease as opposed to conduction system disease in BBB, thereby causing abnormal depolarisation of myocardium itself, which probably explains worse cardiovascular outcomes in this subgroup.

This study adds to our growing evidence that isolated RBBB is not a trivial finding. However, this study also raises several important questions.

Question 1: what is the mechanism of the adverse cardiovascular outcomes in a patient with isolated RBBB?

We know from the major cardiac resynchronisation therapy (CRT) studies that electrical dyssynchrony caused by LBBB contributes to cardiomyopathy. Regardless of whether electrical dyssynchrony precedes or follows the mechanical dyssynchrony in cardiomyopathy, resynchronisation has been shown to help.

Right bundle, as opposed to a left bundle, supplies much smaller mass of myocardium. RBBB consequently creates much less mechanical dyssynchrony compared with LBBB. Response to resynchronisation in patients with RBBB is poor, and CRT in patients with RBBB is, at best, a controversial therapy.

As such, there are multiple hypothetical explanations for adverse cardiovascular outcomes in RBBB as listed below, but any of them would require further study:

  1. Are the conduction system disease and the future development of clinical myocardial disease a sign of a common underlying affliction? The adverse cardiovascular outcomes in patients with isolated RBBB could mean an associative finding, as opposed to a causal relationship. Processes such as sarcoidosis, infiltrative cardiomyopathies and myocarditis could be subclinical/smouldering at the outset, and may present with conduction system abnormalities much before development of cardiomyopathy.

  2. BBBs could be a set up for ventricular tachycardias (VT) such as BB re-entrant VTs, fascicular re-entrant VTs and fascicular premature ventricular complexes (PVCs). These in turn could lead adverse cardiac outcomes and increased mortality.

  3. The present study by Rasmussen et al shows that the outcomes are worse with wider QRS complexes. Wider BBBs could mean a progressive conduction disease leading to eventual pacemaker indication, and could be causal in more significant mechanical dyssynchrony, which could lead to cardiomyopathy.

Question 2: how should we manage patients with isolated asymptomatic RBBB?

At this time, it is unclear if we should recommend a surveillance strategy for patients with isolated RBBB; and if yes, at what frequency. The HRs associated with the cardiovascular outcomes in RBBB (heart failure: 1.28 [males], 1.42 [females]; pacemaker implant: 3.26 [males], 3.69 [females]) translate to a very low absolute risk in these patients. This would mean that we would have to monitor a very large number of patients to reduce one event.

At a prevalence level of 2.6% (males) and 1.1% (females), the healthcare burden of monitoring patients with asymptomatic RBBB would be enormous. Additionally, it is possible that these adverse outcomes could probably be captured early just by clinical symptom or sign directed surveillance without the additional healthcare burden of routine monitoring the patients with isolated RBBB. Third, mandating routine follow-up and surveillance of these patients could lead to increased health insurance premiums and/or denial of coverage. Last, but not least, even if we do monitor, we do not know if we could reduce the risk of an adverse event in this population.

These are difficult questions and merit further study.

References

Footnotes

  • Contributors The article was written by both authors.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Provenance and peer review Commissioned; internally peer reviewed.

  • Patient consent for publication Obtained.

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