• bundle branch block
• right bundle branch block
• left bundle branch block
• heart conduction system
• asymptomatic bundle branch block

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).

In their Heart paper, Rasmussen et al 6 present the clinical implications …