Is right bundle branch block serious?

Depends: Right bundle branch block (rbbb) in and of itself is likely not a detrimental issue, however if it happens in the setting of a heart attack, it portends a poor prognosis.

Depends on cause.: May be expected after repair of ventricular septal defect. Otherwise usually related to subendocardial injury on the right side of the interventricular septum. Sick sinus syndrome, coronary artery disease. Serious but if the left bundle goes, treated with pcemaker.