What occurs when someone has a left bundle branch block?

Nothing to severe: Left bundle branch block (lbbb) is a pattern on the ECG that indicates that the heart's electrical waves are following an unusual path. It is common in older age, and usually due to scarring of the pathway. It may cause no symptoms and need no therapy. It can lead to marked slowing that needs a pacemaker. A new LBBB can also be a sign of a heart attack.

Depends --: May be nothing -- but it is known to relate to cardiac enlargement and congestive heart failure.