Can chemotherapy cause left bundle branch block?

Sometimes. A left bundle branch block is a finding on an EKG ( the electrical tracing of your heart.) it shows that something has altered the way your heart conducts electrical impulses. Chemotherapy can do it, but so can heart disease. Your doctor should rule out other causes for a left bundle branch block before assuming chemotherapy is the cause.
Chemo. Many chemotherapies can cause heart problems (ie, adriamycin (doxorubicin) - heart muscle problems; Paclitaxel - very fast heart rate or arrhythmia). Lbbb is not a common side effect but it is possible.