A:
An electrocardiogram (EKG) records the heart's electrical activity. The phrase "left bundle branch block" refers to the electrical signal down the left side of your heart. It might not even be a "block." It could just mean that the signal is slower on the left side of your heart than on the right side.
If you do not have other evidence of heart disease -- like a heart attack, chest pain when exerting yourself, fainting spells, or shortness of breath with mild exertion -– this might not be important at all.
On the other hand, a left bundle branch block can be evidence of damage to the heart from any of a number of causes, including a heart attack or some disease that damaged the heart.
Your doctors were right to cancel your surgery. There is no point in having elective surgery until the risk to you is known. (Elective surgery is not urgent or emergency surgery.)
Left bundle branch blocks make it impossible to interpret the EKG and search for evidence of heart attacks. Other tests (like echocardiography and nuclear cardiology scans) are needed to check if the heart is normal and if there is a risk for future heart attacks.
So, the short answer to your question is that the meaning of your left bundle branch block is not clear, and you should get other tests to clarify its meaning.
Thomas H. Lee, M.D., is the chief executive officer for Partners Community HealthCare Inc. He is a professor of medicine at Harvard Medical School. He is an internist and cardiologist at Brigham and Women's Hospital. Dr. Lee is the chairman of the Cardiovascular Measurement Assessment Panel of the National Committee for Quality Assurance. 
