Left bundle branch block has several possible causes

Q: I recently had an ECG that showed that I have a "left bundle branch block." What does this mean?

A: When your heart beats, it does so in response to electrical signals. Your heart muscle is crisscrossed by a network of electrical pathways. A bundle branch block is caused by an abnormality in one of those pathways.

The electrical signals that orchestrate each heartbeat work this way. A small group of cells called the sinoatrial (SA) node generates an electrical impulse. The signal travels on pathways through the walls of the upper chambers (the atria), then forks into two separate channels and continues down the inside walls of the lower chambers (the ventricles).

The pathway that leads to the left ventricle is called the left bundle branch. The left ventricle pumps oxygen-rich blood throughout the body. The pathway that leads to the right ventricle is called the right bundle branch. The right ventricle pumps oxygen-depleted blood to the lungs, where it picks up oxygen and then enters the left ventricle.

The signals coming down the left and right bundle branches tell the left and right ventricles to "pump now!"

The electrical impulse must travel down the right and left bundle branches at the same speed for the ventricles to contract at about the same time. A block in either branch slows the signal, delaying the heart muscle's response. (I've put an illustration of this process on my website, AskDoctorK.com.)

My colleague Dr. Peter Zimetbaum is a cardiologist at Harvard-affiliated Beth Israel Deaconess Medical Center. He explains that a right bundle branch block is harmless. But a left bundle branch block may signal high blood pressure, thickening or stretching of the heart wall, coronary artery disease or simply advancing age.

When a left bundle branch block appears on an ECG, the next step is usually an echocardiogram. This noninvasive imaging test shows whether the heart structure looks normal. If it does, treatment might entail more aggressive blood pressure control.

Some people with a left bundle branch block eventually need a pacemaker. But most people can function normally without one.

One exception is people who also have a weakened heart muscle associated with advanced heart failure. Individuals in this group may be good candidates for an implanted pacemaker-like device with an added wire to stimulate the left ventricle. This device can coordinate the timing of the contractions on the left and right sides of the heart. This enables the heart to pump more efficiently.

Try not to be overly anxious about your diagnosis of left bundle branch block. Most of my patients with this condition have not seen the condition progress. But do get yearly checkups that include an ECG. And be on the lookout for troublesome symptoms, including shortness of breath, lagging energy or trouble exercising. Such symptoms could be a sign of one of the treatable medical conditions, like heart failure, that is sometimes found in people with left bundle branch block.

• Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.