Harvard Heart Letter

Should an abnormal electrocardiogram be a cause for worry?

Here's the truth about left and right bundle branch blocks that show up on some ECG readings.

Your doctor has told you that you have a bundle branch block. Although the phrase evokes images of a suburban subdivision, bundle branch blocks are abnormalities of the heart's conduction system that show up on a tracing of the electrical activity in the heart, called an electrocardiogram (ECG) reading.

Like the circuitry of a house, your heart muscle is crisscrossed by a network of electrical pathways that carry energy from room to room—or, in the case of the heart, from chamber to chamber. At rest, the sinoatrial node in the right atrium generates 60 to 100 electrical impulses per minute, which initiate the heartbeat. The signal spreads through the walls of the upper chambers, then forks into separate conduits and continues down the interior walls of the ventricles. These nerve channels, known as the left bundle branch and the right bundle branch, deliver the "contract now" message to muscle fibers in the ventricles of the lower chambers. But the heart's intricate circuitry sometimes experiences technical difficulties.

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