What is a Bundle and is it Harmful?

Health Professional

During pre-op testing for my father's back surgery, the EKG showed a "bundle." What exactly is a bundle and what does it mean for his health?

The heart's electrical activity normally starts in the heart's natural pacemaker (called the sinoatrial node), which is situated on the upper right chamber of the heart (atrium). From there the electrical impulse travels to the left upper chamber (atrium) and into the atrioventricular (AV) node. Electrical impulses travel through certain cells that are specially designed for the purpose of carrying those impulses. They carry them faster than regular heart muscle cells and are located near one another. When there are many such cells together in a bunch we call this grouping a node. From the AV node the electrical impulse travels down the bundle of His and divides into the right and left bundle branches. When the electrical impulse carrying cells are arranged in a line to carry many impulses together we call this a bundle, if it is large. If it is smaller, like a branch, we call it a fascicle. The right bundle branch contains one fascicle. The left bundle branch subdivides into two fascicles: the left anterior fascicle and the left posterior fascicle. Ultimately, the fascicles divide further into millions of (Purkinje) fibers that send impulses to the individual cardiac cells, allowing for rapid, coordinated contraction of the lower chambers (ventricles) which send the blood pumping around the arteries.

Bundle branch block can be detected by electrocardiography (ECG) when the duration of the QRS complex is prolonged. This means that the heart muscle takes a bit longer to contract because the electrical impulse that causes the contraction is delayed as it passes through the regular heart muscle. When a bundle branch or fascicle becomes injured (due to underlying heart disease, heart attack, long-term high blood pressure, age dependent calcification or cardiac surgery), it may cease to conduct electrical impulses appropriately. This results in altered pathways for the heart's contraction. Since the electrical impulse no longer uses the preferred pathway across the bundle branch, it may move instead through muscle fibers in a way that both slows the electrical movement and changes the direction of the impulses. Conduction may be slowed or blocked in the left or right bundle branch without causing any symptoms.

Usually, no treatment is needed for either type unless there is an associated cardiac problem. Right bundle branch block is often seen in young people and does not increase mortality or decrease life expectancy. We sometimes see left bundle branch block as well in the young. A study of 20 year olds with this diagnosis back in the 1960's demonstrated no evidence for any heart problem in most. They were Air Force cadets and were able to resume pilot training without a problem once physicians were able to check them and make sure that they did not have a problem that might damage their health or cause problems with public safety. Left bundle branch blocks become more common as we age and may indicate cardiac problems such as long-term high blood pressure, valve disease, cardiac calcifications, coronary arterial disease, cardiomyopathy, or the simple aging process.

The prudent physician will want to make sure that there are no other cardiac problems present before proceeding with an elective operation. This requires a thorough physical examination, a review of old records, and sometimes further testing such as a chest x-ray, an echocardiogram (ultrasound), a period of heart monitoring, a cardiac stress test, a CAT scan (computerized axial tomography is a noninvasive x-ray procedure), or even, on occasion, an angiogram.

When no additional problems are found, life returns to normal and although the bundle branch block remains, it doesn't cause further problems. The most important thing is that he tells whatever doctor he sees in the future about the bundle branch block, which once worked up, will not generally require further attention.