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Both sides of the heart in sync: Why bundle branch block may lead to decreased heart function
A healthy heart is the cornerstone of our overall health. However, some heart problems, especially bundle branch block, can cause a significant decrease in heart function. Bundle branch block occurs when the heart's electrical signals are partially or completely interrupted within its electrical conduction system, a condition that may affect the normal functioning of the heart.
The heart's electrical activity begins in the sinoatrial node (the heart's natural pacemaker), then passes through the atria to the atrioventricular node, then through the bundle of His, and finally divides into the left and right bundle branches.
Anatomy and Physiology of the Heart
The heart's electrical signals begin in the sinoatrial node, located in the upper part of the right atrium. These electrical impulses first pass through the left and right atria and then converge at the atrioventricular node. Next, the electrical signal is transmitted downward through the bundle of His, which is divided into the right and left bundle branches. The right bundle branch is mainly composed of one small branch, while the left bundle branch is further divided into two small branches, namely the left anterior branch and the left posterior branch. More literature divides the left bundle branch into three small branches: left anterior, left posterior, and left ventricular septal branches. Ultimately, these small branches branch into millions of Purkinje fibers that intertwine with individual cardiomyocytes to achieve rapid, coordinated, and synchronized depolarization of the ventricles.
Mechanisms of bundle branch block
When the bundle branches, or small branches of the heart, are damaged due to heart disease, myocardial infarction, or heart surgery, they can no longer conduct electrical signals properly. This condition causes the pathway for ventricular depolarization to change. Instead of taking the preferred route through the bundle branches, the electrical signal may move through the muscle fibers, which not only slows down the signal but also changes the signal. direction of propagation. With prolonged cardiac depolarization, there may be a corresponding decrease in cardiac output.
If heart failure occurs, specialized pacemakers can be used to resynchronize the ventricles, which theoretically helps shorten the QRS time, bring the contractions of the left and right ventricles closer together, and slightly improve the ejection fraction.
Diagnosis
Bundle branch block is usually diagnosed via electrocardiogram (EKG) when the QRS complex lasts longer than 120 milliseconds. Right bundle branch block specifically prolongs the second half of the QRS complex and may shift the heart's electrical axis slightly to the right. Left bundle branch block increases the width of the entire QRS complex and, in most cases, shifts the heart's electrical axis to the left. The electrocardiogram shows a terminal R wave in lead V1 and a sliding S wave in lead I. Another normal feature is appropriate T wave discordance, where the T wave is opposite to the terminal deflection of the QRS complex, indicating that bundle branch block, especially left bundle branch block, may cause the heart to contract at an unequal ratio.
Types of bundle branch blockBundle branch block is further subdivided based on the anatomical location of the defect:
- Right bundle branch block: can be incomplete (IRBBB) or complete (CRBBB)
- Left bundle branch block: can be incomplete (iLBBB) or complete (cLBBB)
In addition, left bundle branch block can be further classified into:
- Left anterior fascicular block: involves only the anterior half of the left bundle branch
- Left posterior fascicular block: involves only the posterior half of the left bundle branch
Some people are born with partial bundle branch block, but more often it develops as heart disease develops. Even if the heart has bundle branch block, the patient can still be active and their ECG appearance may be unremarkable. In contrast, when bundle branch block is complex and diffuse to the bundle branch system or is accompanied by significant ventricular myocardial damage, it may be a serious indicator of underlying cardiac disease. In more severe cases, a pacemaker may be needed to restore optimal electrical signaling to the heart muscle.
As with every part of the cardiac system, the impact of bundle branch block on cardiac function cannot be underestimated. How can we better protect and maintain our heart health in our daily lives?
