Left bundle branch block - what is it?
The heart is an electromechanical organ. This means that electrical excitation and conduction come first, followed by mechanics. During mechanics, the heart relaxes and fills up, the heart muscle contracts and then ejects the blood into the circulation. Electromechanical coupling means this relationship, that an electrical impulse is followed by the mechanical response of the heart.
Left bundle branch block - electrical excitation of the main cardiac chambers
The electrical impulse in physiological sinus rhythm happens in the sinus node, which is located in the upper right atrium. From there, excitation occurs via the atria. After brief filtering in the AV node, the excitation is subsequently transmitted to the main cardiac chambers. In this process, the right tawara muscle excites the right main chamber and the left tawara muscle excites the left main chamber. The tawara legs are specialized cardiac fibers for conduction of excitation in the main chambers. They can be thought of as a tree with a larger main lead and branches into many smaller sub-branches. Consequently, this leads to simultaneous excitation of the entire heart muscle.
Left bundle branch block - Diagnosis
We diagnose left bundle branch block on a 12-lead resting ECG. In this case, there is delayed conduction in the left tawara thigh. On the ECG, we see that there is delayed excitation of the left main ventricle. The QRS complex is wider than 120 ms in a complete left bundle branch block. We distinguish from this an incomplete left bundle branch block, in which the QRS complex is between 110 and 120 ms wide.
Left bundle branch block - what are the causes?
The main causes of left bundle branch block are structural diseases of the heart, especially coronary artery disease and a previously experienced myocardial infarction. In systolic heart failure, i.e. a relevant restriction of the left ventricular pumping force, we find left bundle branch block in about 30% of cases. It is not always clear what is chicken and what is egg. So, heart failure can lead to left bundle branch block, but left bundle branch block can also lead to heart failure if it persists for a long time. In addition, other causes are hypertensive diseases and valvular defects, and rather rarely, left bundle branch block can also be congenital.
Left bundle branch block - what are the consequences?
In left bundle branch block, there is an electromechanical disturbance. This means that the left main chamber of the heart is excited late and in a "roundabout" way. As a result, there is asynchrony between the left and right main chambers of the heart. The excitation reaches the right main chamber earlier and interventricular asynchrony occurs. In addition, however, the left main ventricle also operates asynchronously within itself. In this case, the septal portions of the ventricular septum are excited earlier than those of the lateral wall. Consequently, we call this intraventricular asynchrony. Due to this asynchronous contraction of the left heart, the heart works uneconomically. The cardiac output is therefore reduced and, if the left bundle branch block persists for a longer period of time, it can lead to a deterioration or new pumping weakness of the left heart.
Left bundle branch block - what treatment options?
First of all, it is important to effectively combat the causes of left bundle branch block, i.e. the underlying disease. This includes the drug therapy of a heart failure and the care of a circulatory disorder of the heart. There are no drugs that accelerate the conduction again and thus directly resolve the left bundle branch block. In certain cases, however, a so-called CRT system is used. This is a special pacemaker system that can excite the right ventricle and the left ventricle simultaneously via the coronary sinus access for the left ventricle. This leads to synchronization again and thus to an improvement in the pumping behavior.
Left bundle branch block - what to consider?
Left bundle branch block usually indicates cardiac damage and has a worse prognosis overall than so-called right bundle branch block. We therefore always recommend a cardiological evaluation in the case of left bundle branch block.
Fahy GJ et al. Natural history of isolated bundle branch block. American Journal of Cardiology 1996; 77: 1185-1190.
Cardiopraxis - Cardiologists in Düsseldorf & Meerbusch