3rd degree heart block risk – Special attention should be given to patients with a planned insertion of a pulmonary artery catheter (PAC). The insertion may cause a right bundle branch block (RBBB), which happens in 2-5% of PAC insertions. In combination with a preexisting LBBB, a RBBB can lead to a complete heart block (3rd degree heart block).

Tachycardia and hypertension – Left bundle branch blocks can appear during anesthesia, particularly during hypertensive or tachycardic episodes.

Masked Myocardial Infarction – The appearance of a LBBB during anesthesia may indicate myocardial ischemia. ST-segment and T wave changes associated with a true MI can be masked by a LBBB.

Tachycardia – Supraventricular tachycardia in a patient with a LBBB may falsely appear as ventricular tachycardia.

Assessment – In isolation, this condition is often benign. However, since the presence of a LBBB has a strong connection with serious heart disease, further assessment should be performed to rule out problems not yet diagnosed.

A bundle branch block, in general, is a conduction disturbance at the level of the His-purkinje system. In many cases, this is caused by an occlusion or disturbance of circulation at the left anterior descending artery (LAD), which supplies blood to both the right and left bundle branches.

Two fascicles of the left bundle branch exist: anterior and posterior. If only one is affected, it is called a hemiblock (HB). Disruption of the anterior fascicle is the most common hemiblock. Hemiblock’s have a normal or slightly delayed QRS duration.

A complete LBBB is defined as a QRS complex duration longer than 120 ms without Q waves in leads I, V5, and V6. A complete block will also demonstrate a slurred R wave in lead I, aVL, V5, and V6.

This condition is often an indication of serious heart disease (eg. coronary artery disease, aortic valve disease, hypertension, and cardiomyopathy).