Review Question - QID 103725

The patient’s ECG findings are indicative of a presumably new onset left bundle branch block (LBBB). The criteria to diagnose LBBB on ECG are a heart rhythm that is supraventricular in origin (sinus rhythm in this patient), a QRS duration of greater than 120 ms, a QS or rS complex in lead V1, and an RsR' wave in lead V6.

The cardiac conduction system features two bundle branches, the left and right bundle branches, that conduct electrical signal from the bundle of His to the ventricles. The left bundle branch is made up of the left anterior and posterior bundle branches. These bundle branches may become "blocked", or have a delay in electrical conduction due to a number of factors including ischemia, electrolyte abnormalities, or ventricular strain. A new onset LBBB can be a manifestation of left anterior descending artery occlusion and may mask typical ECG findings associated with myocardial infarction. The combination of chest pain and new onset LBBB is an indication for urgent coronary angiography.

Tan et. al review left bundle branch block. They discuss the etiology of the condition to include ischemia, valvular pathology and electrolyte abnormalities. They further discuss management of LBBB based on patient presentation.

Figure A demonstrates a left bundle branch block on a 12 lead ECG. Note characteristic morphology with negative QRS complexes in leads V1 and V2, with positive QRS complexes in leads V5 and V6.

Incorrect Answers:
Answer 1: First degree atrioventricular block presents with a PR interval greater than 200ms, and QRS duration is generally normal. It is often a benign condition and does not require intervention.

Answer 3: Right bundle branch block presents with a morphology that is distinct is from LBBB. Leads V1 and V2 typically have an upright QRS complex with a RsR' morphology.

Answer 4: Third degree atrioventricular block manifests on electrocardiogram as complete dissociation of P waves and QRS complexes. This patient's electrocardiogram demonstrates one P wave for every QRS complex in synchronicity.

Answer 5: Ventricular tachycardia, which also presents with a wide QRS complex, does not have P waves visible on the electrocardiogram, as is the case with this patient's rhythm.

Bullet Summary:
Left bundle branch block presents on the electrocardiogram with a rhythm that is supraventricular in origin, a QRS duration of greater than 120 ms, a QS or rS complex in lead V1, and an RsR' wave in lead V6.