[Coexistence of left branch block and ectopic rhythm simulating alternating block]

Area of Science:

• Cardiology
• Electrophysiology
• Medical Diagnostics

Background:

• Chronic left bundle branch block (LBBB) can complicate the interpretation of electrocardiograms (ECGs).
• Myocardial infarction (MI) can cause dynamic ECG changes, but rapid QRS morphology shifts are unusual.
• Understanding ventricular activation patterns is crucial for diagnosing cardiac conditions.

Observation:

• A patient with chronic LBBB experienced rapid, transient ECG changes during acute diaphragmatic MI with septal extension.
• Observed sequence included complete LBBB, incomplete right bundle branch block (RBBB), narrow QRS, and then reversal of these patterns.
• Atrial rhythm remained stable throughout the observation period.

Findings:

• The dynamic QRS changes were attributed to the fusion of sinus rhythm with an ectopic rhythm originating distal to the LBBB block.
• Retrograde activation of the right bundle and concealed conduction in the left bundle likely influenced the ectopic rhythm's rate.
• Ventricular activation from a breakthrough point in the blocked left bundle can mimic bundle branch block patterns.

Implications:

• This case highlights the potential for rapid, complex ECG alterations in acute MI superimposed on pre-existing conduction defects.
• Recognizing these dynamic changes is vital for accurate diagnosis and management of MI in patients with LBBB.
• The findings underscore the role of electrotonic effects and concealed conduction in generating transient bundle branch block appearances.