Hemiblock in ECG refers to a block in one of the two fascicles (anterior or posterior) of the left bundle branch. Specifically, it indicates a delay or interruption in the electrical impulse conduction through either the left anterior fascicle or the left posterior fascicle of the heart. This condition is also referred to as left anterior fascicular block (LAFB) or left posterior fascicular block (LPFB).
Understanding Hemiblocks
The left bundle branch divides into two main fascicles:
- Left Anterior Fascicle (LAF): Carries electrical impulses to the anterior and superior portions of the left ventricle.
- Left Posterior Fascicle (LPF): Carries electrical impulses to the posterior and inferior portions of the left ventricle.
When one of these fascicles is blocked (hemiblock), the electrical impulse must travel through the remaining fascicle, which results in a change in the direction of ventricular depolarization and characteristic ECG patterns.
Types of Hemiblocks
There are two primary types of hemiblocks:
-
Left Anterior Fascicular Block (LAFB):
- This is the more common type of hemiblock.
- ECG characteristics often include:
- Left axis deviation (typically between -45° and -90°).
- Small Q waves in leads I and aVL.
- Small R waves in leads II, III, and aVF.
- Normal or slightly prolonged QRS duration.
-
Left Posterior Fascicular Block (LPFB):
- This is a less common type of hemiblock, as the left posterior fascicle is thicker and has a dual blood supply.
- ECG characteristics often include:
- Right axis deviation (typically > +90°).
- Small Q waves in leads II, III, and aVF.
- Small R waves in leads I and aVL.
- Exclusion of other causes of right axis deviation (e.g., right ventricular hypertrophy, pulmonary embolism).
- Normal or slightly prolonged QRS duration.
Causes of Hemiblocks
Hemiblocks can be caused by various factors, including:
- Ischemic heart disease: Coronary artery disease can lead to damage and subsequent block of the fascicles.
- Hypertension: High blood pressure can cause structural changes in the heart that contribute to hemiblocks.
- Cardiomyopathy: Diseases of the heart muscle can disrupt the normal electrical conduction pathways.
- Valvular heart disease: Conditions affecting the heart valves can also lead to changes in the heart's electrical system.
- Fibrosis: Scarring of the heart tissue can impede electrical impulse conduction.
- Congenital heart defects: Some individuals may be born with abnormalities in the heart's electrical system.
- Medications: Some drugs can rarely cause hemiblocks.
Clinical Significance
Isolated hemiblocks may not always be clinically significant, particularly LAFB. However, they can be associated with increased risk of developing complete heart block in the presence of other conduction abnormalities. LPFB is rarer and often associated with more significant underlying cardiac disease. The presence of hemiblock should prompt a thorough cardiac evaluation to identify any underlying heart conditions and assess the overall risk of developing more serious arrhythmias.
Conclusion
In conclusion, a hemiblock on an ECG indicates a conduction delay or block in either the left anterior or left posterior fascicle of the left bundle branch, which can alter the electrical activity and create distinctive patterns on the ECG. Further investigation may be warranted to determine the underlying cause and assess the risk of future complications.
