How Is Bundle Branch Block Diagnosed?
In the depolarization (contraction) cascade–right to left and superior to inferior–that results in cardiac output, a disruption in the electrical conductive pathway that occurs below the atrioventricular (AV) node is called an intraventricular conduction delay (IVCD). The Bundle of His separates into right and left main bundle branch divisions. These divide further into “fascicles.” Disruptions in this circuit result in characteristic symptoms and electrocardiogram (ECG) changes.
Symptoms from Bundle Branch Block (BBB) and Fascicular Block
Delays in the depolarizing wavefront produces varying degrees of dyssynchrony of contractions, which affects ventricular pump efficiency and impairs cardiac output. Symptoms are caused by these inefficiencies in cardiac output:
- Blood pressure lability (instability)
Any such symptoms will require a neurological and cardiovascular investigation. The cardiovascular diagnostics are designed to pursue the presence of arrhythmias via electrocardiogram and functional assessment via cardiac ultrasonography (echocardiogram).
Since bundle branch blocks are electrical disturbances in the conductive pathway of the heart, the ECG stands as the gold standard of making the diagnosis. The familiar waveform of an ECG is altered, especially in that portion which represents the contraction (depolarization) of the ventricles–the QRS complex. Bundle branch blocks will delay conductivity selectively to one ventricle or the other, and one ventricle will lag behind in its contraction compared to the other:
The normal QRS complex, which is the combined ventricular contraction in normal circumstances, becomes wider (takes longer from beginning to end) when one ventricle takes longer than the other to contract. Since the many ECG views (directions of perspective, or “leads”) give a composite dimensional picture, the “axis” of the heart’s contraction can characterize which direction predominates in an imbalance of the usual axis.
The ECG can also discern areas of ischemia or prior myocardial infarction that could contribute to the BBB.
The cardiac ultrasound uses Doppler technology to ascertain flow through the chambers of the heart, which is affected in bundle branch blocks that cause alterations in cardiac output.
Its B (brightness) mode can evaluate structural integrity, diagnosing valve disease that may be associated with BBB, or even thrombus formation related to the atrial fibrillation that is more frequent in those suffering from BBB.
Congenital abnormalities, a major source of right BBB, can be identified, both as deviations in architecture and in disorders of flow dynamics.