Bundle branch block on ECG

Conduction blocks in myocarditis

In inflammatory diseases of the heart muscle (myocarditis), the electrocardiographic picture often includes various rhythm and conduction disorders. Against the background of sinus tachycardia, supraventricular and ventricular arrhythmias, bundle branch blocks as well as atrioventricular (AV) blocks of varying degrees are regularly recorded. An additional sign is a general decrease in the amplitude (voltage) of ECG waves.

The presence of bundle branch block, along with marked left ventricular hypertrophy (greater than 1.4 cm), is an important criterion for caution. The physician must carefully monitor the patient's condition in the presence of concomitant sinoatrial (SA) or AV conduction disorders, as this can lead to severe complications: grade II–III SA and AV blocks, as well as clinically significant sinus bradycardia.

Focal junctional tachycardia

Automatic or non-reentrant junctional tachycardia can originate directly from the bundle of His trunk or from the AV node. A unique feature of this arrhythmia is that neither the atria nor the ventricles are required for its initiation and maintenance.

Key ECG characteristics and pathophysiological aspects:

• QRS complex: may remain narrow or acquire the morphology of a typical bundle branch block.
• RP’ interval: shortened (with stable 1:1 conduction) or complete AV dissociation is recorded.
• Regularity: the rhythm is predominantly regular, but occasionally may mimic atrial fibrillation due to irregular intervals.
• Mechanisms of development: abnormal automaticity or triggered activity (often provoked by beta-adrenergic stimulation and calcium channel blockers).
• Concealed extrasystoles: isolated junctional extrasystoles not conducted to the ventricles can cause intermittent refractoriness of the AV node and episodes of AV block.

Clinical management and diagnostic workup

If a patient presents with 1:1 conduction and a normal PR interval, an integrated approach is required. First, a thorough assessment of symptoms and advanced diagnostics of the underlying heart disease are performed.

The baseline electrocardiogram is subject to retrospective analysis to identify hidden disorders. The specialist must specifically look for signs of nonspecific intraventricular delay, left anterior hemiblock (LAHB), isolated fascicular block, or a combination of these abnormalities.

Alternating block and prognosis

Of particular clinical significance is alternating (alternans) bundle branch block. On the ECG, it manifests as QRS complexes that sequentially change their morphology from left bundle branch block (LBBB) to right bundle branch block (RBBB) patterns.

Such dynamics indicate severe and widespread disease of the conduction system. The prognosis in such cases is serious: there is an extremely high probability of the sudden development of complete heart block. This condition may be accompanied by a very slow escape rhythm or the onset of life-threatening asystole. Subsequent medical management strictly depends on the presence of symptoms in the patient and comorbid conditions that promote the progression of conduction disorders (e.g., muscular dystrophy).

Class and exam questions

What else to study on this topic

• Criteria for diagnosing nonspecific intraventricular conduction delay on the ECG.
• Pathogenesis of triggered activity in intraventricular conduction disorders.
• Differential diagnosis of irregular junctional tachycardia and atrial fibrillation.
• The effect of muscular dystrophies on the progression of heart blocks.
• Methodology for recording the bundle of His potential during intracardiac EPS.

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