Place in conduction disorders structure
Cardiac conduction disorders include various types of pathologies, among which bundle branch blocks and atrioventricular blocks occupy a special place. In clinical practice, such changes are frequently recorded against the background of myocardial inflammatory processes (e.g., in myocarditis), along with sinus tachycardia and other types of arrhythmias.
The presence of such disorders is regarded by physicians as an important caution criterion. Specifically, they are included among exclusion or heightened attention criteria along with marked left ventricular hypertrophy.
Electrocardiographic features of the QRS complex
Ventricular electrical activity undergoes changes in conduction system pathology. Specifically, in non-reentrant (automatic) junctional tachycardia (also referred to as focal junctional tachycardia), the QRS complex may remain narrow or acquire a morphology typical of bundle branch block.
Focal tachycardia itself originates in the AV node or bundle of His, developing due to mechanisms of abnormal automaticity or triggered activity. Heart rate in this condition typically ranges from 110 to 250 beats per minute.
Diagnostic search and risk assessment
Patients with a normal PR interval and 1:1 conduction require an in-depth evaluation. The diagnostic workup necessarily includes:
- Assessment of the underlying heart disease.
- Analysis of clinical symptoms.
- Detailed analysis of the baseline electrocardiogram to identify left anterior hemiblock, nonspecific intraventricular conduction delay, or isolated fascicular disorders.
The prognosis directly depends on comorbid conditions capable of accelerating disease progression, such as muscular dystrophy.
Alternating bundle branch block
Alternating bundle branch block poses a particular danger. On the electrocardiogram, it manifests as a constant alternation of QRS complexes exhibiting left bundle branch block (LBBB) and right bundle branch block (RBBB) morphology.
This phenomenon indicates severe damage to the conduction system. The primary clinical risk lies in the high probability of a sudden transition to complete heart block accompanied by a slow escape rhythm or cardiac arrest (asystole).
Class and exam questions
Where does focal junctional tachycardia originate and what are its ECG features?
Tachycardia originates in the AV node or bundle of His. It is characterized by a heart rate ranging from 110 to 250 beats per minute, a short RP' interval (during 1:1 conduction) or AV dissociation, and QRS complexes may be narrow or appear as typical bundle branch block.
Why is alternating bundle branch block considered dangerous?
Alternating QRS complex morphology (with features of both left and right bundle branch block) indicates profound damage to the conduction apparatus. This carries a high risk of sudden development of complete heart block with asystole or an extremely slow escape rhythm.
What mechanisms underlie focal junctional tachycardia?
The primary mechanisms are abnormal automaticity as well as triggered activity, which is initiated under the influence of beta-adrenergic stimulation and calcium channel blockade.
What else to study on this topic
- Pathophysiology and mechanisms of focal junctional tachycardia
- The role of concealed junctional extrasystoles in the development of AV blocks
- Data from intracardiac electrophysiological study (EPS)
- Caution criteria and ECG effects in myocarditis
- Differential diagnosis of fascicular blocks and conduction delays
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