Intraventricular Conduction

Complications and hemodynamic consequences

Disorders of impulse conduction within the ventricles can lead to serious clinical consequences. Pronounced intraventricular blocks cause asynchronous myocardial contraction, resulting in hemodynamic disturbances. In some cases, this serves as a direct indication for cardiac resynchronization therapy (CRT).

Key threats and complications include:

• Risk of developing ventricular tachyarrhythmias (detected by late potentials marker on signal-averaged ECG).
• Progression of trifascicular block to complete atrioventricular (AV) block.
• Adams-Stokes attacks manifested by syncope due to severe bradycardia or asystole in complete AV block.

If bundle branch block occurs acutely, it is considered a marker of exacerbation of the underlying cardiac disease. This condition requires priority treatment of the underlying heart pathology.

Indications for ECG diagnostics

12-lead ECG registration is necessary to detect conduction disorders in a wide range of patients. Basic diagnostics are indicated when bradycardia is suspected, with subsequent progression to stress testing or ambulatory monitoring if the resting ECG is uninformative.

Mandatory assessment of rhythm, morphology, and QRS duration is performed in the following cases:

1. Chronic heart failure — to assess AV and ventricular conduction, scar changes, and hypertrophy.
2. Dilated cardiomyopathy — to diagnose sinoatrial, atrioventricular, and ventricular blocks, as well as to determine indications for CRT or pacemaker implantation.
3. Infective endocarditis — for monitoring rhythm disturbances.
4. Suspected active myocarditis — prolongation of the P-Q interval and intraventricular blocks are sometimes the only objective evidence of myocardial involvement.
5. Sarcoidosis, abdominal aortic aneurysm, hemorrhagic fever with renal syndrome (HFRS) (with auscultatory changes) — to exclude concomitant conduction pathologies.

Of particular clinical significance is a newly diagnosed (or presumed newly diagnosed) complete left bundle branch block (LBBB) in the setting of myocardial infarction. It is equivalent to a diagnostically significant ST-segment elevation and serves as an ECG criterion for deciding on thrombolytic therapy.

Myocardial scintigraphy as an adjunctive method

When the ECG lacks a convincing picture of focal changes due to intraventricular conduction disorders (e.g., in bundle branch blocks), imaging becomes the method of choice in case of diagnostic difficulties.

• Agent used: technetium radioisotope ($^{99m}\text{Tc}$).
• Time parameters: visualization of the focus is possible 6–12 hours after the onset of anginal status (pain attack).
• Persistence of changes: the positive picture persists for 6–14 days.

Contraindications in conduction disorders

The presence of certain degrees of heart block is, in itself, a strict contraindication or limitation for a number of medical interventions and drug prescriptions.

Antiarrhythmic drugs (such as phenytoin, amiodarone, and class IC agents) are contraindicated in:

• second- and third-degree SA and AV blocks;
• severe intraventricular conduction disturbances (QRS duration >140 ms);
• sinus bradycardia (in the absence of an implanted pacemaker);
• QT interval >500 ms (for amiodarone).

Caution is required in left ventricular (LV) hypertrophy greater than 1.4 cm and pre-existing bundle branch blocks. A QRS widening of more than 25% from baseline serves as the criterion for drug discontinuation.

Exercise ECG testing is contraindicated in complete LBBB, WPW syndrome, and paced rhythm. It is important to note that intraventricular conduction abnormalities, LV hypertrophy, atrial fibrillation, electrolyte imbalance, and the use of cardiac glycosides can yield false-positive test results.

Physiotherapy exercise (e.g., for concomitant coxarthrosis) is contraindicated in third-degree AV block, unstable angina, uncontrolled arrhythmia, and recent ischemic ECG changes.

Permanent cardiac pacing is not recommended if the AV block resolves spontaneously following myocardial infarction. Transient sinus node dysfunction and AV block are common in the acute phase of infarction (especially inferior wall MI) and are associated with nodal ischemia or increased vagal tone.

Class and exam questions

What else to study on this topic

• Markers of late potentials on signal-averaged ECG
• Indications for cardiac resynchronization therapy (CRT) in heart failure
• Diagnostic algorithm for suspected bradycardia with a non-diagnostic resting ECG
• Causes of false-positive results in exercise ECG testing
• Features of transient sinus node dysfunction in inferior wall myocardial infarction

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.