Cardiac Conduction System Disorders

Introduction to Conduction System Disease

• A wide QRS complex (>120 ms) indicates abnormal ventricular activation due to conduction pathway problems, which can signify serious cardiac conditions including ventricular tachycardia, bundle branch blocks, or accessory pathway conduction that may increase mortality risk, according to the American College of Cardiology 1
• Conduction system diseases include atrioventricular (AV) block with impaired conduction in the His-Purkinje system, as stated by the European Heart Journal 2, and bundle branch blocks (QRS >140 ms with RBBB or >160 ms with LBBB pattern), as noted by the American College of Cardiology 1

Abnormal Ventricular Activation

• Ventricular tachycardia (VT) is a type of abnormal ventricular activation originating from ventricular tissue, as described by the American College of Cardiology 1
• Pre-excitation syndromes (WPW) involve accessory pathway conduction bypassing normal AV node delay, according to Circulation 3
• Severe heart failure can cause abnormal ventricular activation, as stated by the American College of Cardiology 1

Diagnostic Criteria for Ventricular Tachycardia

• QRS pattern with negative concordance across precordial leads is a diagnostic criterion for ventricular tachycardia, as noted by the American College of Cardiology 1
• RS interval >100 ms in any precordial lead is a highly specific diagnostic criterion for ventricular tachycardia, as stated by the American College of Cardiology 1
• The following ECG criteria are highly suggestive of VT:
• AV dissociation with ventricular rate faster than atrial rate is a key diagnostic criterion for VT, as recommended by the American College of Cardiology 1
• QRS width criteria suggest VT when QRS >140 ms with right bundle branch block (RBBB) pattern or QRS >160 ms with left bundle branch block (LBBB) pattern 5, 1, 6

Arrhythmia Risk and Management

• In pre-excitation syndromes, a wide QRS complex indicates risk of rapid conduction during atrial fibrillation, according to Circulation 3
• Ignoring clinical context, such as history of myocardial infarction, can increase the likelihood of misdiagnosing ventricular tachycardia, as stated by the American College of Cardiology 1
• When in doubt, treating as ventricular tachycardia is recommended, as noted by the American College of Cardiology 1
• Ventricular tachycardia (VT) is a life-threatening condition that requires immediate intervention, with fusion complexes being pathognomonic for VT and necessitating treatment with DC cardioversion if hemodynamically unstable or antiarrhythmic medications if stable, according to the American College of Cardiology 1
• Immediate DC cardioversion is recommended for unstable patients (those with hypotension, altered mental status, chest pain, or heart failure) 1
• For stable patients, first-line treatment includes IV amiodarone or procainamide, with caution against using calcium channel blockers or adenosine unless specifically indicated 1
• Evaluating for underlying structural heart disease and considering an electrophysiology study for definitive diagnosis and ablation are important steps in the management of VT 1

Diagnostic Differentiation from Other Conditions

• SVT typically has rates of 260-300 bpm in infants, while sinus tachycardia is almost always <230 bpm 7
• VT can have rates of 200-500 bpm with slight variation over several beats 7
• SVT typically has extremely regular R-R intervals after the first 10-20 beats, while VT may have more variation 7
• QRS complex identical to sinus rhythm suggests SVT rather than VT 4
• SVT with pre-existing bundle branch block can be mistaken for VT 4

Common Pitfalls in Diagnosis

• Ignoring clinical context, such as a history of previous myocardial infarction, can lead to misdiagnosis of VT as SVT, highlighting the need for comprehensive patient evaluation 1
• Failure to recognize fusion complexes is a critical mistake, as these are definitive indicators of VT 1
• Inadequate ECG analysis, focusing solely on rate or QRS width without examining for fusion complexes, AV dissociation, and other specific morphological features, can lead to incorrect diagnoses 1
• Overreliance on single criteria can lead to misdiagnosis, as no single criterion is 100% sensitive or specific 4