Electrical Conduction Blocks in the Heart

Introduction to Conduction Blocks

The American Heart Association notes that disruption of electrical impulse transmission through the specialized conduction system (AV node, His bundle, bundle branches, or fascicles) can result in bradycardia, abnormal QRS morphology, or complete dissociation between atrial and ventricular activity 1
The American College of Cardiology recommends that patients with electrical conduction blocks be evaluated for symptoms, including syncope, presyncope, dizziness, fatigue, or exercise intolerance 2

First-degree AV block is characterized by a PR interval >200 ms, with all impulses conducted, according to the American Heart Association 3
Second-degree AV block is defined as some atrial impulses failing to conduct to ventricles (Mobitz I or II), as noted by the American College of Cardiology 3
Right bundle branch block (RBBB) is identified by a QRS ≥120 ms with rSR' pattern in V1-V2 and wide S waves in leads I and V6, as described by the European Society of Cardiology 1
Left anterior fascicular block (LAFB) is characterized by left axis deviation with QRS <120 ms, larger R waves in leads I and aVL, according to the American Heart Association 5

The European Society of Cardiology states that the location of coronary occlusion determines the type and prognosis of conduction disturbances, with AV block occurring above the His bundle in the vast majority of cases 6, 7
The American College of Cardiology recommends that patients with inferior MI and AV block be monitored for transient block, which usually resolves within 7 days, and is well-tolerated with narrow QRS escape rhythm >40 bpm 6, 7
Anterior MI with conduction block is associated with unstable wide QRS escape rhythm and extremely high mortality (up to 80%) due to extensive myocardial necrosis, as noted by the European Society of Cardiology 6, 7

The American Heart Association recommends that patients with newly detected bundle branch blocks undergo transthoracic echocardiography to evaluate for structural heart disease 2, 1
The American College of Cardiology states that isolated fascicular blocks (especially LAFB alone) are generally benign and do not require pacing 5
Bundle branch blocks in structurally normal hearts typically have a benign prognosis, according to the European Society of Cardiology 8, 2

The American College of Cardiology recommends that coronary blockages require revascularization to restore blood flow, while electrical blocks require pacing only when symptomatic, persistent (>14 days post-MI), or high-risk patterns present 3, 4
The American Heart Association notes that transient AV block without intraventricular conduction defects does not require permanent pacing 3, 4
Consideration of ICD or CRT-D is recommended in post-MI patients with LVEF ≤35% who have pacing indications, as stated by the American College of Cardiology 3