What is the difference between blocks of the heart, including coronary artery (CA) blockages and electrical conduction blocks, such as atrioventricular (AV) blocks or bundle branch blocks?

Difference Between Blocks of the Heart

"Heart blocks" refer to two fundamentally different pathologies: coronary artery blockages (mechanical obstruction of blood flow) and electrical conduction blocks (disruption of the heart's electrical signaling system), which have distinct mechanisms, presentations, and treatments.

Coronary Artery Blockages vs. Electrical Conduction Blocks

Coronary Artery (CA) Blockages

• Mechanical obstruction of blood flow through coronary arteries due to atherosclerotic plaque, thrombus, or vasospasm 1
• Results in myocardial ischemia or infarction when oxygen supply cannot meet myocardial demand 1
• Presents with chest pain, ST-segment changes on ECG, elevated cardiac biomarkers, and regional wall motion abnormalities 1
• Treatment involves revascularization (thrombolysis, angioplasty, stenting, or bypass surgery) 2

Electrical Conduction Blocks

• Disruption of electrical impulse transmission through the specialized conduction system (AV node, His bundle, bundle branches, or fascicles) 3
• Results in bradycardia, abnormal QRS morphology, or complete dissociation between atrial and ventricular activity 4
• Presents with syncope, presyncope, fatigue, or may be asymptomatic with characteristic ECG patterns 5
• Treatment involves pacemaker implantation for symptomatic or high-risk blocks 6

Types of Electrical Conduction Blocks

Atrioventricular (AV) Blocks

• First-degree AV block: PR interval >200 ms, all impulses conducted 6
• Second-degree AV block: Some atrial impulses fail to conduct to ventricles (Mobitz I or II) 6
• Third-degree (complete) AV block: No atrial impulses reach the ventricles; complete dissociation between atrial and ventricular activity 4

Bundle Branch Blocks

• Right bundle branch block (RBBB): QRS ≥120 ms with rSR' pattern in V1-V2 and wide S waves in leads I and V6 3
• Left bundle branch block (LBBB): QRS ≥120 ms with broad, notched R waves in lateral leads and absent septal Q waves 7

Fascicular Blocks (Hemiblocks)

• Left anterior fascicular block (LAFB): Left axis deviation with QRS <120 ms, larger R waves in leads I and aVL 8
• Left posterior fascicular block (LPFB): Right axis deviation, less common than LAFB 2

Critical Intersection: When Both Occur Together

Conduction Blocks Complicating Acute Myocardial Infarction

The location of coronary occlusion determines the type and prognosis of conduction disturbances:

Inferior MI with AV Block

• AV block occurs above the His bundle in the vast majority of cases 6
• Usually transient (resolves within 7 days), well-tolerated with narrow QRS escape rhythm >40 bpm 6
• Associated with low mortality when isolated 6
• Permanent pacing generally not indicated if block resolves within 14 days 6

Anterior MI with Conduction Block

• AV block occurs below the AV node in the His-Purkinje system 6
• Associated with unstable wide QRS escape rhythm and extremely high mortality (up to 80%) due to extensive myocardial necrosis 6
• Intraventricular conduction disturbances (bundle branch blocks) reflect extensive myocardial damage rather than isolated electrical problems 6
• Permanent pacing indicated for persistent second- or third-degree AV block with bundle branch block after MI 6

High-Risk Combinations

• Left bundle branch block combined with advanced second- or third-degree AV block carries particularly ominous prognosis 6
• Right bundle branch block combined with left anterior or posterior fascicular block (bifascicular block) has increased risk of progression to complete AV block 5, 2
• Mobitz II with bundle branch block and third-degree AV block with wide QRS post-MI have similarly poor prognosis 6

Clinical Pitfalls and Key Distinctions

Evaluation Approach

• Always obtain transthoracic echocardiography in newly detected bundle branch blocks to evaluate for structural heart disease 5, 3
• Assess for symptoms including syncope, presyncope, dizziness, fatigue, or exercise intolerance in all conduction blocks 5
• Monitor for progression: Bifascicular blocks carry increased risk of advancing to complete AV block requiring pacemaker 5, 8

Prognostic Implications

• Isolated fascicular blocks (especially LAFB alone) are generally benign and do not require pacing 8
• Bundle branch blocks in structurally normal hearts typically have benign prognosis 6, 5
• Bundle branch blocks with MI identify high-risk patients even in the thrombolytic era, with mortality remaining elevated 6
• Conduction disturbances post-MI carry poor prognosis primarily due to extent of myocardial injury rather than the conduction block itself 6

Treatment Distinctions

• Coronary blockages require revascularization to restore blood flow 2
• Electrical blocks require pacing only when symptomatic, persistent (>14 days post-MI), or high-risk patterns present 6
• Transient AV block without intraventricular conduction defects does not require permanent pacing 6
• Consider ICD or CRT-D in post-MI patients with LVEF ≤35% who have pacing indications 6