what is the pathology behind heart block and it's types

Reading File

Finding Sources

Reading File

Finding Sources

Reading File

Loading Image

Reading File

Loading Image

Reading File

Loading Image

I now have comprehensive textbook material and ECG images to compose a thorough answer. Here it is:

Heart Block - Pathology and Types

What is Heart Block?

Heart block is a disturbance of impulse conduction - distinct from interference, which is a normal physiologic phenomenon caused by refractoriness after a preceding impulse. Block can occur at multiple levels: between the sinus node and atrium (SA block), between atria and ventricles (AV block), within the atria (intra-atrial), or within the ventricles (intraventricular). The term "heart block" most commonly refers to atrioventricular (AV) block.

AV block exists when an atrial impulse is conducted with delay or is not conducted at all to the ventricle, at a time when the AV junction is not physiologically refractory. The site of block can be in the AV node, the His bundle, or the bundle branches. - Braunwald's Heart Disease, p. 1315

Classification Overview

AV block is classified by severity into three degrees:

1. First-Degree AV Block

Pathophysiology

Every atrial impulse still reaches the ventricles, but conduction is delayed. The PR interval exceeds 0.20 seconds in adults (values as long as 1.0 second have been recorded). The delay can reside in:

The AV node (prolonged A-H interval) - most common
The His-Purkinje system (prolonged H-V interval)
Both sites simultaneously
Rarely, intra-atrial conduction delay

If the QRS is normal in contour, the AV delay almost always sits in the AV node. If the QRS shows a bundle branch block (BBB) pattern, delay may be at the AV node or His-Purkinje system, and a His bundle electrogram is needed to localize it. - Braunwald's Heart Disease, p. 1315

Causes

Idiopathic degeneration of the conduction system (Lenegre's disease, Lev's disease)
Drugs: beta-blockers, calcium channel blockers, digoxin
Enhanced vagal tone (e.g., trained athletes)
Inferior MI (right coronary artery territory, which supplies the AV node)
Electrolyte disturbances (e.g., hypokalemia)
Infections: Lyme disease, endocarditis, myocarditis
Muscular dystrophies (myotonic muscular dystrophy)
Infiltrative disease: amyloidosis, sarcoidosis
Post-ablation procedures - Frameworks for Internal Medicine, p. 47

ECG

Constant PR interval >200 ms; every P wave followed by a QRS.

Prognosis

Generally benign. Some evidence of slightly increased risk of atrial fibrillation and more serious conduction abnormalities over time. Pacemaker is only considered if PR >300 ms with hemodynamic symptoms.

2. Second-Degree AV Block

Some (but not all) atrial impulses reach the ventricles. There are two subtypes:

2a. Mobitz Type I (Wenckebach Block)

ECG

Progressive lengthening of the PR interval with each successive beat until a P wave is completely blocked (dropped QRS). After the dropped beat, the PR interval resets to its shortest value and the cycle repeats. The RR interval progressively shortens before the dropped beat. - Frameworks for Internal Medicine, p. 48

Originally described by Wenckebach using jugular venous tracings (A-C prolongation leading to dropped beats).

Pathophysiology

Block location: within the AV node (suprahisian)
The AV node progressively fatigues with each impulse until it completely fails to conduct, then recovers
Associated with inferior MI (RCA territory, which supplies the AV node in ~90% of people) - look for ST elevation in leads II, III, aVF

Symptoms

Usually asymptomatic. Dyspnea and light-headedness are rare.

Prognosis

Generally benign; rarely progresses to complete block. Vagal-mediated forms are physiologic (athletes, during sleep).

2b. Mobitz Type II Block

ECG

Conducted beats with a constant PR interval followed by a sudden dropped QRS - no preceding PR prolongation. The QRS is often wide because block is below the His bundle. - Frameworks for Internal Medicine, p. 48

Pathophysiology

Block location: below the AV node, within the His-Purkinje system or bundle branches (infrahisian)
Associated with anterior MI (LAD territory, which supplies the His-Purkinje system and bundle branches) - look for ST elevation in V1-V4
Because block is infrahisian, it carries a much worse prognosis than Mobitz I

Symptoms

Frequently symptomatic: dyspnea, palpitations, light-headedness, syncope.

Prognosis

High rate of progression to complete (third-degree) AV block and increased mortality. Permanent pacemaker is typically indicated.

2c. High-Grade (Advanced) AV Block

When two or more consecutive P waves are blocked. The term distinguishes intermittent severe block from complete block, as some ventricular conduction still occurs. - Braunwald's Heart Disease, p. 1317

2d. 2:1 AV Block

Every other P wave is blocked - a special case of second-degree block where Mobitz I vs. II cannot be distinguished from the ECG pattern alone (consecutive PR intervals are not available for comparison). His bundle electrogram is needed to localize.

3. Third-Degree (Complete) AV Block

Definition

No atrial impulses reach the ventricles. The atria and ventricles beat completely independently - AV dissociation. P-P intervals are regular; R-R intervals are regular; but there is no relationship between P waves and QRS complexes. - Braunwald's Heart Disease, p. 1317

Pathophysiology

Block can be at three levels:

Level	Block site	QRS	Escape rate	Notes
AV nodal	Above His bundle	Narrow (normal)	40-60 bpm	Usually congenital; atropine increases rate
Intrahisian	Within His bundle	Can be narrow or wide	40-60 bpm	Rarely identified without invasive studies
Infrahisian	Below His bundle (bundle branches)	Wide (abnormal)	<40 bpm	Usually acquired; worse prognosis

In acquired complete block (most common in adults), block is distal to the His bundle due to trifascicular conduction disease. Each P wave is followed by a His deflection, but ventricular escape complexes are NOT preceded by a His deflection. - Braunwald's Heart Disease, p. 1317

Complete AV block - His bundle electrogram patterns

Above: His bundle electrograms in complete AV block, showing the A (atrial), H (His), and V (ventricular) deflections with long intervals (1380-1390 ms escape intervals visible) and eventually complete ventricular standstill (6.8 second pause in panel C). - Braunwald's Heart Disease

Causes of Complete AV Block

Degenerative fibrosis of the conduction system (idiopathic - most common in elderly)
Ischemic heart disease (inferior MI: AV nodal block; anterior MI: infrahisian block)
Infiltrative disease: amyloidosis, sarcoidosis, hemochromatosis
Infectious: Chagas disease, Lyme disease, rheumatic fever
Surgical trauma: post-cardiac surgery (especially congenital heart disease repair)
Congenital (associated with maternal anti-Ro/SSA antibodies in neonates; also with L-loop TGA, AV septal defects)
Drugs: digoxin toxicity, beta-blockers, calcium channel blockers, amiodarone
Tumors, polymyositis, myxedema, calcific aortic stenosis
Hereditary: SCN5A gene mutations (same gene responsible for LQT3) - Braunwald's Heart Disease, p. 1317-1318

Symptoms

Syncope (Stokes-Adams attacks), near-syncope, fatigue, dyspnea, heart failure symptoms.

Treatment

Permanent pacemaker implantation is indicated.

Summary Table

Type	PR Interval	Dropped QRS?	Location of Block	Associated MI Territory	Prognosis
1st degree	>200 ms, constant	None	AV node (usually)	Inferior (mild)	Benign
2nd degree - Mobitz I	Progressively lengthens	Yes (periodic)	AV node	Inferior (RCA)	Usually benign
2nd degree - Mobitz II	Constant, then sudden drop	Yes (sudden)	His-Purkinje	Anterior (LAD)	High risk - pacemaker
High-grade	Variable	2+ consecutive P waves blocked	Variable	Variable	Serious
3rd degree (complete)	AV dissociation	All P waves blocked	AV node / His / infrahisian	Both	Emergency - pacemaker

Key Pathological Mechanisms Underlying Heart Block

Fibrosis / degeneration of the conduction system (Lenegre's disease = primary conduction fibrosis; Lev's disease = calcification spreading from aortic/mitral valves)
Ischemia/infarction: AV node is supplied by the RCA (occlusion causes nodal block); His-Purkinje system is supplied by the septal perforators of the LAD (occlusion causes infrahisian block)
Infiltration: amyloid, sarcoid granulomas directly disrupt conduction tissue
Inflammation: Lyme carditis can cause all degrees of AV block; autoimmune (anti-Ro antibodies in congenital block)
Drug effects: Drugs that slow AV nodal conduction (beta-blockers, CCBs, digoxin, adenosine)
Genetic channelopathies: SCN5A mutations impair sodium channel function in conduction tissue
Phase 3 / Phase 4 block: Rate-dependent block due to incomplete repolarization recovery or spontaneous diastolic depolarization in diseased tissue - Braunwald's Heart Disease, p. 1318

Sources: Braunwald's Heart Disease (10th ed.), Frameworks for Internal Medicine, Goldman-Cecil Medicine

This is a shared conversation. Sign in to Orris to start your own chat.