Heart Block

Table of Contents

  1. Overview
  2. Types and Classification
  3. Causes and Triggers
  4. Symptoms
  5. ECG and Diagnostic Findings
  6. Treatment
  7. Natural and Lifestyle Approaches
  8. Complications and Prognosis
  9. When to Seek Emergency Care
  10. Key Research Papers
  11. Connections
  12. Featured Videos

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1. Overview

Heart block — also called atrioventricular (AV) block — is a disorder of the cardiac conduction system in which the electrical impulse generated by the sinoatrial (SA) node in the right atrium is delayed or blocked from reaching the ventricles through the AV node and His-Purkinje system. The result is a slowing or dissociation of atrial and ventricular electrical activity, with varying clinical consequences ranging from asymptomatic to life-threatening hemodynamic collapse.

Under normal conditions, the electrical impulse travels from the SA node → atria → AV node → Bundle of His → left and right bundle branches → Purkinje fibers → ventricular myocardium, producing a normal PR interval of 120–200 ms on the surface ECG. A PR interval exceeding 200 ms (one large box on standard ECG paper) defines first-degree AV block. More advanced block produces dropped beats or complete AV dissociation.

Heart block is classified by severity — first, second (Mobitz type I or II), and third degree — each with distinct ECG patterns, clinical significance, and management implications. Understanding these distinctions is essential because:

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2. Types and Classification

First-Degree AV Block

Defined as a PR interval >200 ms (one large box, or 5 small boxes, on standard 25 mm/sec ECG paper) on every beat, with all P waves conducted to the ventricles. The PR interval prolongation reflects delayed conduction through the AV node, but every atrial impulse eventually reaches the ventricles. No beats are "dropped."

First-degree AV block is not truly a "block" in the sense of any impulse failing to conduct — it is a conduction delay. The PR interval may be markedly prolonged (300–400 ms or more) without symptoms. It is seen in trained athletes (vagal tone), with age-related fibrosis, and in many systemic conditions affecting the AV node.

Second-Degree AV Block — Mobitz Type I (Wenckebach)

Classic Wenckebach pattern (named for Dutch physician Karel Frederick Wenckebach who described it in 1906): progressive PR interval prolongation with each successive beat until one P wave fails to conduct (dropped QRS). After the dropped beat, the cycle resets and PR interval shortens back toward baseline before the progressive prolongation begins again.

Key ECG features of Mobitz I:

Anatomical location: AV nodal block. The AV node is richly innervated by the vagus nerve and responds to changes in autonomic tone. Wenckebach is often benign, physiologic, and reversible.

Second-Degree AV Block — Mobitz Type II

Named after Woldemar Mobitz who described the two types in 1924. In Mobitz type II, P waves are conducted with a constant, unchanging PR interval until one P wave suddenly fails to conduct (dropped QRS) without any preceding PR prolongation. The block occurs below the AV node — in the His bundle or bundle branches.

Key ECG features of Mobitz II:

Anatomical location: Infranodal block (His bundle or bundle branches). This is the dangerous distinction: infranodal block is inherently unstable, unpredictably progresses to complete heart block, and the escape rhythm (from ventricular tissue below the block) is slow (20–40 bpm), wide-complex, and unreliable. (PMID: 6850024)

2:1 AV Block — The Diagnostic Dilemma

When every other P wave is blocked (2:1 conduction), it is mathematically impossible to distinguish Mobitz I from Mobitz II on ECG alone — because you cannot observe PR interval behavior with only one conducted beat per cycle. Clues to distinguish:

Third-Degree (Complete) AV Block

Complete absence of AV conduction — no atrial impulses reach the ventricles. The atria and ventricles beat independently (AV dissociation). The ventricles are maintained by an escape pacemaker located below the site of block:

ECG in complete heart block: Regular P waves at the atrial rate (e.g., 80 bpm); regular QRS complexes at the escape rate (e.g., 40 bpm); P waves and QRS complexes bear no relationship to each other (dissociated, "marching through" independently). No PR interval relationship is maintained. (PMID: 7790957)

Lenegre-Lev Disease

The most common cause of isolated progressive cardiac conduction disease (PCCD) in adults without structural heart disease is Lenegre-Lev disease — idiopathic fibrosis and sclerosis of the cardiac conduction system. Lenegre disease (Jean Lenegre, 1964) describes primary sclerodegeneration of the bundle branches; Lev disease (Maurice Lev, 1964) describes fibrosis encroaching on the conduction system from the cardiac fibrous skeleton (aging, calcific aortic stenosis, mitral annular calcification). Clinically, the two processes are indistinguishable, and the eponym is often combined. Both lead to progressive bundle branch block progressing to complete heart block over years to decades. SCN5A gene variants (sodium channel mutations) cause familial forms of progressive conduction disease. (PMID: 10888438)

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3. Causes and Triggers

Structural and Degenerative Causes

Infectious Causes

Autoimmune Causes

Drug-Induced Heart Block

Metabolic and Electrolyte Causes

Infiltrative Causes

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4. Symptoms

First-Degree AV Block

Typically asymptomatic. Occasionally, patients with very prolonged PR intervals (>300 ms) may experience symptoms from the timing mismatch between atrial and ventricular contractions ("pacemaker syndrome" physiology): reduced cardiac output, fatigue, or reduced exercise tolerance. This is termed "AV dyssynchrony" and is rare.

Second-Degree Mobitz I (Wenckebach)

Usually asymptomatic or minimally symptomatic, particularly in the setting of lower resting heart rates. Some patients note:

Second-Degree Mobitz II

More likely to be symptomatic than Mobitz I due to the unpredictable nature of dropped beats and the infranodal site of block:

Third-Degree (Complete) Heart Block

Symptoms depend critically on the rate and reliability of the escape rhythm:

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5. ECG and Diagnostic Findings

Surface ECG — The Primary Diagnostic Tool

The 12-lead ECG is central to diagnosing and classifying heart block. Key measurements:

ECG Pattern by Type

Holter Monitoring and Event Recorders

Paroxysmal or intermittent heart block — particularly relevant for unexplained syncope — may not be captured on a resting 12-lead ECG. Ambulatory monitoring options:

Electrophysiology Study (EPS)

Invasive catheter recording of His bundle electrograms (H-V interval) precisely localizes the block site:

Additional Investigations

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6. Treatment

First-Degree AV Block

No treatment required in asymptomatic patients. Address reversible causes (drug review, electrolyte correction). Counseling and reassurance. Follow-up ECG monitoring for progression. Athletes with first-degree block and vagal physiology require no treatment; block may resolve with detraining.

Second-Degree Mobitz I (Wenckebach)

Management is generally expectant for asymptomatic patients with Wenckebach at the AV node level:

Second-Degree Mobitz II

Mobitz II requires pacemaker implantation in virtually all cases regardless of symptoms, because of the high risk of unpredictable progression to complete heart block:

Complete (Third-Degree) Heart Block

Complete heart block requires pacemaker therapy. Acute management:

Pacemaker Types

Reversible Causes — Treat the Underlying Disease

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7. Natural and Lifestyle Approaches

Important Caveat

Heart block requiring pacemaker therapy is a structural electrical problem that cannot be reversed by lifestyle or supplementation alone. Natural approaches serve as adjuncts — supporting overall heart health, addressing modifiable risk factors, and supporting recovery — not as alternatives to pacemaker implantation when it is indicated.

Electrolyte Support

Avoiding Drug and Supplement Interactions

Patients with any degree of heart block should be aware that many over-the-counter medications and supplements can worsen conduction:

Exercise and Activity

Exercise guidance depends on the type and degree of block:

Vagal Maneuver Awareness

Patients with AV nodal block (first-degree, Wenckebach) should be aware that activities that increase vagal tone transiently worsen AV nodal conduction: Valsalva maneuver, breath-holding, bearing down during bowel movements (avoid constipation), and carotid sinus pressure. These maneuvers do not affect infranodal block.

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8. Complications and Prognosis

Progression of Block

The most critical prognostic concern is progression to higher-degree block, particularly from Mobitz II to complete heart block. This progression can be sudden (within minutes during acute MI), subacute (over days in infective endocarditis), or gradual (over years in Lenegre-Lev disease). Infranodal disease detected on EPS (HV interval >70 ms or block below the His bundle with provocative testing) identifies patients at high risk for progression. (PMID: 6850024)

Sudden Cardiac Death

The primary fatal complication of unrecognized or untreated high-degree AV block is sudden cardiac death from asystole or ventricular fibrillation during a Stokes-Adams attack. In previously healthy patients with complete heart block, death may occur before medical evaluation if the escape rhythm is absent or unreliable.

Pacemaker-Induced Cardiomyopathy

Right ventricular apex pacing (the most common pacing site) produces dyssynchronous ventricular contraction — the paced ventricle activates from the apex rather than through the normal His-Purkinje network — effectively creating an LBBB pattern. This dyssynchrony causes LV dysfunction over years in approximately 20–30% of patients who require frequent RV pacing. His bundle pacing and left bundle branch pacing are emerging strategies that preserve synchronous ventricular activation. (PMID: 23265516)

Pacemaker Complications

Prognosis

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9. When to Seek Emergency Care

Call emergency services immediately for any of the following:

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10. Key Research Papers

The following peer-reviewed studies represent key evidence in heart block research and management:

PubMed searches for further reading:

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Connections

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