Mobitz type I (Wenckebach) and Mobitz type II AV block represent the two principal clinical subcategories of second-degree atrioventricular (AV) block, distinguished by distinct anatomical sites of electrical conduction failure between the atria and the ventricles 1,2. Second-degree AV block occurs when atrial electrical impulses are intermittently prevented from conducting to the ventricles, resulting in dropped ventricular beats 1,3. Mobitz type I (Wenckebach) AV block is generally characterized by a benign, reversible functional delay localized within the AV node 1,1.5. Conversely, Mobitz type II AV block involves structural pathology within the distal His-Purkinje conduction system, carrying a high risk of sudden progression to complete heart block and hemodynamic collapse 1,1.1,1.3. Precise differentiation via electrocardiography is critical for guiding therapeutic interventions, emergency resuscitation, and permanent device selection 1,1.3.

Etiology & Risk Factors

  • Mobitz Type I (Wenckebach) Etiologies: Often functional or transient, frequently resulting from high vagal tone in healthy young adults or conditioned athletes during rest 1,1.5. Pathological causes include inferior wall myocardial infarction (affecting the right coronary artery and the nodal branch), digitalis toxicity, beta-blockers (e.g., metoprolol), non-dihydropyridine calcium channel blockers (e.g., diltiazem, verapamil), amiodarone, acute myocarditis (such as Lyme carditis), hyperkalemia, and post-cardiac ablation or surgical trauma near the triangle of Koch 1,1.5.
  • Mobitz Type II Etiologies: Almost invariably associated with structural organic heart disease affecting the infra-nodal His-Purkinje conduction axis 1,1.1,1.3. Common etiologies include anterior myocardial infarction (anterior STEMI due to proximal left anterior descending artery occlusion causing extensive septal necrosis), idiopathic fibrosis and sclerosis of the conduction system (Lenegre or Lev disease), infiltrative cardiomyopathies (amyloidosis, sarcoidosis, hemochronmatosis, scleroderma), muscular dystrophies (e.g., myotonic dystrophy), and calcific aortic stenosis extending into the fibrous skeleton of the heart 1,1.1,1.4.
  • General Risk Factors: Advanced age, underlying coronary artery disease, structural heart disease, chronic hypertension, and exposure to polypharmacy involving multiple negative dromotropic agents 1,1.1,1.4.

Pathophysiology

The pathophysiological mechanisms underlying second-degree AV blocks dictate their distinct electrophysiological behaviors and clinical trajectories 1,1.3. In Mobitz type I (Wenckebach), the conduction delay occurs at the level of the AV node 1,1.3,1.5. AV nodal cells```html

Mobitz type I (Wenckebach) and Mobitz type II AV block are distinct forms of second-degree atrioventricular (AV) block defined by the intermittent failure of electrical impulses to conduct from the atria to the ventricles 1. While Mobitz Type I is typically a benign, functional phenomenon localized to the AV node, Mobitz Type II reflects structural infragian conduction disease associated with a high propensity for progression to complete heart block and Stokes-Adams syncope 1.

Etiology & Risk Factors

  • Mobitz Type I (Wenckebach): Enhanced vagal tone (athletes, sleep), inferior wall myocardial infarction (affecting the right coronary artery supplying the AV node), acute myocarditis, digitalis toxicity, hyperkalemia, and medications that slow AV nodal conduction (beta-blockers, verapamil, diltiazem) 1.
  • Mobitz Type II: Idiopathic fibrosis of the conduction system (Lenègre or Lev disease), structural heart disease, anterior wall myocardial infarction (reflecting extensive damage to the bundle of His and bundle branches), infiltrative cardiomyopathies (sarcoidosis, amyloidosis), and post-cardiac surgery 1.

Pathophysiology

In Mobitz Type I block, decremental conduction occurs within the AV node. Each successive atrial impulse encounters increasingly refractory nodal tissue, leading to progressive prolongation of the PR interval until an impulse fails entirely to conduct, producing a dropped QRS complex. Following the dropped beat, the AV node resets, and the cycle repeats. In contrast, Mobitz Type II block involves an all-or-none conduction failure localized within the bundle of His or the bundle branches. The PR interval of conducted beats remains constant, and the block occurs abruptly without prior warning or progressive PR elongation 1.

Mobitz type I (Wenckebach) and Mobitz type II AV block — Pathophysiological Cascade & Clinical Flowchart 1 Conduction Impairment Structural or functional blockade within the AV node (Mobitz I) or His-Purkinje system (Mobitz II). 2 Electrical Delay or Failure Progressive decrement in nodal cell action potential upstroke (Mobitz I) or abrupt conduction failure in diseased bundle branches (Mobitz II). 3 Intermittent AV Dissociation Periodic failure of atrial impulses to depolarize the ventricles, resulting in dropped ventricular beats. 4 Electrocardiographic Signatures Progressive PR lengthening with R-R shortening (Mobitz I) or fixed PR with sudden non-conducted P waves (Mobitz II). 5 Hemodynamic Compromise & Pacing Prevention of fatal asystole, syncope, and cardiac arrest through definitive pacemaker therapy.

Clinical Presentation & Classic Clinical Signs

  • History & Cardinal Symptoms: Many patients with Mobitz Type I are completely asymptomatic. When symptomatic—or in Mobitz Type II, which is rarely asymptomatic—patients report fatigue, dizziness, presyncope, syncopal episodes, and signs of heart failure resulting from decreased cardiac output during pauses.
  • Physical Examination & Named Clinical Signs: Irregularly irregular or regularly irregular bradycardia depending on the block ratio. Jugular venous pulsations may reveal intermittent cannon 'a' waves if an atrial contraction coincides with a closed tricuspid valve during the blocked beat. First heart sound (S_1) intensity may vary in Mobitz Type I as the PR interval changes.

Diagnostic Workup & Interpretation (Laboratory, Imaging & ECG/Pathology)

Diagnostic Modality / Test Expected Clinical & Radiological / Laboratory Finding
Gold-Standard Investigation12-lead ECG demonstrating characteristic PR interval and QRS drop patterns
Laboratory PanelBasic metabolic panel (evaluating potassium and renal function), cardiac biomarkers (troponin to rule out acute myocardial infarction), and thyroid-stimulating hormone (TSH)
Imaging / Radiology (X-ray, US, CT, MRI)Transthoracic echocardiogram to assess structural heart disease, left ventricular ejection fraction, and wall motion abnormalities
ECG / Histopathology / ProceduralMobitz I: Progressive PR lengthening with a grouped beating pattern and narrow QRS. Mobitz II: Constant PR interval with sudden dropped QRS complexes and typically wide QRS intervals

Staging, Classification & Clinical Scoring Systems

Criterion / Stage / Score Component Clinical Threshold, Points & Management Implication
Mobitz Type I (Wenckebach)Benign anatomical level (AV node); usually reversible if secondary to drugs, ischemia, or increased vagal tone; observation sufficient if asymptomatic
Mobitz Type IIHigh-risk anatomical level (Infranodal/His-Purkinje); unpredictable progression to complete heart block; absolute indication for permanent pacemaker implantation

Differential Diagnosis

Differential Condition Distinguishing Clinical, Biochemical & Imaging Features
First-Degree AV BlockProlonged PR interval greater than 200 ms, but every P wave is successfully conducted to the ventricles
Third-Degree (Complete) Heart BlockComplete dissociation between atrial and ventricular activity with independent P wave and QRS rates
Blocked Premature Atrial Contractions (PACs)Early P wave occurring prematurely that fails to conduct due to a refractory AV node, mimicking a dropped second-degree block
Sinus Arrest / Sinoatrial Exit BlockFailure of sinus node impulse generation or exit, resulting in dropped P waves and QRS complexes simultaneously

Management & Treatment Protocol

  • Initial Stabilization & Resuscitation: Assess hemodynamic stability. Apply continuous cardiac monitoring, secure intravenous access, and prepare for emergent temporary transcutaneous pacing if the patient is unstable (hypotensive, altered mental status, signs of shock).
  • First-Line Medical / Pharmacological Therapy (with exact adult dosing): For symptomatic bradycardia due to Mobitz Type I, administer Atropine 0.5 mg to 1.0 mg IV push, repeated every 3 to 5 minutes to a maximum total dose of 3 mg. Note that atropine is frequently ineffective in Mobitz Type II because the block is infranodal. If atropine fails, initiate an epinephrine infusion (2 mcg/min to 10 mcg/min titration) or dopamine infusion (5 mcg/kg/min to 20 mcg/kg/min).
  • Surgical / Interventional Indications: Permanent pacemaker insertion is indicated for symptomatic Mobitz Type II block, high-grade AV block, or asymptomatic Mobitz Type II block in the setting of wide QRS complexes or neuromuscular disorders regardless of symptoms.

Complications & Prognosis

  • Key acute complications include hemodynamic collapse, ventricular arrhythmias, and sudden cardiac death resulting from progression to complete heart block 1,3.
  • Prognosis for Mobitz Type I is generally favorable when transient and reversible, whereas Mobitz Type II carries a guarded prognosis without permanent pacing due to high mortality rates from unpredictable progression to asystole 1,3.