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For further information, see CMDT Part 10-29: AV Block

Key Features

Essentials of Diagnosis

  • Conduction disturbance between the atrium and ventricle that can be physiologic (due to enhanced vagal tone) or pathologic

  • Block occurs in the AV node (first-degree, second-degree Mobitz type I) or below the AV node (second-degree Mobitz type II, third-degree)

  • Symptomatic AV block or block below the AV node in the absence of a reversible cause usually warrants permanent pacemaker implantation

General Considerations

  • Can be physiologic (due to increased vagal tone) or pathologic (due to underlying heart disease such as ischemia, myocarditis, fibrosis of the conduction system, or after cardiac surgery)

  • There are three categories of AV block:

    • First-degree (PR interval > 200 msec with all atrial impulses conducted)

    • Second-degree (intermittent blocked beats)

    • Third-degree (complete heart block, in which no atrial impulses are conducted to the ventricles)

  • Second-degree AV block is further subclassified into

    • Mobitz type I (Wenckebach), in which the AV conduction time (PR interval) progressively lengthens before the blocked beat

    • Mobitz type II, in which there are intermittently nonconducted atrial beats not preceded by lengthening AV conduction

  • AV dissociation

    • Occurs when an intrinsic ventricular pacemaker is firing at a rate faster than or close to the sinus rate, such that atrial impulses arriving at the AV node when it is refractory may not be conducted

    • This phenomenon does not necessarily indicate AV block

Clinical Findings

First-degree and second-degree Mobitz type I block

  • Rarely produces symptoms

  • Normal, physiologic block of this type

    • Occurs in response to increases in parasympathetic output

    • Is commonly seen during sleep, with carotid sinus massage, or in well-trained athletes

    • May also occur as a medication effect, including

      • Calcium channel blockers

      • β-Blockers

      • Digitalis

      • Antiarrhythmics

  • Pathologic causes that should be ruled out include

    • Myocardial ischemia or infarction

    • Inflammatory processes (ie, Lyme disease)

    • Fibrosis

    • Calcification

    • Infiltration (ie, amyloidosis or sarcoidosis)

Second-degree Mobitz type II block and third-degree complete heart block

  • Almost always due to pathologic disease involving the infranodal conduction system

  • Common symptoms include

    • Fatigue

    • Dyspnea

    • Presyncope or syncope

  • With complete heart block,

    • The ventricular escape rate is usually slow (< 50 beats/min)

    • Severity of symptoms may vary depending on the rate and stability of the escape rhythm

Intraventricular conduction block

  • Relatively common

  • May be transient (ie, related to increases in heart rate) or permanent

  • Right bundle branch block is often seen in patients with structurally normal hearts

  • Left bundle branch block is more often a marker of underlying cardiac disease, including

    • Ischemic heart disease

    • Inflammatory or infiltrative disease

    • Cardiomyopathy

    • Valvular heart disease


  • When only 2:1 AV block is present on the ECG, the differentiation between Mobitz type I or Mobitz type II is ...

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