Atrioventricular (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 atrioventricular (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 considered 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

DIAGNOSIS

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