Atrioventricular Blocks

Definition, Etiology, PathogenesisTop

Atrioventricular (AV) block is an impairment or a blockade of conduction from the atria to the ventricles.

Types of AV block:

1) First-degree AV block: Every atrial impulse is conducted to the ventricles but the conduction time is prolonged >200 milliseconds.

2) Second-degree AV block: Some impulses are not conducted to the ventricles.

3) Third-degree (complete) AV block: No atrial impulses are conducted to the ventricles. The atria and ventricles are controlled by independent pacemakers with a ventricular escape rhythm that is slower than atrial rhythm.

Classification of AV blocks on the basis of location:

1) Proximal: At the level of the AV node.

2) Distal: Below the AV node.

First-degree AV block may result from abnormal conduction in the atrium, AV node, or (rarely) in the His bundle and Purkinje fibers. Mobitz type I (Wenckebach) second-degree AV block is almost always located within the AV node, while Mobitz type II second-degree block or advanced heart block is located below the node. Third-degree AV blocks may be proximal (located in the AV node) or distal.

Cardiac conduction system: see Figure 1 in Disorders of Automaticity and Conduction.

Causes: Congenital blocks, myocardial infarction (MI) or ischemia, degenerative processes of the conduction system (Lenègre disease, Lev disease), cardiomyopathy, myocarditis, complications of surgery or intravascular procedures, heart tumor, systemic disease (particularly sarcoidosis and connective tissue disease), drugs (beta-blockers, verapamil and diltiazem, digitalis, class I antiarrhythmic drugs [see Table 1 in Cardiac Arrhythmias], amiodarone), hypothyroidism, autonomic dysfunction, hyperkalemia, ablation of the AV junction. First-degree AV block and Mobitz type I second-degree AV block may also be caused by increased vagal tone, which is often observed in athletes and occasionally also in healthy individuals at night.

Clinical Features and Natural HistoryTop

AV blocks may be transient (eg, following acute MI), paroxysmal, or permanent. Signs and symptoms: see Disorders of Automaticity and Conduction. In third-degree AV block physical examination may reveal variable intensity of the first heart sound. In proximal third-degree AV block heart rates are 40 to 60 beats/min and increase during exercise; in distal third-degree AV blocks heart rates are usually lower (typically 20-40 beats/min).

DiagnosisTop

Diagnostic Criteria

Electrocardiography (ECG):

1) In patients with first-degree AV block, the PR interval is >200 milliseconds (see Figure 3 in Disorders of Automaticity and Conduction).

2) In patients with Mobitz type I (Wenckebach) second-degree AV block, there is progressive PR interval prolongation for several beats preceding the nonconducted P wave. In the following P-QRS complex the PR interval is shorter (normal or near-normal) than the PR interval before the block. This occurs periodically, creating the appearance of “grouped beating” (see Figure 3 in Disorders of Automaticity and Conduction). In patients with Mobitz type II second-degree AV block, the block occurs without the prior progressive PR interval increase and the following PR interval remains unchanged. In patients with advanced second-degree AV block, the block is sustained for ≥2 cycles (ie, 2 consecutive P waves with no QRS). A special form of second-degree AV block is the 2:1 block, which may be Mobitz type I or type II block; if the PR interval of the conducted P wave is prolonged and the QRS complex is narrow and has normal morphology, Mobitz type I second-degree AV block is more likely. In 70% of cases second-degree AV block with a narrow QRS complex is proximal in the conducting system, while 80% of cases of second-degree AV block with a wide QRS complex are distal and associated with a high risk of progression to third-degree AV block.

3) In patients with third-degree AV block, P waves and QRS complexes are independent and ventricular rate is slower than atrial rate. In proximal block the origin of escape rhythm is located above the His bundle bifurcation, QRS complexes are narrow, and their rate is 40 to 60/min (see Figure 3 in Disorders of Automaticity and Conduction). In a distal block QRS complexes are wide, may be polymorphic, and their rate is 20-40/min; rhythm is less stable and episodes of torsades de pointes may occur if there are concomitant VPBs, due to bradycardia-induced QT prolongation.

TreatmentTop

1. Management of symptomatic bradycardia: see Figure 2 in Disorders of Automaticity and Conduction.

2. Chronic first-degree AV block and Mobitz type I second-degree AV block usually require no treatment. Attempt to discontinue drugs that increase the AV conduction time. Assess for chronotropic response (appropriate heart rate increase with exercise). If episodes happen mostly at nighttime, consider testing for sleep apnea. Repeated follow-up is recommended.

3. Indications for pacemaker implantation:

1) Persistent bradycardia: Third-degree AV block or Mobitz type II second-degree AV block, regardless of symptoms.

2) Periodic or paroxysmal third-degree or second-degree AV block (including AF with slow or regular conduction to ventricles).

3) The recommendation is weaker in Mobitz type I second-degree AV block that is symptomatic or has been located below the His bundle using electrophysiologic study. The recommended first-choice pacing mode is DDD, or VVI in patients with AF (in AF there is no need for atrial sensing or pacing). Before making the decision to implant a pacemaker, make sure that the AV block is not caused by a transient or reversible cause, such as myocardial infarction, electrolyte disturbances, drugs, perioperative hypothermia, or myocarditis.