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Emergency and Acute Medicine – Atrioventricular Blocks
Overview
Atrioventricular (AV) blocks represent impaired electrical conduction between the atria and ventricles at the level of the AV node or the His–Purkinje system. They are classified by severity and physiologic impact. First-degree AV block involves delayed AV nodal conduction without loss of ventricular beats and is generally benign, occurring in about 1.6% of healthy adults. Second-degree AV block is defined by intermittent failure of atrial impulses to conduct to the ventricles. Mobitz type I (Wenckebach) typically reflects AV nodal disease with progressive PR interval prolongation until a dropped QRS complex and is usually benign, though it may accompany inferior wall myocardial infarction. Mobitz type II reflects infranodal disease with fixed PR intervals and sudden dropped beats, carries a high risk of progression to complete heart block, and has a worse prognosis, particularly with acute myocardial infarction. Third-degree AV block, or complete heart block, is characterized by total AV dissociation with independent atrial and ventricular rhythms and is never benign. Symptoms are more severe when the block is distal in the conduction system. Toxin-induced complete block may resolve once the offending agent is removed.
Etiology And Risk Factors
AV block results from structural conduction system disease, increased refractory periods, or marked shortening of supraventricular cycle length. Inferior wall myocardial infarction commonly causes first-degree and Mobitz type I blocks that are often transient and benign, whereas anterior wall infarction is associated with Mobitz type II block and increased mortality due to ventricular arrhythmias and heart failure. Chronic coronary artery disease may cause fibrosis around the AV node. Drug-related causes include digoxin, beta-blockers, calcium channel blockers, amiodarone, procainamide, class IC antiarrhythmics, and clonidine. Other causes include congenital AV block, valvular disease, surgical trauma following coronary bypass or valve surgery, increased vagal tone, infectious etiologies such as Lyme disease, myocarditis, endocarditis, rheumatic fever, tuberculosis, syphilis, diphtheria, Chagas disease, and toxoplasmosis. Infiltrative and systemic diseases include sarcoidosis, amyloidosis, hemochromatosis, cardiomyopathy, collagen vascular disease, myxedema, hypothermia, and electrolyte disturbances, particularly hyperkalemia.
Pediatric Considerations
AV block occurs in children and is often asymptomatic, though neonatal mortality is highest. Causes include congenital heart disease, maternally acquired antibodies, and infectious myocarditis or rheumatic fever. Toxic ingestions must always be considered in children presenting with new conduction abnormalities.
Clinical Presentation
First-degree AV block is usually asymptomatic. Mobitz type I block may present with pulse irregularity. Mobitz type II and complete heart block commonly cause exercise intolerance, palpitations, chest pain, presyncope or syncope, dyspnea, orthopnea, and altered mental status. Physical findings range from no abnormalities in first-degree block to irregular pulse, hypotension, mental status changes, cyanosis, jugular venous distention, and pulmonary rales in high-grade blocks.
Diagnostic Evaluation
A 12-lead ECG is essential to classify the block and assess for ischemia. First-degree AV block shows a PR interval greater than 0.20 seconds. Mobitz type I demonstrates progressive PR prolongation with cyclic dropped QRS complexes, usually with a narrow QRS. Mobitz type II shows constant PR intervals with intermittent dropped ventricular beats and may have a wide QRS if infranodal. Third-degree block shows independent atrial and ventricular rhythms with fixed PP and RR intervals and variable PR intervals. QRS width depends on the level of the escape rhythm.
Adjunctive Testing
Laboratory studies should target reversible causes and include electrolytes, calcium, magnesium, cardiac enzymes—particularly in Mobitz type II and complete heart block—and digoxin levels when applicable. Chest radiography may reveal cardiomyopathy or heart failure. Echocardiography can identify structural or valvular disease.
Differential Diagnosis
Consider accelerated junctional rhythm, idioventricular rhythm, sinus bradycardia, and sinoatrial block.
Prehospital Management
Unstable patients with Mobitz type II or complete heart block require immediate transcutaneous pacing. Atropine should be avoided in Mobitz type II block and is contraindicated in complete heart block with a wide QRS, as it may worsen conduction. Measures should be taken to avoid increased vagal tone.
Initial Stabilization
Patients with hypoperfusion manifested by hypotension, chest pain, dyspnea, or altered mental status require immediate transcutaneous pacing. Atropine may be used for symptomatic sinus bradycardia or complete heart block with a narrow QRS complex.
Emergency Department Management
First-degree AV block requires no acute treatment; AV nodal blocking agents should be avoided, and evaluation for myocardial infarction, electrolyte abnormalities, or medication toxicity should be performed when indicated. Mobitz type I block usually needs no treatment; atropine may be given if symptomatic. Mobitz type II block requires temporary pacing, either transcutaneous or transvenous, and atropine should not be used. Complete heart block mandates emergent pacing; atropine may provide transient benefit only in narrow-QRS escape rhythms. If toxin-mediated, targeted therapy is required, including digoxin-specific antibodies for digoxin toxicity and glucagon or calcium for beta-blocker or calcium channel blocker overdose.
Medications
Atropine may be given at 0.5–1.0 mg IV every 5 minutes as needed, with pediatric dosing of 0.01–0.03 mg/kg. Digoxin-specific antibody dosing depends on serum level and body weight, with 10 vials often used empirically for severe toxicity. Glucagon is administered at 5–10 mg IV, and calcium chloride at 250–500 mg IV, with weight-based pediatric dosing.
Disposition And Follow-Up
Patients with Mobitz type II or complete heart block require admission to a monitored setting. Asymptomatic first-degree and Mobitz type I block patients may be discharged with outpatient cardiology follow-up.
Clinical Safety Insights
Prompt ECG acquisition in symptomatic patients is critical. Once high-grade AV block is identified, pacing must not be delayed. A thorough history, including medication and toxin exposure, is essential to identify reversible causes. Common errors include misinterpreting ECGs, failing to recognize high-grade block, delaying pacing, and not arranging timely cardiology consultation for permanent pacemaker evaluation.
Overview
Atrioventricular (AV) blocks represent impaired electrical conduction between the atria and ventricles at the level of the AV node or the His–Purkinje system. They are classified by severity and physiologic impact. First-degree AV block involves delayed AV nodal conduction without loss of ventricular beats and is generally benign, occurring in about 1.6% of healthy adults. Second-degree AV block is defined by intermittent failure of atrial impulses to conduct to the ventricles. Mobitz type I (Wenckebach) typically reflects AV nodal disease with progressive PR interval prolongation until a dropped QRS complex and is usually benign, though it may accompany inferior wall myocardial infarction. Mobitz type II reflects infranodal disease with fixed PR intervals and sudden dropped beats, carries a high risk of progression to complete heart block, and has a worse prognosis, particularly with acute myocardial infarction. Third-degree AV block, or complete heart block, is characterized by total AV dissociation with independent atrial and ventricular rhythms and is never benign. Symptoms are more severe when the block is distal in the conduction system. Toxin-induced complete block may resolve once the offending agent is removed.
Etiology And Risk Factors
AV block results from structural conduction system disease, increased refractory periods, or marked shortening of supraventricular cycle length. Inferior wall myocardial infarction commonly causes first-degree and Mobitz type I blocks that are often transient and benign, whereas anterior wall infarction is associated with Mobitz type II block and increased mortality due to ventricular arrhythmias and heart failure. Chronic coronary artery disease may cause fibrosis around the AV node. Drug-related causes include digoxin, beta-blockers, calcium channel blockers, amiodarone, procainamide, class IC antiarrhythmics, and clonidine. Other causes include congenital AV block, valvular disease, surgical trauma following coronary bypass or valve surgery, increased vagal tone, infectious etiologies such as Lyme disease, myocarditis, endocarditis, rheumatic fever, tuberculosis, syphilis, diphtheria, Chagas disease, and toxoplasmosis. Infiltrative and systemic diseases include sarcoidosis, amyloidosis, hemochromatosis, cardiomyopathy, collagen vascular disease, myxedema, hypothermia, and electrolyte disturbances, particularly hyperkalemia.
Pediatric Considerations
AV block occurs in children and is often asymptomatic, though neonatal mortality is highest. Causes include congenital heart disease, maternally acquired antibodies, and infectious myocarditis or rheumatic fever. Toxic ingestions must always be considered in children presenting with new conduction abnormalities.
Clinical Presentation
First-degree AV block is usually asymptomatic. Mobitz type I block may present with pulse irregularity. Mobitz type II and complete heart block commonly cause exercise intolerance, palpitations, chest pain, presyncope or syncope, dyspnea, orthopnea, and altered mental status. Physical findings range from no abnormalities in first-degree block to irregular pulse, hypotension, mental status changes, cyanosis, jugular venous distention, and pulmonary rales in high-grade blocks.
Diagnostic Evaluation
A 12-lead ECG is essential to classify the block and assess for ischemia. First-degree AV block shows a PR interval greater than 0.20 seconds. Mobitz type I demonstrates progressive PR prolongation with cyclic dropped QRS complexes, usually with a narrow QRS. Mobitz type II shows constant PR intervals with intermittent dropped ventricular beats and may have a wide QRS if infranodal. Third-degree block shows independent atrial and ventricular rhythms with fixed PP and RR intervals and variable PR intervals. QRS width depends on the level of the escape rhythm.
Adjunctive Testing
Laboratory studies should target reversible causes and include electrolytes, calcium, magnesium, cardiac enzymes—particularly in Mobitz type II and complete heart block—and digoxin levels when applicable. Chest radiography may reveal cardiomyopathy or heart failure. Echocardiography can identify structural or valvular disease.
Differential Diagnosis
Consider accelerated junctional rhythm, idioventricular rhythm, sinus bradycardia, and sinoatrial block.
Prehospital Management
Unstable patients with Mobitz type II or complete heart block require immediate transcutaneous pacing. Atropine should be avoided in Mobitz type II block and is contraindicated in complete heart block with a wide QRS, as it may worsen conduction. Measures should be taken to avoid increased vagal tone.
Initial Stabilization
Patients with hypoperfusion manifested by hypotension, chest pain, dyspnea, or altered mental status require immediate transcutaneous pacing. Atropine may be used for symptomatic sinus bradycardia or complete heart block with a narrow QRS complex.
Emergency Department Management
First-degree AV block requires no acute treatment; AV nodal blocking agents should be avoided, and evaluation for myocardial infarction, electrolyte abnormalities, or medication toxicity should be performed when indicated. Mobitz type I block usually needs no treatment; atropine may be given if symptomatic. Mobitz type II block requires temporary pacing, either transcutaneous or transvenous, and atropine should not be used. Complete heart block mandates emergent pacing; atropine may provide transient benefit only in narrow-QRS escape rhythms. If toxin-mediated, targeted therapy is required, including digoxin-specific antibodies for digoxin toxicity and glucagon or calcium for beta-blocker or calcium channel blocker overdose.
Medications
Atropine may be given at 0.5–1.0 mg IV every 5 minutes as needed, with pediatric dosing of 0.01–0.03 mg/kg. Digoxin-specific antibody dosing depends on serum level and body weight, with 10 vials often used empirically for severe toxicity. Glucagon is administered at 5–10 mg IV, and calcium chloride at 250–500 mg IV, with weight-based pediatric dosing.
Disposition And Follow-Up
Patients with Mobitz type II or complete heart block require admission to a monitored setting. Asymptomatic first-degree and Mobitz type I block patients may be discharged with outpatient cardiology follow-up.
Clinical Safety Insights
Prompt ECG acquisition in symptomatic patients is critical. Once high-grade AV block is identified, pacing must not be delayed. A thorough history, including medication and toxin exposure, is essential to identify reversible causes. Common errors include misinterpreting ECGs, failing to recognize high-grade block, delaying pacing, and not arranging timely cardiology consultation for permanent pacemaker evaluation.
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