What is the emergency department work‑up for a patient with third‑degree (complete) atrioventricular block?

Emergency Department Work-Up for Third-Degree (Complete) Heart Block

Immediately assess hemodynamic stability and prepare for transcutaneous pacing while simultaneously obtaining a 12-lead ECG, establishing IV access, and evaluating for reversible causes—this is a cardiovascular emergency that can rapidly progress to asystole and requires urgent intervention. 1, 2

Immediate Stabilization and Monitoring

• Place the patient on continuous cardiac monitoring with pulse oximetry and frequent blood pressure measurements to detect hemodynamic deterioration 3, 2
• Establish IV access immediately and prepare transcutaneous pacing pads on the patient while completing the initial assessment 3, 2
• Assess for signs of poor perfusion including altered mental status, syncope, presyncope, chest pain, acute heart failure, hypotension, or shock—these indicate need for immediate intervention 3, 1
• Obtain a 12-lead ECG to confirm third-degree AV block, determine QRS morphology (narrow vs. wide), and identify acute MI or ischemia 3, 1, 2

Critical ECG Analysis

• Determine the anatomic level of block by examining the escape rhythm: narrow QRS (40-60 bpm) suggests AV nodal block with junctional escape, while wide QRS (20-40 bpm) indicates infranodal (His-Purkinje) block with ventricular escape 1, 2
• Infranodal blocks are high-risk and may progress rapidly to asystole with unreliable escape rhythms, requiring more urgent pacing 1, 4
• Look for evidence of acute MI, particularly anterior MI which typically causes infranodal block with poor prognosis, versus inferior MI which usually causes AV nodal block that may respond to atropine 4

Essential Laboratory and Imaging Work-Up

• Complete blood count (CBC) to evaluate for anemia or infection 2
• Comprehensive metabolic panel including electrolytes (potassium, calcium, magnesium) to identify reversible metabolic causes 2, 5
• Cardiac troponin to evaluate for acute coronary syndrome 2
• Thyroid function tests (TSH, free T4) as hypothyroidism and hyperthyroidism can cause AV block 1, 5
• Chest radiograph to assess for heart failure, cardiomegaly, or pulmonary pathology 2
• Lyme serology (IgG/IgM with western blot confirmation) especially in younger patients without structural heart disease or in endemic areas, as Lyme carditis is a reversible cause 1, 5

Evaluation for Reversible Causes

Rule out the following reversible etiologies before proceeding to permanent pacing: 1, 5

• Acute myocardial infarction (check troponin, ECG for STEMI)
• Drug toxicity (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics)
• Electrolyte abnormalities (hyperkalemia, hypomagnesemia)
• Lyme disease (particularly in younger patients)
• Myocarditis (consider viral serologies, inflammatory markers)
• Infiltrative diseases (sarcoidosis, amyloidosis—may require echocardiography)
• Hypothyroidism or hyperthyroidism

Pharmacologic Management

For symptomatic AV nodal-level block (narrow QRS escape rhythm): 3, 1

• Atropine 0.5 mg IV bolus, repeat every 3-5 minutes to maximum total dose of 3 mg 3
• Do NOT use doses <0.5 mg as this may paradoxically worsen block via central vagal stimulation 3, 6
• Atropine is ineffective for infranodal blocks (wide QRS escape) and should not delay pacing 3, 1

If atropine fails or for infranodal block: 1

• Beta-adrenergic agonists (dopamine, epinephrine, or isoproterenol infusion) may be used as a temporizing measure when ischemia risk is low 1

Pacing Strategy

For hemodynamically unstable patients or those not responding to atropine: 3, 1, 2

• Initiate transcutaneous pacing immediately as a bridge to transvenous pacing 3, 1
• Prepare for temporary transvenous pacing in symptomatic patients or those with high-risk features 1, 2
• Use externalized permanent active-fixation leads if prolonged temporary pacing is anticipated 1

Consultation and Disposition

• Emergent cardiology consultation for all patients with third-degree AV block 2
• Interventional cardiology consultation for consideration of temporary transvenous pacemaker placement and permanent pacemaker implantation 1, 2
• Admit to intensive care unit with continuous telemetry monitoring until definitive pacing is established 2

High-Risk Features Requiring Urgent Intervention

Do NOT discharge patients with any of the following, even if asymptomatic: 1

• Escape rate <40 bpm
• Ventricular (wide QRS) escape rhythm
• Documented asystolic pauses ≥3 seconds
• Third-degree AV block with atrial fibrillation and pauses ≥5 seconds

Critical Pitfalls to Avoid

• Never delay transcutaneous pacing for atropine administration in hemodynamically unstable patients 3
• Do not use atropine for infranodal (wide QRS) blocks—it is ineffective and wastes critical time 3, 1
• Do not assume reversibility without testing—88% of patients still require permanent pacing even after treating reversible causes 1
• In acute MI with anterior wall involvement, atropine may worsen ischemia and is contraindicated for infranodal block 3
• Avoid discharging younger patients without structural heart disease until Lyme carditis is excluded, as this is a reversible cause 5