What is the emergency management for a patient with third-degree atrioventricular (AV) block on ECG?

Emergency Management of Third-Degree Atrioventricular Block

Initiate transcutaneous pacing immediately for any hemodynamically unstable patient with third-degree AV block, without waiting for atropine response, as this is the definitive emergency intervention that can prevent cardiac arrest. 1

Immediate Stabilization (First 5 Minutes)

Initial Assessment

• Apply transcutaneous pacing pads while completing your assessment—do not delay this step. 1
• Assess hemodynamic stability by checking for syncope, presyncope, hypotension (systolic BP <90 mmHg), altered mental status, chest pain, acute heart failure, or signs of shock. 1, 2
• Obtain a 12-lead ECG immediately to confirm third-degree AV block and determine QRS width (narrow vs. wide), as this distinguishes AV-nodal from infranodal block and determines whether atropine will work. 1
• Establish IV access and place patient on continuous cardiac monitoring with pulse oximetry. 1

Determine Block Location by QRS Morphology

• Narrow QRS escape rhythm (<120 ms): Block is at the AV node level—may respond to atropine. 3, 1
• Wide QRS escape rhythm (≥120 ms): Block is infranodal (His-Purkinje system)—will NOT respond to atropine and requires immediate pacing. 3, 1

Pharmacologic Management (Only for AV-Nodal Block)

Atropine Protocol

• For narrow-QRS escape rhythms only: Give atropine 0.5–1 mg IV bolus, repeat every 3–5 minutes up to a total maximum dose of 3 mg. 1, 4
• Critical pitfall: Never give doses <0.5 mg, as this may paradoxically worsen bradycardia through central vagal stimulation. 1, 4
• Do NOT use atropine for wide-QRS escape rhythms—it is completely ineffective for infranodal blocks and wastes critical time. 3, 1

When Atropine Fails or Is Contraindicated

• If atropine fails in AV-nodal block AND coronary ischemia is unlikely, consider β-adrenergic agonists (dopamine 5–10 µg/kg/min, dobutamine, epinephrine, or isoproterenol) only as a temporary bridge while arranging pacing. 1
• In acute inferior MI with AV-nodal block, IV aminophylline may be considered as adjunctive therapy. 1
• These medications are NOT primary therapy—they only buy time until pacing is established. 1

Pacing Strategy (Definitive Emergency Treatment)

Transcutaneous Pacing

• Start immediately for any of the following: 1
  • Hemodynamically unstable patient (regardless of QRS width)
  • Wide-QRS escape rhythm (infranodal block)
  • No response to atropine after 1.5–2 mg
  • Escape rate <40 bpm
  • Pauses ≥3 seconds
• Do NOT postpone transcutaneous pacing to give atropine in unstable patients—this is a critical error. 1

Transvenous Pacing

• Arrange temporary transvenous pacing for patients who remain symptomatic or unstable after transcutaneous pacing, serving as a bridge to permanent pacemaker. 1
• For anticipated prolonged temporary pacing, use an externalized permanent active-fixation lead rather than standard passive-fixation temporary leads. 1

Rule Out Reversible Causes

Before committing to permanent pacing, systematically exclude: 1, 5

• Acute myocardial infarction (check troponin, ECG for STEMI)
• Drug toxicity: β-blockers, calcium-channel blockers, digoxin, antiarrhythmics
• Electrolyte abnormalities: hyperkalemia, hypomagnesemia
• Lyme carditis (check history, serology if endemic area)
• Myocarditis or infiltrative disease (sarcoidosis, amyloidosis)
• Thyroid disorders (TSH, free T4)

Important: Even after correcting reversible causes, 88% of patients with third-degree AV block still require permanent pacemaker implantation. 2

Disposition and Definitive Management

Indications for Permanent Pacemaker (Class I)

• All symptomatic patients with third-degree AV block require permanent pacemaker implantation. 3, 1
• Asymptomatic patients with any of the following high-risk features: 3, 1
  • Documented asystole ≥3.0 seconds
  • Escape rate <40 bpm
  • Escape rhythm below the AV node (wide QRS)
  • Atrial fibrillation with pauses ≥5 seconds

Critical Pitfall

• Never discharge an asymptomatic patient with third-degree AV block who has escape rate <40 bpm, ventricular escape rhythm, or pauses ≥3 seconds without pacemaker placement—this carries high risk of sudden death. 1, 2

Immediate Consultation

• Contact interventional cardiology or electrophysiology immediately for all patients with third-degree AV block. 1
• Admit to intensive care unit with continuous telemetry monitoring until permanent pacemaker is placed. 1

Special Considerations

Post-Myocardial Infarction

• Temporary pacing is indicated for medically refractory, symptomatic bradycardia. 1
• Observe for resolution before deciding on permanent pacing, as block may be transient. 1
• If block persists beyond the observation period, permanent pacing is required. 1

Infranodal vs. AV-Nodal Block Prognosis

• Infranodal blocks (wide QRS) can progress rapidly and unpredictably to asystole—require continuous monitoring and urgent pacing regardless of symptoms. 3, 1, 2
• AV-nodal blocks (narrow QRS) have more stable junctional escape rhythms but still require close monitoring and cardiology evaluation. 3, 1