What is the management for a patient with new onset third-degree (complete) heart block and bradycardia (heart rate 39)?

Management of New Onset Third-Degree Heart Block with Heart Rate 39

For new onset third-degree (complete) heart block with bradycardia at 39 bpm, immediately prepare for transcutaneous pacing (TCP) or transvenous pacing as the definitive treatment, while avoiding reliance on atropine since third-degree AV block—especially with infranodal block—is unlikely to respond to vagolytic therapy. 1

Immediate Assessment and Stabilization

Rapidly assess hemodynamic stability by evaluating for signs of poor perfusion including: 1

• Acute altered mental status
• Ischemic chest discomfort
• Acute heart failure
• Hypotension (systolic BP <80-90 mmHg)
• Signs of shock

Initiate basic supportive measures immediately: 1

• Maintain patent airway and assist breathing as necessary
• Administer supplemental oxygen if hypoxemic
• Establish IV access
• Attach cardiac monitor and pulse oximetry
• Obtain 12-lead ECG to assess QRS width and identify underlying rhythm

Critical Decision Point: QRS Width Determines Block Location

Examine the QRS complex width on the ECG—this is crucial for prognosis and treatment: 1, 2

• Wide QRS complex (>120 ms): Indicates infranodal block (His-Purkinje system), which carries high mortality risk and will NOT respond to atropine 1, 2
• Narrow QRS complex: Suggests AV nodal level block, though still requires pacing in most cases 2

Pharmacologic Therapy: Limited Role

Atropine should be avoided or used with extreme caution in third-degree AV block: 1

• The 2010 AHA Guidelines explicitly state to "avoid relying on atropine in type II second-degree or third-degree AV block or in patients with third-degree AV block with a new wide-QRS complex" 1
• Atropine is ineffective for infranodal blocks because these are not mediated by vagal tone 1
• If attempted in hemodynamically unstable patients while preparing for pacing, use 0.5 mg IV every 3-5 minutes (maximum 3 mg total), but do not delay pacing 1
• Atropine may paradoxically worsen the block in some cases 1

Alternative pharmacologic bridge therapy (only while preparing for pacing): 1, 3

• Beta-adrenergic support (dopamine or epinephrine infusion) may be considered as temporizing measure 1, 3
• These are NOT definitive treatments—only bridges to pacing 3

Definitive Treatment: Pacing

Transcutaneous pacing (TCP) should be initiated immediately for symptomatic patients: 1

• TCP is the preferred initial intervention for hemodynamically unstable third-degree AV block 1
• TCP is painful in conscious patients and should be considered a temporizing measure only 1
• Prepare for transvenous pacing as the next step 1

Permanent pacemaker implantation is a Class I indication: 1, 2

• The 2018 ACC/AHA/HRS Guidelines state that "in patients with acquired second-degree Mobitz type II atrioventricular block, high-grade atrioventricular block, or third-degree atrioventricular block not attributable to reversible or physiologic causes, permanent pacing is recommended regardless of symptoms" 1
• Non-randomized studies demonstrate permanent pacing improves survival, particularly in patients with syncope 2

Identify and Treat Reversible Causes

While stabilizing, evaluate for potentially reversible etiologies: 1, 4

• Acute myocardial infarction (present in 55.5% of AVB cases) 4, 5
• Medication toxicity (beta-blockers, calcium channel blockers, digoxin)
• Electrolyte disturbances (hyperkalemia, hypomagnesemia)
• Myocarditis or infiltrative diseases 4
• Lyme disease (in endemic areas)

Obtain laboratory studies: 6

• Complete blood count
• Comprehensive metabolic panel including magnesium
• Cardiac troponin
• Thyroid function tests if clinically indicated

Common Pitfalls to Avoid

Do not rely on atropine as primary therapy for third-degree AV block, especially with wide QRS—this is a critical error that delays definitive treatment 1

Do not delay pacing for pharmacologic trials in hemodynamically unstable patients—atropine administration should not delay TCP implementation 1

Do not assume the block is reversible even if caused by acute MI—approximately 8% of post-MI patients develop complete heart block, and many require permanent pacing 6, 4

Disposition and Follow-up

Immediate cardiology consultation for transvenous pacemaker placement and consideration of permanent pacemaker 1, 6

Admit to intensive care unit with continuous cardiac monitoring 6

If ischemic etiology, coordinate with interventional cardiology for potential percutaneous coronary intervention, though pacing takes priority over revascularization in unstable patients 4, 3