What is the best course of treatment for an adult patient with a history of cardiovascular disease presenting with a heart block as evidenced by an electrocardiogram (ECG)?

Heart Block on ECG: Treatment Algorithm

For an adult with cardiovascular disease presenting with heart block on ECG, immediate treatment depends on hemodynamic stability: unstable patients require atropine 0.5 mg IV (repeated every 3-5 minutes to maximum 3 mg) followed by transcutaneous pacing if unresponsive, while stable patients need urgent risk stratification based on the degree and location of block to determine need for permanent pacing. 1

Immediate Assessment and Stabilization

Step 1: Assess Hemodynamic Stability

Determine if the patient has signs of poor perfusion directly caused by bradycardia 1:

• Acute altered mental status
• Ischemic chest discomfort
• Acute heart failure
• Hypotension or other signs of shock
• Increased work of breathing (tachypnea, retractions, paradoxical abdominal breathing) 1

Step 2: Initial Management for ALL Patients

• Maintain patent airway and assist breathing as necessary 1
• Oxygen if hypoxemic 1
• Cardiac monitor to identify rhythm 1
• IV access 1
• Obtain 12-lead ECG immediately (but don't delay therapy) 1
• Monitor blood pressure and pulse oximetry continuously 1

Treatment Based on Stability

For UNSTABLE Patients (Symptomatic Bradycardia)

Atropine remains the first-line drug 1:

• Dose: 0.5 mg IV every 3-5 minutes 1
• Maximum total dose: 3 mg 1
• Critical warning: Doses <0.5 mg may paradoxically worsen bradycardia 1

Important caveats about atropine 1, 2:

• Use cautiously in acute coronary ischemia/MI (increased heart rate may worsen ischemia) 1
• Will be ineffective in cardiac transplant patients (lack vagal innervation) 1
• May cause AV block and nodal rhythm with large doses 2

If atropine fails or is inappropriate 1:

• Transcutaneous pacing (Class IIa) - initiate immediately 1
• IV dopamine or epinephrine infusion for rate acceleration 1
• Transvenous pacing if drugs and transcutaneous pacing fail 1

For STABLE Patients: Risk Stratification by Block Type

First-Degree AV Block (PR >0.20 seconds)

• Generally benign but not always 1, 3
• 40.5% of patients with first-degree AV block progress to higher-grade block requiring pacemaker 3
• Consider ambulatory ECG monitoring if symptoms suggest intermittent higher-grade block 1
• Permanent pacing reasonable if PR >240 ms with clearly attributable symptoms 1

Second-Degree AV Block - Mobitz Type I (Wenckebach)

• Block at AV node level 1
• Often transient and asymptomatic 1
• Permanent pacing reasonable only if symptoms clearly attributable to the block 1

Second-Degree AV Block - Mobitz Type II

• High-risk: Block below AV node in His-Purkinje system 1
• Potential to progress to complete heart block 1
• Asymptomatic Mobitz II requires permanent pacing (Class I) 1
• Especially dangerous in acute MI setting - requires immediate intervention 1

Third-Degree (Complete) Heart Block

• No impulses pass between atria and ventricles 1, 4
• Cardiovascular emergency requiring prompt recognition 4
• Ventricular rate depends on escape rhythm location 4:
  • AV node/high His-Purkinje: narrow QRS, 40-60 bpm 4
  • Ventricular escape: wide QRS, 20-40 bpm 4
• Permanent pacing indicated (Class I) if associated with 1:
  • Symptomatic bradycardia
  • Congestive heart failure
  • Asystole or escape rate <40 bpm
  • Confusional states clearing with temporary pacing

Bifascicular/Trifascicular Block

• Bifascicular block with intermittent complete heart block and symptoms: permanent pacing required (Class I) 1
• Alternating bundle branch block: immediate permanent pacing required (Class I) 1
• Syncope with HV interval ≥70 ms or infranodal block at EPS: permanent pacing indicated (Class I) 1
• Annual progression rate to complete heart block: 4% when associated with heart disease 5

Special Populations Requiring Permanent Pacing

Neuromuscular diseases 1:

• Kearns-Sayre syndrome with any conduction disorder: permanent pacing reasonable (Class IIa) - high risk of sudden death 1
• Lamin A/C mutations with PR >240 ms and LBBB: permanent pacing reasonable (Class IIa) 1
• Myotonic dystrophy type 1 with PR >240 ms or QRS >120 ms: permanent pacing may be considered (Class IIb) 1

Post-Myocardial Infarction Considerations

Temporary pacing during acute MI does NOT automatically indicate need for permanent pacing 1:

• Prognosis related to extent of myocardial injury, not the AV block itself 1
• Permanent pacing indicated if 1:
  • Persistent advanced block at AV node (Class II)
  • Complete heart block with complications listed above (Class I)

Critical pitfall: Bifascicular block prior to acute MI carries 65% one-year mortality 5

Diagnostic Workup for Stable Patients

Mandatory initial testing 1, 6:

• Transthoracic echocardiography to exclude structural heart disease (Class I) 1, 6
• Extended ambulatory ECG monitoring (24-48 hours minimum) to detect intermittent higher-grade block 1, 6

Consider electrophysiology study (EPS) 1:

• Symptoms suggestive of intermittent bradycardia with conduction disease on ECG (Class IIa) 1
• Syncope with bundle branch block to evaluate HV interval 1

Advanced imaging if echocardiogram unrevealing 1, 6:

• Cardiac MRI, CT, or nuclear studies (Class IIa) 1, 6

Disposition

• Unstable patients: ICU admission with cardiology consultation 4
• Stable patients with high-risk features: cardiology referral for pacemaker evaluation 1, 6
• All newly detected LBBB: mandatory cardiology referral regardless of symptoms 6