What medications are used to treat a 3rd degree heart block?

Treatment of Third-Degree Heart Block

Immediate Pharmacologic Management

Atropine is the first-line medication for acute symptomatic third-degree AV block when the block is at the AV nodal level, but it is likely ineffective and should not be relied upon for infranodal (His-Purkinje) blocks with wide QRS escape rhythms. 1

Atropine Administration

• Dosing: Administer 0.5-1.0 mg IV bolus every 3-5 minutes to a maximum total dose of 3 mg 1, 2
• Mechanism: Reverses cholinergic-mediated decreases in heart rate by blocking vagal tone at the AV node 1, 3
• When effective: Third-degree block at the AV nodal level (narrow QRS escape rhythm, typically 40-60 bpm) 1
• When ineffective: Infranodal blocks (wide QRS escape rhythm, typically 20-40 bpm) where the block is in the His-Purkinje system 1, 2

Critical Atropine Caveats

• Doses <0.5 mg may paradoxically worsen bradycardia through a parasympathomimetic effect 1
• Contraindicated after cardiac transplantation due to risk of complete AV block or sinus arrest with asystole 1, 4
• Use cautiously in acute MI as increased heart rate may worsen ischemia or increase infarct size 1
• Do not delay pacing in patients with poor perfusion while attempting atropine 1, 2

Alternative Pharmacologic Agents

When atropine fails or is contraindicated, beta-adrenergic agonists may be considered as temporizing measures while preparing for pacing 1, 2:

• Isoproterenol: Beta-agonist that increases heart rate and AV conduction 1
• Dopamine: Vasopressor with chronotropic effects 1
• Dobutamine: Inotrope with chronotropic properties 1
• Epinephrine: Combined alpha and beta effects for hemodynamic support 1, 2

Important limitation: These agents should only be used in patients with low likelihood of coronary ischemia, as they increase myocardial oxygen demand 1

Special Circumstance: Inferior MI

In acute inferior myocardial infarction with third-degree AV block, intravenous aminophylline may be considered to improve AV conduction 1. This is specific to inferior MI where the block is typically at the AV nodal level due to right coronary artery occlusion 5.

Temporary Pacing

Transcutaneous Pacing (TCP)

Transcutaneous pacing should be initiated immediately in unstable patients with third-degree AV block who do not respond to atropine 1, 2:

• Apply as a bridge to transvenous pacing 1, 2
• Painful in conscious patients; consider sedation 1
• May provide inconsistent capture 1
• Can be initiated while IV access is being established in unstable patients 1

Transvenous Pacing

Temporary transvenous pacing is the definitive temporary solution for symptomatic third-degree AV block refractory to medical therapy 1, 2:

• More reliable than transcutaneous pacing 1
• For prolonged temporary pacing needs, use an externalized permanent active fixation lead rather than standard passive fixation temporary lead 1
• Indicated while evaluating for reversible causes or preparing for permanent pacemaker 1, 2

Definitive Management: Permanent Pacemaker

Permanent pacemaker implantation is indicated for all patients with acquired third-degree AV block regardless of symptoms 2, 6. This is a Class I indication with the following specific scenarios 6:

• Third-degree AV block with bradycardia and symptoms (heart failure, syncope, ventricular arrhythmias) 6
• Asymptomatic third-degree AV block with asystole ≥3.0 seconds or escape rate <40 bpm 6
• Asymptomatic persistent third-degree AV block with cardiomegaly or LV dysfunction 6
• Third-degree AV block during exercise in absence of ischemia 6
• Post-catheter ablation of AV junction 6
• Postoperative AV block not expected to resolve 6
• Neuromuscular diseases with AV block (myotonic muscular dystrophy, Kearns-Sayre syndrome) 6

Management of Reversible Causes

Before proceeding to permanent pacing, evaluate and treat potentially reversible causes 1, 2, 6:

Reversible Causes Requiring Medical Therapy First

• Lyme carditis: Initiate parenteral antibiotics (ceftriaxone) immediately; provide temporary pacing support as needed 2, 6
• Drug toxicity: Discontinue offending agents (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics); support with temporary pacing 1, 6
• Electrolyte abnormalities: Correct hyperkalemia, hypomagnesemia, hypercalcemia 2, 6
• Acute inferior MI: Often transient; may resolve with reperfusion; temporary pacing while observing 1, 5

Special Considerations for Permanent Pacing Despite Reversibility

In selected patients on chronic stable doses of medically necessary antiarrhythmic or beta-blocker therapy who develop symptomatic third-degree AV block, proceed to permanent pacing without drug washout 1, 2. This is reasonable when the medication cannot be discontinued due to other indications 1.

For cardiac sarcoidosis with third-degree AV block, proceed to permanent pacing with defibrillator capability without waiting for reversibility due to high risk of sudden death and disease progression 1, 2.

For amyloidosis or progressive neuromuscular diseases, permanent pacing is warranted despite transient resolution due to inevitable progression 6.

Clinical Algorithm Summary

1. Assess hemodynamic stability (hypotension, altered mental status, chest pain, dyspnea) 2, 7
2. Determine block location from ECG (narrow QRS = nodal; wide QRS = infranodal) 1, 7
3. If nodal block with instability: Atropine 0.5-1.0 mg IV (repeat q3-5min, max 3 mg) 1, 2
4. If no response or infranodal block: Initiate transcutaneous pacing immediately 1, 2
5. Prepare for transvenous pacing if TCP ineffective or prolonged support needed 1, 2
6. Evaluate for reversible causes while providing temporary support 1, 2
7. Proceed to permanent pacemaker for all acquired third-degree AV block unless clearly reversible and resolving 2, 6