What is the treatment for bradycardia (slow heart rate) due to sinoatrial (SA) block?

Treatment of Bradycardia from Sinoatrial Block

Atropine 0.5-1 mg IV is the first-line treatment for symptomatic bradycardia due to sinoatrial block, repeated every 3-5 minutes up to a maximum of 3 mg, while simultaneously identifying and correcting any reversible causes. 1

Immediate Management Algorithm

Step 1: Assess Reversibility

Before administering any medication, you must evaluate for reversible causes that require immediate correction 1:

• Medications: Beta blockers, calcium channel blockers, digoxin, antiarrhythmic drugs 1
• Electrolyte abnormalities: Hyperkalemia and hypokalemia 1
• Metabolic derangements: Hypothyroidism, hypoglycemia 1
• Cardiac ischemia: Acute MI or ongoing ischemia 1
• Infections: Lyme disease 1
• Sleep apnea 1

Step 2: Atropine Administration

For symptomatic or hemodynamically compromised patients 1, 2:

• Initial dose: 0.5-1 mg IV bolus 1
• Repeat dosing: Every 3-5 minutes as needed 1
• Maximum total dose: 3 mg 1
• Target heart rate: Approximately 60 bpm 1
• Mechanism: Blocks muscarinic acetylcholine receptors, facilitating sinoatrial conduction and increasing sinus node automaticity 1, 2
• Duration of action: Half-life approximately 2 hours 1

Step 3: Recognize Atropine Contraindications and Cautions

Absolute contraindication 1:

• Heart transplant patients without evidence of autonomic reinnervation (can cause paradoxical bradycardia or high-degree AV block) 1

Use with extreme caution 1, 3:

• Acute coronary ischemia or MI (increased heart rate may worsen ischemia or increase infarct size) 1
• Avoid initial doses of 1.0 mg or cumulative doses exceeding 2.5 mg over 2.5 hours in MI patients, as these correlate with ventricular tachycardia, ventricular fibrillation, and increased PVCs 3

Step 4: Second-Line Pharmacologic Agents

If atropine is ineffective or contraindicated, consider alternative agents 1:

• Isoproterenol 1
• Dopamine 1
• Epinephrine 1
• Dobutamine 1

Step 5: Temporary Pacing

Transcutaneous pacing is indicated for unstable patients who do not respond to atropine 1:

• This is a temporizing measure while preparing for transvenous pacing 1
• Should be initiated promptly in hemodynamically unstable patients 4

Step 6: Definitive Treatment

Permanent pacemaker implantation is the definitive treatment for persistent symptomatic bradycardia from SA block that is not attributable to reversible causes 1, 4:

• This is the only long-term solution for persistent symptomatic bradycardia 5
• Indicated when symptoms recur despite correction of reversible factors 5

Critical Pitfalls to Avoid

Paradoxical worsening with atropine: In patients with infranodal (His-Purkinje level) blocks, atropine can paradoxically worsen bradycardia or cause ventricular standstill 6. While SA block typically responds favorably to atropine, be prepared to immediately escalate to epinephrine infusion and transcutaneous pacing if deterioration occurs 6.

Excessive dosing in acute MI: Higher initial doses (1.0 mg vs. 0.5-0.6 mg) or cumulative doses exceeding 2.5 mg are associated with ventricular arrhythmias and should be avoided 3.

Mistaking atropine for definitive therapy: Atropine is a temporary bridge, not a long-term solution 5. Patients with recurrent symptomatic SA block require permanent pacing 1, 5.