Treatment of Bradycardia from Sinoatrial Block
For symptomatic bradycardia due to sinoatrial (SA) block, atropine 0.5-1 mg IV is the reasonable first-line treatment, repeated every 3-5 minutes up to a maximum of 3 mg, while simultaneously evaluating and treating any reversible causes. 1
Initial Assessment and Reversible Causes
Before initiating pharmacologic therapy, evaluation and treatment of reversible causes is mandatory in all symptomatic patients with sinus node dysfunction (including SA block). 1
Key reversible causes to identify and address include: 1
- Medications: Beta blockers, calcium channel blockers, digoxin, antiarrhythmic drugs
- Electrolyte abnormalities: Hyperkalemia, hypokalemia
- Metabolic derangements: Hypothyroidism, hypothermia, hypoglycemia
- Acute myocardial ischemia or infarction
- Infections: Lyme disease, viral illnesses
- Sleep apnea
- Drug toxicity or overdose
Acute Pharmacologic Management
First-Line: Atropine
Atropine is reasonable for patients with SA block causing symptomatic bradycardia or hemodynamic compromise (Class IIa recommendation). 1, 2
Dosing protocol: 1
- Initial dose: 0.5-1 mg IV bolus
- Repeat: Every 3-5 minutes as needed
- Maximum total dose: 3 mg
- Target: Achieve minimally effective heart rate (approximately 60 bpm) 2
Mechanism: Atropine blocks muscarinic acetylcholine receptors, facilitating sinoatrial conduction and increasing sinus node automaticity with a half-life of approximately 2 hours. 1, 2, 3
Critical Precautions with Atropine
Do NOT use atropine in heart transplant patients without evidence of autonomic reinnervation (Class III: Harm). 1, 2 Atropine can cause paradoxical bradycardia or high-degree AV block in these patients. 1
Use with caution in acute coronary ischemia or MI, as increased heart rate may worsen ischemia or increase infarct size. 1, 2
Avoid doses <0.5 mg, which may paradoxically worsen bradycardia. 1
Second-Line Agents (When Atropine Fails or Is Contraindicated)
In patients at low likelihood of coronary ischemia who remain symptomatic despite atropine, beta-agonists or vasopressors may be considered (Class IIb recommendation): 1, 2
- Isoproterenol: 20-60 mcg IV bolus followed by 10-20 mcg doses, or infusion of 1-20 mcg/min based on heart rate response 1
- Dopamine: 5-20 mcg/kg/min IV, starting at 5 mcg/kg/min and increasing by 5 mcg/kg/min every 2 minutes 1
- Epinephrine: 2-10 mcg/min IV or 0.1-0.5 mcg/kg/min IV titrated to desired effect 1
- Dobutamine: May be considered as alternative 1
Important caveat: Dopamine doses >20 mcg/kg/min may cause vasoconstriction or arrhythmias; monitor for ischemic chest pain. 1
Temporary and Definitive Pacing
Transcutaneous pacing (TCP) is reasonable for unstable patients who do not respond to atropine (Class IIa recommendation). 1
- TCP should be viewed as a temporizing measure only while preparing for transvenous pacing 1
- TCP is painful in conscious patients and may not achieve consistent capture 1
For persistent symptomatic bradycardia not attributable to reversible causes, permanent pacemaker implantation is the definitive treatment. 2, 4
Clinical Pitfalls to Avoid
Atropine may paradoxically worsen bradycardia in certain heart blocks. Patients with infranodal blocks (at the His-Purkinje level) are at increased risk of adverse events, including ventricular standstill, following atropine administration. 5 SA block typically responds favorably, but be prepared to escalate therapy immediately if deterioration occurs.
Excessive atropine dosing causes serious adverse effects. Higher initial doses (≥1.0 mg) or cumulative doses exceeding 2.5 mg over 2.5 hours correlate with ventricular tachycardia/fibrillation, sustained sinus tachycardia, increased PVCs, and toxic psychosis. 6
Most patients with sinus node dysfunction (including SA block) are stable and minimally symptomatic, requiring no acute therapy; evaluation can be performed in the outpatient setting. 1 Reserve aggressive treatment for those with clear symptoms or hemodynamic compromise.