Atropine for Bradycardia

Atropine given intravenously blocks vagal slowing of the sinoatrial and atrioventricular nodes, raising the heart rate in bradycardia from raised vagal tone, inferior myocardial infarction, drugs and some poisonings. It is cheap, available in all emergency stores, and is also the key drug in organophosphate poisoning.

It does not work well when the block is distal to the atrioventricular node (Mobitz type 2 or third degree block with a wide QRS), in a transplanted heart, or in most overdoses of beta blockers and calcium channel blockers.

Classification. Antimuscarinic; anticholinergic belladonna alkaloid.

Mechanism. Competitively blocks muscarinic M2 receptors in the heart, removing vagal tone, which speeds sinoatrial discharge and atrioventricular conduction.

Pharmacodynamics. IV onset within 1 to 2 minutes, peak at 2 to 4 minutes, duration 30 to 60 minutes. Doses under 500 micrograms in adults can cause paradoxical slowing from central vagal stimulation.

Pharmacokinetics. Half life 2 to 4 hours in adults (up to 10 hours in children under 2 and older adults). About 50% metabolised in the liver; 30 to 50% excreted unchanged in urine. Crosses the blood brain barrier and the placenta.

Symptomatic bradycardia with adverse signs (shock, syncope, heart failure, ischaemia), adult.

• 1 mg IV bolus (some practice starts with 500 micrograms).

• Repeat every 3 to 5 minutes if needed; maximum total 3 mg.

• If there is no response, move to transcutaneous pacing or an infusion: adrenaline 2 to 10 micrograms/minute or dopamine 5 to 20 micrograms/kg/minute.

Bradycardia in organophosphate or carbamate poisoning (East Africa), adult. 1 to 2 mg IV (up to 3 mg), repeated every 5 to 10 minutes and doubled each time until the chest is clear and the heart rate above 80 beats/minute; then an infusion of 10 to 20% of the loading total each hour. Needs intensive monitoring.

Children. 20 micrograms/kg IV (minimum 100 micrograms, maximum 500 micrograms for a single dose in a child; maximum 1 mg in an adolescent); repeat once after 3 to 5 minutes; maximum total 1 mg in a child and 2 mg in an adolescent.

Contraindications. None in life threatening bradycardia. Use with great care in acute myocardial infarction (tachycardia increases oxygen demand), glaucoma, urinary retention, pyloric stenosis, paralytic ileus, and myasthenia gravis (unless on neostigmine).

Warnings. Doses under 500 micrograms in adults can slow the heart paradoxically. Not effective after heart transplant (denervated heart) and may cause high degree block in Mobitz type 2 or third degree block with a wide QRS. In such patients go straight to pacing. Tachycardia, fever, dry mouth, urinary retention, delirium in older adults. Total dose of 3 mg gives full vagal block; do not exceed it.

Adverse effects. Dry mouth, blurred vision, mydriasis, flushing, tachycardia, urinary retention, constipation, hyperthermia (especially in children), confusion and agitation, rarely ventricular arrhythmias.

Interactions. Other anticholinergics (tricyclic antidepressants, antihistamines, antipsychotics): additive effects. Pralidoxime: used together in organophosphate poisoning. Beta blockers, calcium channel blockers: bradycardia may not respond to atropine. Digoxin: bradycardia from toxicity responds in part.

Special groups.

• Pregnancy: acceptable in emergencies; crosses the placenta and can raise foetal heart rate.

• Breastfeeding: single doses compatible; larger doses may reduce milk supply.

• Patients of African ancestry: no dose change.

• Older adults: usual emergency doses apply; risk of delirium and urinary retention afterwards.

• Kidney impairment: no change for single doses.

• Liver impairment: no change for single doses.

Formulations. Injection 0.6 mg/mL ampoule and 1 mg/mL ampoule; prefilled syringe 1 mg in 10 mL (100 micrograms/mL).

Administration. Rapid IV bolus through a secure line, followed by a flush of 0.9% sodium chloride; or intraosseous. May be given by endotracheal tube in 2 to 3 times the dose if no access exists (rare).

Baseline. 12 lead ECG to identify the rhythm; blood pressure; look for reversible causes (inferior infarction, drugs, hypothermia, raised intracranial pressure, hypothyroidism, hyperkalaemia).

Monitoring. Continuous ECG and blood pressure; response in heart rate at 2 to 3 minutes; ready pacing equipment. In organophosphate poisoning, check the chest for secretions, pupils, and heart rate every 5 to 10 minutes.

Stopping. Single doses only; no taper. Repeat only as needed up to 3 mg.

Common prescribing errors.

• Giving less than 500 micrograms in an adult.
• Giving it to a transplanted heart.
• Relying on it for Mobitz type 2 or complete block with a wide QRS.
• Giving above 3 mg in routine bradycardia.
• Delaying pacing.
• Treating beta blocker overdose with atropine alone instead of glucagon, calcium and insulin with glucose.