Why we use it
Atropine is a competitive antagonist at muscarinic acetylcholine receptors. By blocking parasympathetic input to the sinus and AV nodes, it lifts the brake on conduction — the SA node fires faster and the AV node conducts more readily. In the lab we keep it close because vagal reactions are common and sometimes dramatic during catheter manipulation, sheath placement, and ablation near vagally-innervated structures.
Indication in the lab
- Vagal reactions during groin access (femoral nerve / venous manipulation)
- Bradycardia during transseptal — needle tenting and puncture can trigger a strong vagal response
- Ablation near ganglionated plexi in AF ablation can produce sudden, profound bradycardia or asystole
- Inferior wall and right coronary territory ablations — vagal reflexes are common
- Pre-induction in patients with high resting vagal tone or known vasovagal history
- Crash cart for any case — symptomatic sinus bradycardia, AV block at nodal level
Dose and route
- Standard adult: 0.5 mg IV every 3–5 minutes as needed, to a total of 3 mg.
- Cardiac arrest / asystole: 1 mg IV push (older protocols; modern ACLS deemphasizes routine use in arrest).
- Pediatric: 0.02 mg/kg IV, minimum dose 0.1 mg (smaller doses can paradoxically worsen bradycardia).
- IV push, fast — this is not a slow-titration drug.
The “minimum 0.5 mg” rule for adults exists because tiny doses can produce a central vagomimetic effect that worsens bradycardia.
Onset and duration
- Onset: less than 1 minute IV.
- Peak: 2–4 minutes.
- Duration: 30–60 minutes (vagolytic effect); longer for some other anticholinergic effects.
Monitoring
- Continuous ECG — looking for the HR to climb and AV conduction to improve
- BP — usually rises with HR, but watch for paradoxical hypotension if the bradycardia was actually compensatory
- Mental status in awake patients (anticholinergic delirium is real, especially in elderly)
- Pupils, dry mouth, urinary retention as expected side effects
When atropine won’t work
Atropine works at the SA and AV node — both vagally innervated, both nodal. It does not fix conduction problems below the AV node.
- Infranodal AV block (Mobitz II, complete heart block with wide escape) — atropine may worsen it by speeding the sinus rate while the block holds, dropping the ventricular rate.
- Post-cardiac transplant — denervated heart, no vagal tone to block.
- Severe hypothermia — limited efficacy.
In these settings, go straight to pacing.
Side effects to watch for
- Tachycardia — dose-dependent; sometimes exaggerated, especially in younger patients
- Dry mouth, blurred vision, mydriasis — predictable anticholinergic effects
- Urinary retention — especially in older men with prostatic disease
- Confusion / agitation / delirium — central anticholinergic syndrome, more in the elderly
- Worsening ischemia if HR climbs too fast in a patient with CAD
- Anhidrosis — patients can overheat in long cases
Reversal
No routine reversal. Symptomatic anticholinergic toxicity from overdose is managed supportively; physostigmine is a centrally-acting reversal but rarely needed in our setting.
Common pitfalls
- Sub-therapeutic dosing (0.2–0.3 mg) and watching the heart slow further.
- Reaching for atropine when the patient has Mobitz II — pacing pads should already be on.
- Forgetting that the long duration means the patient may have a dry mouth and blurred vision in recovery for an hour.
- Giving atropine after the vagal reaction has already resolved on its own — adds unnecessary tachycardia.