Atropine
The patient's heart rate is 38 and they're pale and diaphoretic. The first drug you reach for isn't a cardiac stimulant — it's a blocker. Understanding why is the key to atropine.
Core Concept
Atropine is an anticholinergic (parasympatholytic) that works by blocking acetylcholine at muscarinic receptors on the SA and AV nodes. The vagus nerve normally slows heart rate via acetylcholine release; atropine removes that brake, allowing the intrinsic sympathetic tone to increase rate. This makes it the first-line drug for symptomatic bradycardia — specifically sinus bradycardia and AV blocks at the nodal level. Per ACLS, the dose is 0.5 mg IV push every 3–5 minutes, up to a maximum of 3 mg. Doses below 0.5 mg can paradoxically worsen bradycardia by partially stimulating the vagal center before fully blocking it. Atropine is ineffective in infranodal (Mobitz Type II) and third-degree heart block with a wide QRS because these blocks occur below where vagal tone has influence. In those cases, transcutaneous pacing or epinephrine/dopamine infusion is needed instead. Atropine is also used preoperatively to reduce secretions and as an antidote for organophosphate or nerve agent poisoning, where massive cholinergic excess causes the SLUDGE presentation (Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis).
Watch Out For
Don't confuse atropine (increases HR by blocking parasympathetic input) with epinephrine (increases HR by directly stimulating sympathetic beta-1 receptors) — atropine removes the brake, epinephrine hits the gas. Students often think atropine works for all bradycardic rhythms, but it is ineffective in infranodal blocks (Mobitz II, complete heart block with wide QRS). Never give less than 0.5 mg IV — paradoxical bradycardia from sub-therapeutic dosing is a classic NCLEX trap.
Clinical Pearl
Atropine lifts the vagal foot off the brake — it doesn't press the gas pedal. If the block is below the node, atropine can't reach it. Think: 'No vagus, no atropine.'
Test Your Knowledge
3 quick questions — see how well you understood Atropine