Atropine is an anticholinergic (specifically an antimuscarinic) that works by inhibiting the parasympathetic nervous system. It can be administered IV, IM, SC, and even down an endotracheal tube! However, since atropine crosses the blood-brain barrier, it can cause central anticholinergic syndrome – a spectrum of confusion and delirium, especially in the elderly and treated with physostigmine. Its other effects are similar to those of glycopyrrolate.
Although glycopyrrolate is my “go-to” antimuscarinic in the ICU and OR, I reach for atropine in cases of sudden, profound bradycardia. In fact, I always have this syringe alongside succinylcholine on my workstation in case of emergency. Interestingly, a low dose of atropine (< 0.4 mg IV in adults) may result in paradoxical bradycardia by blocking the negative feedback of inhibitory presynaptic muscarinic receptors that limit the parasympathetic nervous system. Therefore, whenever I use atropine, I almost give the entire syringe (0.8 mg IV dosed every few minutes if needed).
Atropine (crosses placenta) is often used instead of glycopyrrolate (does NOT cross the placenta) to mitigate fetal bradycardia when reversing neuromuscular blocking agents with neostigmine (crosses placenta). Furthermore, meperidine (Demerol), a weak opioid analgesic, has structural similarities to atropine and, therefore, some anticholinergic properties itself. Don’t be surprised if your patient’s heart rate increases when administering Demerol! Remember that atropine will likely not work on fresh heart transplants since these denervated organs no longer have vagal tone in the immediate post-op period. Use direct adrenergic agents (i.e., dobutamine) instead!
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