Atropine lives in your protocol for various ailments including symptomatic bradycardia and for rarer occurrences of organophosphate poisoning. You also may not be aware that it also has off label utilizations to minimize secretions in the intubated patient. We will dive into this pharmacological agent in depth and help answer the questions around how it works and why it is to be delivered rapid via IV push with brady arrythmias.
Mechanism of Action
Atropine is commonly classified as an anticholinergic or anti-parasympathetic (parasympatholytic) drug. More precisely, however, it is termed an antimuscarinic agent since it antagonizes the muscarine-like actions of acetylcholine. When mentoring students, I regularly find because of its effect on heart rate that they automatically assume it works directly on the sympathetic nervous system, but this is not the case. The best way I can describe a parasympatholytic is that drugs of this class "speed you up", but working against or inhibiting the system that is supposed to "slow you down".
It's Also A Vagolytic -- What's That Mean?
Think about cranial nerve X and everything it innervates directly... you guessed it, it blocks organs that the vagus nerve help control in one way or another. They include, but aren't limited to the:
Pharynx, Larynx, muscles of the soft palate, the GI tract (stomach + intestines)
Why Does It Have To Be Given Fast?
Remember that Atropine is in part of a competitive landscape with Acetylcholine (ACH) - the main neurotransmitter of the parasympathetic nervous system. If Atropine is given slowly (or in low doses) to correct a bradycardia, it could paradoxically accentuate the parasympathetic actions of ACH - so therefore ACH would be considered the victor and thus the heart rate would continue to be slow or potentially get worse. Certainly not the desired patient outcome. When given correctly and at the appropriate dose, it works immediately and is relatively safe when used at therapeutic doses. Its effects, however, may be transient in nature -- especially in higher degree heart blocks. Per AHA, Atropine should only be considered a temporizing measure while awaiting a transcutaneous pacemaker for patients with symptomatic high-degree AV block.
Don't Forget About Anticholinergics!!
Acetylcholine works on three different receptors that merit attention in nerve agent poisonings. Atropine is only useful to counter muscarinic effects (pralidoxime and benzodiazepines act on the others). Remember 2-PAM Chloride?? If there are local symptoms just in the eyes or respiratory tract, atropine is not indicated. Intravenous (IV) atropine indications include patients with hypersalivation, bronchial secretions, or bradycardia. Large doses and repeat doses will likely be required. Odds are that you and even your mutual aid agencies may not have enough medication in severe cases. For example, ingestions especially require higher doses (up to 20 mg). Titrate to effect by monitoring the patient’s ability to clear excess secretions. Keep in mind that pupils and heart rate are poor indications of appropriate dosing in these patients.
AHA Has This to Say (adults/peds)
Adults: If the bradycardia is persistent and the patient is symptomatic with signs of AMS, hypotension, or signs of shock, give Atropine in a 1 mg bolus (repeat to a 3 mg max if necessary) If this is ineffective consider TCP and/or vasopressors.
Peds: Atropine is less of a priority and comes after compressions and Epinephrine in this algorithm.
Be Cautious When...
Clinicians of all types need to exercise caution in patients with coronary heart disease, acute myocardial ischemia, congestive heart failure, tachycardia, or hypertension, as the increased cardiac demand and possible further worsening of tachycardia and hypertension can prove detrimental to patient outcomes. Although relative contraindications can be overridden by clinical need in certain unstable patients.
I hope you enjoyed this blog post -- in my humble opinion, it's never too late for a pharmacology review. After all, we owe it to the patient's and the communities we serve. They expect that we are on the top of our game when they call.
August 28, 2023
Author: Joshua Ishmael, MBA, MLS(ASCP)CM, NRP
Pass with PASS, LLC.