While atropine can be used independently for anti-salivation effects, it most commonly is secondary to anticholinergic or antimuscarinic poisoning which is discussed below. It is not formally recommended for routine use in controlled airways, though can be used off-label for minimizing secretions in the intubated patient.
Acetylcholine works on three different receptors that must be addressed in nerve agent poisonings. Atropine is only useful to counter muscarinic effects (pralidoxime and benzodiazepines address the others). If there are local symptoms to the eyes or respiratory tract, atropine is not indicated. Intravenous (IV) atropine is indicated for patients with hypersalivation, bronchial secretions, or bradycardia. Large doses and repeat doses may be required. Ingestions especially require higher doses (up to 20 mg). Titrate to effect by monitoring the patient’s ability to clear excess secretions. Pupils and heart rate are poor indications of appropriate dosing in these patients.
Atropine is the first-line therapy (Class IIa) for symptomatic bradycardia in the absence of reversible causes. Treatments for bradydysrhythmias are indicated when there is a structural disease of the infranodal system or if the heart rate is less than 50 beats/min with unstable vital signs. Approximately 20% of bradydysrhythmias are due to endogenous cardiac electrical systems. The structural disease may or may not require resuscitation and should be closely monitored with medication and pacing readily available. If there is no improvement in the clinical state after repeat doses of atropine, additional treatments with atropine are unlikely to be effective. However, transient improvements with repeat dosing are an indication to continue treatment with atropine (which may exceed standard cumulative dosing maximums). Pediatric bradycardia is rarely cardiac and often secondary to hypoxia and hypoventilation. If bradycardia persists despite adequate respiratory support, atropine is indicated.
Rapid Sequence Intubation (RSI) Pretreatment
Although not recommended as a routine agent, atropine may be used 3-5min prior to initiation of RSI to prevent bradycardia. In the setting of post-intubation related bradycardia, atropine is indicated. Post induction bradycardia is seen more commonly in the pediatric population due to the predominance of vagal response, even without the use of succinylcholine.
Atropine/Diphenoxylate (Lomotil) is an antimotility agent that can be used in the treatment of diarrhea as a second-line therapy. It may potentiate barbiturates, alcohol, or tranquilizers and therefore must be used with caution. This allows the central acting opiate effect and capitalization on the anticholinergic side effect of constipation to slow motility.
Under current ACLS protocols, atropine is indicated for symptomatic bradycardia and not in a pulseless patient. All indications for atropine in the pulseless patient have been removed by the AHA.
Atropine is not indicated in beta-blocker-induced bradycardias or hypotension, though its use is unlikely to be harmful. Glucagon is first line to treat beta-blockade-induced symptoms.
Pregnancy Class B: It does cross the placenta and may lead to fetal tachycardia; however, it does not cause fetal abnormalities. 
Atropine is an antimuscarinic that works through competitive inhibition of postganglionic acetylcholine receptors and direct vagolytic action. This leads to parasympathetic inhibition of the acetylcholine receptors in smooth muscle. The end effect of increased parasympathetic inhibition allows for preexisting sympathetic stimulation to predominate creating increased cardiac output and other associated antimuscarinic side effects as described herein.
Atropine can be administered by intravenous (IV), subcutaneous, intramuscular, or endotracheal (ET) method; IV is preferred. For ET administration dilute 1 mg to 2 mg in 10 mL of sterile water or normal saline prior to administration. For pediatric ET, double the dose and dilute in 3 to 5 mL.
In general, dosing of atropine can be repeated every 5minutes until a maximum of 0.04 mg/kg is reached.
Dosing in adults to greater than 0.5mg and slow IV pushes have been associated with paradoxical bradycardia (though likely transient) and ventricular fibrillation (VF).
The most common adverse effects are related to the antimuscarinic properties including xerostomia, blurred vision, photophobia, tachycardia, flushing, and hot skin. Constipation, difficulty with urination, and anhidrosis can occur especially in at-risk populations (most notably, the elderly). In rare cases, delirium or coma may occur. Hypersensitivity reactions may occur and are usually limited to a skin rash that could progress to exfoliation.
Atropine decreases the rate of mexiletine absorption that can be prevented by combined IV delivery of metoclopramide with atropine before anesthesia.
Atropine does not carry a Black Box Warning nor any absolute indications. There are multiple conditions that carry a cautionary status, however, relative contraindications are overridden by the clinical need, especially in the unstable or poisoning patient.
Use with 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.
Furthermore, caution is advised in use with elderly patients, chronic lung disease patients, acute angle glaucoma, obstructive diseases (uropathy, toxic megacolon, paralytic ileus, pyloric stenosis, prostatic hypertrophy), myasthenia gravis, or in situations with environmental heat exposure.
By understanding the adverse reactions, the above cautionary situations are easily understood by compounding effects on preexisting conditions.
Tachycardia is the most common side effect; titrate dose to effect when treating bradydysrhythmia in patients with CAD. Atropine may precipitate acute angle glaucoma, pyloric obstruction, urinary retention due to benign prostatic hyperplasia, or viscid plugs in patients with chronic lung diseases.
Overdose can lead to increased antimuscarinic side effects presenting with dilated pupils, warm, dry skin, tachycardia, tremor, ataxia, delirium, and coma. In extreme toxicity, circulatory collapse secondary to respiratory failure may occur after paralysis and coma. Ten milligrams or less may be fatal to a child, while there is no known adult fatal dose.
All Datura species have endogenous atropine and other assorted anticholinergic alkaloids. This includes Jimson weed and angel’s trumpet, which is typically seen in warm or temperate climate areas. It may be accidental ingestion or intentionally smoked or ingested for the goal effect of hallucinations. While treatment for ingestions is similar to other toxic overdoses of anticholinergics, whole-bowel irrigation is recommended after a large quantity of seed ingestion. Avoid phenothiazines in these patients. Physostigmine is the basis of reversal.
Treatment of overdose includes a short-acting barbiturate or diazepam as needed for convulsions or excitement. Avoid dosing for sedation as it can cause respiratory collapse. Physostigmine is useful as an antidote to treat delirium and coma. Repeat doses may be required as physostigmine has a shorter half-life. Monitor respiratory effort and supplement, if necessary. 
Atropine is only used to treat a few conditions, but this alkaloid is life-saving. All healthcare workers should be aware of how to use atropine, the dose, and method of administration. In patients with bradycardia and poisoning from cholinergic agents, atropine can immediately reverse the slow heart rate and reduce the mortality. Nurses should always make sure that atropine is available in the cardiac arrest cart. More importantly, there should be a syringe and needle also available as the drug can only be administered intravenously. Similarly, the anesthesia nurse should ensure that atropine is available in the operating room at all times. On almost every cardiology and cardiac surgery floor, atropine may be required, and hence it should be readily available. The pharmacist is the person responsible for ensuring that stock supplies of atropine are available in each department. (Level V)
Atropine is a very useful drug to reverse bradycardia from many causes. It works immediately and is relatively safe when used at therapeutic doses. There are countless reports on the number of lives saved through the use of atropine in patients with bradycardia or organophosphate poisoning. (Level V)