Atropine has long been used to treat abnormally slow heart rate in emergency situations by altering the way electrical signals move through the heart.

Bradycardia is an especially slow heartbeat and a risk factor for fainting and other health problems. Atropine, an anticholinergic medication (a drug that blocks the neurotransmitter acetylcholine) used to treat a wide range of health problems, is a first-line treatment for bradycardia.

Atropine is typically used if there are no readily reversible causes of bradycardia, such as an imbalance of electrolytes. Though atropine is considered generally safe and well-tolerated, it can trigger some potentially serious side effects, such as tachycardia and an abnormally fast heart rate.

Keep reading to learn more about atropine and how this powerful medication treats a common heart rhythm disturbance.

Atropine is a tropane alkaloid, a type of strong anticholinergic compound found in certain plants. Atropine is derived from plants in the Solanaceae family, such as Atropa belladonna (deadly nightshade) and Hyoscyamus niger (henbane).

Like other anticholinergic drugs, atropine blocks the action of acetylcholine, a chemical messenger (neurotransmitter) that transmits signals between nerves and muscles, particularly in the parasympathetic nervous system. The parasympathetic nervous system is a network of nerves that helps the body rest and conserve energy, particularly during recovery from a stressful situation.

Atropine has a wide range of uses, including treating bradycardia. It’s also used as a pre-anesthetic medication to reduce salivation during surgery and as an antidote for poisoning by organophosphates or nerve agents.

Bradycardia is defined as a resting heart rate of less than 60 beats per minute (bpm) in adults. The American Heart Association notes that the average resting heart rate for most adults is between 60 and 100 bpm, though extremely fit athletes, such as long-distance runners, may have a resting heart rate of less than 60 bpm without any accompanying health problems.

Bradycardia has several potential triggers, including damage to the heart’s electrical conduction system, electrolyte imbalances (particularly potassium and/or calcium), medication side effects, or a response to certain diseases, such as hypothyroidism or chronic obstructive pulmonary disease (COPD).

In some cases, bradycardia is asymptomatic and doesn’t pose any major health concerns. Treatment is necessary when bradycardia leads to symptoms such as:

  • dizziness
  • weakness
  • fatigue
  • syncope (fainting)

Complications of bradycardia can include:

  • cardiac arrest
  • chest pain (angina)
  • heart failure

Atropine is typically used to treat symptomatic bradycardia, particularly in emergency settings when the slow heart rate is compromising blood flow and oxygen delivery to vital organs. In these cases, atropine is administered intravenously (in a vein) to quickly counteract the abnormal electrical activity in the heart.

Atropine works by blocking muscarinic receptors in the heart. Muscarinic receptors are protein receptors found throughout the body that are involved with several functions, including circulation and respiration.

Under normal circumstances, the parasympathetic nervous system regulates the heart through the vagus nerve. The vagus nerve releases acetylcholine to bind to muscarinic receptors in the heart, reducing the heart rate. This is helpful when the heart needs to slow down after exertion or stress. But if the vagus nerve releases too much acetylcholine, the heart rate can drop too much.

By blocking the muscarinic receptors, atropine prevents the slowing effect of acetylcholine, thereby promoting an increase in heart rate.

If repeat doses of atropine aren’t effective in improving the heart rate initially, it’s unlikely subsequent atropine treatment will be effective either.

While atropine is widely regarded as safe and effective, its use carries potential risks, especially when administered inappropriately or in individuals with certain underlying conditions. Some of the risks and adverse effects include:

  • Tachycardia: One of the most common side effects of atropine is an increase in heart rate (tachycardia) as the drug blocks parasympathetic nervous system activity. This can worsen the situation in patients with preexisting tachyarrhythmias (rapid heart rhythms).
  • Increased oxygen demand by the heart: Atropine increases heart rate, which may cause the heart muscle to consume more oxygen. This could potentially exacerbate conditions like coronary artery disease, where the oxygen supply is already limited. This is particularly concerning in patients recovering from a heart attack.
  • Anticholinergic side effects: Atropine’s anticholinergic properties may lead to dry mouth, blurred vision, urinary retention, constipation, and confusion, especially in older adults. These side effects are generally mild and temporary, but they can be distressing.
  • Toxicity in overdose: Although rare, atropine overdose can occur, leading to symptoms such as delirium, hallucinations, and life threatening arrhythmias (heart rhythm disturbances). This requires immediate medical intervention.

The outlook for individuals following atropine administration for bradycardia largely depends on the underlying cause of the bradycardia and how quickly treatment is administered.

If the source of the bradycardia is a problem with the vagus nerve, atropine is often effective. The same is true if bradycardia is a complication of mild atrioventricular (AV) block — a condition in which electrical signals from the atria to the ventricles are blocked or disrupted.

In people with high degree or complete AV block, atropine may not be effective. And in some cases, it can worsen the bradycardia. Atropine provides only temporary relief, and further treatment may be necessary.

In critically ill patients, atropine may serve as an important bridge to more long-term therapies, such as the placement of a temporary pacemaker or permanent pacemaker.

Overall, the outlook after atropine use for bradycardia is positive in many instances, with most people experiencing a rapid improvement in heart rate and symptoms. However, people with underlying structural heart disease or severe conduction system abnormalities may require more extensive treatment.

Atropine remains an important tool in the management of symptomatic bradycardia, offering rapid relief by increasing heart rate and helping to establish a healthy rhythm moving forward.

While it is generally safe, its use must be carefully monitored, especially in patients with complex arrhythmias or underlying heart disease. With proper dosing and evaluation of a person’s other health risks, atropine can provide effective short-term management.

Long-term treatment, however, may require addressing the root cause of the bradycardia.