Atropine is a medication with a long history, deriving from the Atropa belladonna plant, commonly known as deadly nightshade. This naturally occurring alkaloid can be isolated from various members of the Solanaceae family. The plant’s extracts have been recognized for their potent properties since ancient times. The name “belladonna” itself, meaning “beautiful woman” in Italian, refers to its historical use for cosmetic pupil dilation.
How Atropine Affects the Heart
Atropine exerts its effects by acting as an anticholinergic drug, specifically a muscarinic receptor antagonist. In the heart, the vagus nerve, part of the parasympathetic nervous system, releases acetylcholine as its neurotransmitter. Acetylcholine normally binds to muscarinic receptors, particularly the M2 subtype, located on cells in the sinoatrial (SA) node and atrioventricular (AV) node. This binding slows the heart rate by reducing the firing rate of pacemaker cells and decreasing the speed of electrical impulse conduction through the AV node.
Atropine works by competitively blocking acetylcholine from binding to these muscarinic receptors, effectively blocking the vagal nerve’s influence on the heart. This blockade leads to an increased firing rate of the SA node, which is the heart’s natural pacemaker, and enhanced conduction velocity through the AV node. The overall result is an increase in heart rate and improved electrical signal transmission within the heart.
When Atropine is Used for the Heart
Atropine is frequently used in emergency medical situations to address specific cardiac conditions, primarily a slow heart rate, known as bradycardia. It is considered a first-line medication for symptomatic sinus bradycardia, where the heart beats too slowly and causes symptoms such as low blood pressure or poor organ perfusion. Atropine accelerates the heart rate to a more stable level.
The medication is a component of Advanced Cardiac Life Support (ACLS) protocols for managing symptomatic bradycardia. The standard initial dose is typically 1 milligram (mg) administered intravenously (IV) or intraosseously (IO), repeatable every 3 to 5 minutes. A maximum total dosage of 3 mg is generally recommended.
Atropine may also be beneficial in certain types of atrioventricular (AV) block, where electrical signals from the atria to the ventricles are slowed or disrupted. However, it is generally not effective for Mobitz type II second-degree heart block or third-degree (complete) AV block. In these specific types of heart block, other interventions like transcutaneous pacing are often considered.
Important Considerations for Atropine Use
While atropine is an effective medication, its use requires careful consideration due to potential side effects and specific contraindications. Common side effects are largely related to its anticholinergic properties and include dry mouth, blurred vision, dilated pupils, and light sensitivity. Other effects include dry eyes, constipation, urinary retention, and a flushed appearance.
Paradoxically, very low doses of atropine (below 0.5 mg) can sometimes cause a temporary slowing of the heart rate. Clinicians must exercise caution when administering atropine to patients with existing heart conditions, such as coronary heart disease or acute myocardial ischemia, as it can increase the heart’s oxygen demand. Atropine is a prescription medication and typically administered by healthcare professionals, especially in emergency settings, to ensure appropriate dosing and monitoring for adverse reactions.