Adenosine is an organic nucleoside found in every cell of the human body, where it serves as a building block for RNA and is central to cellular energy transfer and signaling pathways. Separate from its natural role, adenosine is also used as a fast-acting intravenous medication. Synthetically produced but identical to the substance the body makes, this drug is used in controlled hospital settings for its effects on the cardiovascular system for therapeutic and diagnostic purposes.
Primary Medical Use for Arrhythmias
The primary therapeutic application for the drug adenosine is to terminate a specific type of abnormal heart rhythm known as supraventricular tachycardia, or SVT. SVT is a condition where the heart suddenly begins to beat extremely fast, often well over 150 beats per minute, due to an abnormal electrical circuit in the heart’s upper chambers. This rapid heart rate can cause symptoms like palpitations, dizziness, and shortness of breath, and often requires medical intervention to stop.
In an emergency setting, adenosine is a first-line treatment to resolve these episodes by interrupting the faulty electrical pathway. Medical personnel use it to treat re-entrant arrhythmias that depend on the atrioventricular (AV) node to continue, which is the case for the most common forms of SVT. Its high success rate, often converting the rhythm back to normal within seconds, allows for the rapid restoration of a normal sinus rhythm.
Mechanism of Action
The effectiveness of adenosine as an antiarrhythmic drug stems from its direct action on the heart’s electrical conduction system. When injected, it targets A1 receptors in the atrioventricular (AV) node, the electrical gateway between the heart’s upper and lower chambers. Activation of these receptors initiates a cascade of events inside the cardiac cells.
This process causes potassium channels to open and inhibits calcium influx, which makes the cardiac cells hyperpolarized and less electrically excitable. This action slows conduction and results in a temporary block of electrical signals passing through the AV node. For the few seconds this block is in effect, it functions like a “reset button,” interrupting the abnormal electrical circuit that causes SVT and allowing the heart’s natural pacemaker to regain control.
Adenosine has an extremely short half-life of less than 10 seconds, meaning its full effects last only for about 30 seconds. This rapid metabolism occurs as red blood cells and endothelial cells quickly absorb and break down the compound. Because it works and disappears so quickly, the medication must be administered as a very rapid intravenous push to ensure a sufficient concentration reaches the heart.
The Patient Experience and Side Effects
For a person receiving adenosine, the experience is often intense but brief. The administration procedure involves a fast push of the drug into an intravenous line, followed by a saline flush to propel the medication to the heart. This rapid delivery is necessary due to the drug’s short half-life.
Within seconds, patients commonly experience a set of side effects. A feeling of chest pressure or discomfort is common, along with a sudden shortness of breath, a sensation of warmth or flushing, and a headache. These feelings are a direct consequence of the drug’s mechanism—the temporary stopping of the AV node and dilation of blood vessels.
Medical staff expect these reactions as a sign that the drug is working. The side effects resolve as quickly as they appear, vanishing within 30 to 60 seconds. While the experience can be unsettling, patients are continuously monitored, and the discomfort is followed by the relief of the heart returning to a normal rhythm. The drug is avoided in patients with certain high-degree heart blocks or severe, active asthma, as it can cause bronchoconstriction.
Use in Diagnostic Cardiac Testing
Beyond its emergency use for arrhythmias, adenosine serves a different function in diagnostic cardiology. It is used in pharmacologic, or chemical, cardiac stress tests for patients who are unable to perform adequate physical exercise. This test helps doctors evaluate blood flow to the heart muscle to identify potential blockages in the coronary arteries.
In this application, adenosine is not given as a rapid push. Instead, it is administered as a slow, continuous intravenous infusion over several minutes. The purpose of the infusion is to dilate blood vessels by working on the A2A receptors in the coronary arteries, causing them to widen and increase blood flow to the heart muscle, mimicking the effect of vigorous exercise.
While the adenosine infusion simulates exercise, a radioactive tracer is injected and a specialized camera takes pictures of the heart. In healthy arteries, the vessel widens and allows a large amount of the tracer to reach the heart muscle. In areas supplied by a narrowed artery, the vessel cannot dilate properly, and less tracer gets through. This highlights areas of reduced blood flow that may indicate coronary artery disease.