Adenosine is used primarily to treat supraventricular tachycardia (SVT), a type of abnormally fast heart rhythm that originates above the ventricles. It works by briefly slowing electrical conduction through the AV node, the gateway between the upper and lower chambers of the heart, effectively “resetting” the rhythm. Adenosine also has a second, very different clinical role: as a vasodilator during cardiac stress testing for patients who can’t exercise.
The Primary Indication: Supraventricular Tachycardia
Adenosine is the first-line drug for two specific types of SVT that involve a short-circuit loop through or near the AV node. In AV nodal reentrant tachycardia (AVNRT), the electrical signal circles within the AV node itself. In AV reentrant tachycardia (AVRT), the signal loops between the AV node and an extra electrical pathway. In both cases, the AV node is a critical part of the circuit, and adenosine breaks the loop by temporarily blocking conduction through it.
This is an important distinction. Adenosine slows conduction through the AV node, but it does not affect accessory pathways or abnormal rhythms that don’t depend on the AV node. For arrhythmias like atrial fibrillation, atrial flutter, or ventricular tachycardia, adenosine is not the appropriate treatment. It can, however, be useful diagnostically in some of these situations: by briefly slowing the ventricular rate, it can unmask the underlying atrial rhythm on a monitor, helping clinicians figure out exactly what type of arrhythmia they’re dealing with.
How Adenosine Works
Adenosine acts on receptors in the AV node to open potassium channels and block calcium from entering cells. Potassium flowing out of the cell makes it harder for the cell to fire, while reduced calcium influx suppresses the type of electrical activity that AV node cells rely on. The combined effect is a dramatic, temporary slowdown in conduction through the AV node. This pause is often enough to interrupt the reentrant circuit causing SVT and allow the heart’s normal pacemaker to take over.
The drug is broken down by the body almost immediately, with a half-life measured in seconds. This ultra-short duration is both its greatest advantage and the reason it must be given as a rapid IV push followed by a saline flush. If it’s injected too slowly or into a peripheral site far from the heart, it gets metabolized before it ever reaches the AV node.
Adenosine in Cardiac Stress Testing
For patients who can’t walk on a treadmill due to mobility issues, severe deconditioning, or other limitations, adenosine serves as a pharmacological stand-in for exercise during nuclear cardiac imaging. When infused at a controlled rate, it acts on receptors in coronary arteries to cause vasodilation, increasing blood flow to 3 to 5 times the resting level. Healthy coronary arteries dilate fully and show strong radiotracer uptake on imaging. Arteries narrowed by blockages can’t increase flow to the same degree, creating visible perfusion defects that indicate coronary artery disease.
In this context, adenosine is administered as a slow infusion (not a rapid push) over 4 to 6 minutes. The radiotracer is injected near the end of the infusion. Low-level exercise performed simultaneously, when possible, improves image quality and is well tolerated.
How It Compares to Other SVT Treatments
Calcium channel blockers like verapamil are an alternative to adenosine for SVT, and both approaches convert the rhythm successfully at similar rates: roughly 90% to 93% of the time. The key difference is speed. A Cochrane review of seven trials found that adenosine restored normal rhythm in an average of 44 seconds, compared to about 394 seconds (over 6 minutes) for calcium channel blockers.
That speed comes with a tradeoff in comfort. About 12% of patients given adenosine experience chest tightness, compared to essentially none in the verapamil group. Flushing is also far more common with adenosine. Major adverse events, however, were rare with both drugs. Adenosine’s ultra-short duration means its side effects resolve within seconds, while calcium channel blockers have longer-lasting effects that can be problematic if the initial diagnosis turns out to be wrong.
Who Should Not Receive Adenosine
Adenosine is contraindicated in three situations:
- Second- or third-degree heart block (unless the patient has a functioning pacemaker), because adenosine slows AV conduction further and could cause a dangerous pause.
- Sick sinus syndrome or symptomatic slow heart rate (again, unless a pacemaker is in place), for the same reason.
- Known hypersensitivity to the drug.
Beyond these absolute contraindications, adenosine should be avoided in patients with asthma or active bronchospasm. It can trigger bronchoconstriction through mast cell activation and histamine release, and mild to moderate worsening of asthma symptoms has been reported. Patients with other obstructive lung diseases like emphysema or chronic bronchitis require caution as well, as respiratory compromise has occurred during adenosine infusions in this group.
Drug Interactions That Affect Adenosine
Caffeine and theophylline are both competitive blockers of adenosine receptors. Caffeine binds to the same receptor sites that adenosine needs to reach, which can reduce or completely block the drug’s effect. Theophylline is even more potent at blocking adenosine receptors than caffeine. Patients who have recently consumed significant amounts of caffeine or who take theophylline-based medications may need higher doses or may not respond at all.
On the other side of the equation, the heart medication dipyridamole blocks the cellular uptake of adenosine, which intensifies and prolongs its effects. Patients taking dipyridamole typically need a much lower dose. Beta-blockers and calcium channel blockers can also have additive effects on slowing the heart, increasing the risk of profound bradycardia. Digoxin use with adenosine has been rarely associated with ventricular fibrillation, particularly when verapamil is also on board.
Older adults generally require more caution, as they are more likely to have underlying conduction system disease or to be taking medications that interact with adenosine.
Dosing in Adults and Children
In adults, the standard ACLS protocol starts with a 6 mg rapid IV push. If the arrhythmia doesn’t convert within 1 to 2 minutes, a second dose of 12 mg is given. A third 12 mg dose can follow if needed. Each dose must be pushed as fast as possible into a vein as close to the heart as possible (typically the antecubital fossa), immediately followed by a rapid saline flush to propel the drug centrally before it’s broken down.
In children, dosing is weight-based. The first dose is 0.05 to 0.1 mg/kg, with a maximum of 6 mg. If a second dose is needed after 1 to 2 minutes, it increases to 0.2 mg/kg, up to a maximum of 12 mg. The same rapid-push technique applies.
What the Patient Feels
Adenosine is notorious for producing a brief but intense set of sensations. Patients commonly report a sudden feeling of chest pressure or tightness, facial flushing, and shortness of breath. Some describe a “sense of impending doom” that lasts only a few seconds. A brief pause in the heartbeat (transient asystole) is actually the intended effect, and while it can feel alarming, it typically lasts only a few seconds before normal rhythm resumes. These sensations resolve almost as quickly as they begin because the drug is cleared from the bloodstream within 10 to 30 seconds. Patients are generally warned about these effects beforehand, which can make the experience less frightening.