The provider should give adenosine, 6 mg by rapid IV push, when a patient has narrow-complex supraventricular tachycardia (SVT) without signs of hemodynamic compromise. This is the standard first-line drug recommendation from the American Heart Association's advanced cardiovascular life support guidance. The critical boundary: if the patient shows hypotension, altered mental status, ischemic chest pain, acute heart failure, or shock, the correct move is synchronized cardioversion, not a drug.
Adenosine's Mechanism and the 6 mg Starting Dose
Adenosine works by activating A1 receptors in the atrioventricular node, causing transient blockade of conduction through the AV node. That brief block interrupts the reentrant circuit that sustains most narrow-complex SVTs, most commonly atrioventricular nodal reentrant tachycardia (AVNRT). The drug's half-life is under 10 seconds, so the effect is fleeting and the rhythm often resumes if the circuit isn't broken. This is why the push must be fast and followed immediately by a saline flush.
The recommended first dose is 6 mg given as a rapid IV bolus over 1 to 3 seconds into a large peripheral vein, followed by a 20 mL saline flush. If the rhythm doesn't convert within 1 to 2 minutes, a second dose of 12 mg is given the same way. Some protocols allow a third 12 mg dose. The 6 mg/12 mg/12 mg sequence appears in the American Heart Association's ACLS algorithms and in the adenosine label approved by the U.S. Food and Drug Administration.
A common failure mode: giving adenosine through a small distal IV or a slow push. The drug reaches the heart already partially metabolized, and the rhythm doesn't convert. The clinician then assumes the drug failed when the delivery did. Use the largest, most proximal vein available, ideally the antecubital fossa. If the patient has a central line, that's even better.
Adenosine is generally safe in stable SVT because its effect lasts seconds. Transient flushing, chest discomfort, dyspnea, and a brief sense of "impending doom" are common and resolve quickly. The more serious risk is bronchospasm in patients with severe asthma, and the drug can precipitate atrial fibrillation in a minority of patients. Those considerations matter, but they don't change the first-line status in a stable patient without severe reactive airway disease.
Why Synchronized Cardioversion Waits When the Patient Is Stable
The dividing line in SVT management is hemodynamic stability, and it's assessed at the bedside, not on the monitor. A patient with a heart rate of 180 bpm who is alert, talking, with a normal blood pressure and no chest pain is stable. That patient gets a drug. A patient with the same rate who is hypotensive, confused, or reporting ischemic chest pain gets electricity.
Synchronized cardioversion for narrow-complex SVT typically starts at 50 to 100 joules biphasic, with escalation if the first shock fails. The shock is synchronized to the QRS complex to avoid delivering energy during the T wave, which can trigger ventricular fibrillation. That's a real risk, and it's why the sync mode must be confirmed before discharge. In an unstable patient, the risk of waiting on a drug that takes 30 to 60 seconds to work outweighs the shock risk.
Stability is not a permanent label. A patient who arrives stable can deteriorate while you're drawing up adenosine. Reassess after each intervention. If the blood pressure is falling, mental status is changing, or chest pain is escalating, shift to cardioversion. The AHA's 2020 advanced cardiovascular life support update frames this as a continuous decision, not a one-time triage.
One edge case worth naming: patients with pre-excited atrial fibrillation, such as Wolff-Parkinson-White syndrome presenting with an irregular wide-complex tachycardia. That's not a narrow-complex SVT, and adenosine is contraindicated because it can accelerate conduction down the accessory pathway and precipitate ventricular fibrillation. Confirm the rhythm is narrow and regular before reaching for adenosine, and if there's any doubt about pre-excitation, get expert help.
Vagal Maneuvers Before the First Syringe
Before adenosine, try vagal maneuvers. A modified Valsalva maneuver — the patient bears down for 15 seconds, then is laid flat with legs raised for 15 seconds — converts roughly one in three SVTs in the REVERT trial published in The Lancet in 2015. The standard Valsalva converts far fewer. The modified version is safe, free, and takes under a minute, which is why the AHA places it ahead of drugs in the stable SVT algorithm.
Carotid sinus massage is an alternative in patients without carotid bruit, prior stroke, or known carotid disease. It carries a small stroke risk in the wrong patient, so screen first. The Valsalva has no such risk and is the better default in most emergency departments. In the REVERT trial, the modified Valsalva converted 43% of patients versus 17% with the standard maneuver, a difference that changed practice.
Document the attempt and the result. If the Valsalva converts the rhythm, adenosine is unnecessary. If it fails, proceed to the 6 mg dose. This sequence matters because it's the difference between a patient who leaves without a drug exposure and one who doesn't, and because the vagal attempt takes less time than mixing and pushing adenosine.
A practical detail: coaching the Valsalva well is harder than it sounds. Patients often strain for 3 seconds and stop. Count aloud with them and watch the strain. A poorly performed Valsalva fails for the same reason a poorly delivered adenosine fails — technique, not pharmacology.
Comparing the Drug Options for Stable Narrow-Complex SVT
Adenosine is first-line, but it isn't the only option. The alternatives matter in specific situations, and the comparison below reflects the AHA's stable SVT algorithm and standard pharmacology references.
| Drug | Typical dose | Onset | Key advantage | Key limitation |
|---|---|---|---|---|
| Adenosine | 6 mg IV push, then 12 mg, then 12 mg | Under 30 seconds | Fastest diagnostic and therapeutic effect; half-life under 10 seconds | Bronchospasm risk; transient unpleasant effects; fails with poor IV access |
| Diltiazem | 15–20 mg IV over 2 minutes | 2–5 minutes | Longer duration; useful if SVT recurs | Contraindicated in heart failure, hypotension, and pre-excitation |
| Verapamil | 5–10 mg IV over 2 minutes | 2–5 minutes | Effective AV nodal blocker | More negative inotropy than diltiazem; avoid in reduced ejection fraction |
| Metoprolol | 2.5–5 mg IV over 2 minutes | 5–10 minutes | Useful when SVT is catecholamine-driven | Slower onset; avoid in decompensated heart failure or bronchospasm |
| Amiodarone | 150 mg IV over 10 minutes | Minutes | Broad-spectrum; used when other agents fail | Not first-line for stable narrow-complex SVT; slower and more toxic |
The verdict: adenosine wins for the first attempt in a stable patient without severe asthma, because it's the fastest and self-terminating. Diltiazem is the practical second choice if adenosine fails or the rhythm recurs, provided the blood pressure and ejection fraction allow it. Beta-blockers suit patients whose SVT is clearly driven by sympathetic tone, such as thyroid storm or postoperative states.
A detail the comparison tables in many resources omit: the dose escalation in adenosine is not weight-based in adults. The 6 mg and 12 mg doses are fixed, and the second dose is larger because the first may be partially metabolized before reaching the heart. In children, the dose is weight-based at 0.1 mg/kg, capped at 6 mg for the first dose.
Preparing the Push: Access, Flush, and Monitoring
Get the setup right before the drug is in the room. Adenosine is unforgiving of a slow push, and the setup determines whether the push is fast. The steps below follow standard emergency department practice and the administration instructions in the FDA-approved adenosine label.
1: Place a large-bore IV, 18 gauge or larger, in an antecubital vein. A 20 gauge in the hand is a last resort. 2: Attach a three-way stopcock or a dual-port setup so the adenosine and the saline flush can be pushed back-to-back without a pause. 3: Connect the patient to continuous cardiac monitoring, pulse oximetry, and a blood pressure cuff, and have the defibrillator at the bedside with pads on. 4: Draw up 6 mg of adenosine in one syringe and 20 mL of normal saline in another, both ready before you start. 5: Warn the patient about flushing, chest tightness, and a brief sense of breathlessness, then push the adenosine over 1 to 3 seconds and immediately follow with the saline flush. 6: Watch the monitor for conversion during the flush; the effect begins within seconds and the rhythm may convert before the flush finishes.
If the rhythm doesn't convert, wait 1 to 2 minutes and repeat with 12 mg. If the second dose fails, the next decision is a third 12 mg dose or a switch to diltiazem, depending on the patient's blood pressure and comorbidities.
The failure-handling rule: if two doses fail, stop and reassess the diagnosis. Not every narrow-complex tachycardia is AVNRT or AVRT. Atrial flutter with 2:1 block can look regular at about 150 bpm and won't convert with adenosine, though the drug may unmask the flutter waves. Atrial tachycardia may slow transiently and reveal its origin. If the rhythm doesn't break, the monitor strip during adenosine is diagnostic information, not just a failed treatment.
When Adenosine Is the Wrong Choice
Adenosine has a short list of contraindications, and they matter more than the drug's popularity suggests. Severe asthma or active bronchospasm is the main one, because adenosine can trigger bronchoconstriction through mast cell activation. In those patients, diltiazem or a beta-blocker is preferred, assuming no other contraindication.
Pre-excited atrial fibrillation is an absolute contraindication. Adenosine blocks the AV node, which can push conduction down the accessory pathway and accelerate the ventricular rate, potentially into ventricular fibrillation. Any irregular wide-complex tachycardia should be treated as pre-excited AF until proven otherwise, and adenosine should stay in the drawer.
Heart transplant patients are exquisitely sensitive to adenosine because the transplanted heart lacks vagal tone and has denervated AV nodal responses. Standard doses can cause prolonged asystole. Use a reduced dose, typically 1 to 3 mg, and have pacing available. This is a real edge case that shows up on boards and in transplant units, and it's the kind of detail that separates a safe clinician from a confident one.
Other cautions: sick sinus syndrome without a pacemaker, because adenosine can cause prolonged sinus pause; and concurrent use of dipyridamole or carbamazepine, which potentiate adenosine's effect and may warrant a lower dose. The FDA label lists these interactions, and they're worth checking before the push in a patient on either drug.
Narrow-Complex SVT in Pregnancy and Other Special Populations
Pregnancy doesn't rule out adenosine. The drug is considered the preferred agent for stable SVT in pregnancy because it has a very short half-life and doesn't cross the placenta in meaningful amounts, according to the AHA's cardiac arrest guidance for pregnancy. The same 6 mg starting dose applies, and the same push technique. Diltiazem and beta-blockers carry more fetal considerations.
In older adults, the dose doesn't change, but the response can. The transient asystole after adenosine can last longer in patients with conduction disease, so pacing capability at the bedside matters more. A patient with a baseline bundle branch block or a prolonged PR interval deserves a longer look at the monitor after the push.
In pediatric patients, the weight-based dose applies: 0.1 mg/kg for the first dose, up to 6 mg, then 0.2 mg/kg up to 12 mg. The push technique is the same, but the flush volume should be weight-appropriate. Children often need a smaller flush, and the IV is often smaller, so the "rapid push" becomes a relative term. Coordinate with the pediatric team if available.
The through-line across all these populations: adenosine remains first-line in stable narrow-complex SVT unless a specific contraindication applies. Pregnancy, age, and weight change the dose or the monitoring, not the drug class.
What the Post-Adenosine Monitor Strip Tells You
The adenosine effect is diagnostic as well as therapeutic, and the rhythm strip during and after the push carries information that a 12-lead taken afterward may not. If the rhythm converts to sinus and stays there, the diagnosis was AVNRT or AVRT. If the rhythm breaks and immediately returns, the circuit is intact and the dose or the delivery may have been inadequate.
If the rate slows and reveals flutter waves, the diagnosis is atrial flutter. That's a useful finding, because flutter usually needs rate control or cardioversion rather than repeated adenosine. If the rhythm slows and reveals a focal atrial tachycardia, the P-wave morphology can point to the origin, and the patient may need a different long-term plan.
Capture the strip. Print it or save it to the record. The converted rhythm, the pause after conversion, and any atrial activity during the block are all part of the diagnostic picture. Cardiologists use that strip to decide whether the patient needs an electrophysiology study. A clinician who doesn't save it wastes the best diagnostic moment the drug provides.
Documenting the Decision and Handing Off Care
Documentation for a stable SVT encounter should record the rhythm, the rate, the blood pressure, the stability assessment, the vagal maneuver attempt, the adenosine doses and response, and the post-conversion rhythm. That record supports the next clinician's decision and protects the patient if the rhythm recurs. It also matters for billing and for any quality review.
The handoff should name the working diagnosis. "Stable narrow-complex SVT, converted with 6 mg adenosine, now sinus at 88, blood pressure 124/76" is a handoff. "SVT, treated" is not. The difference is whether the receiving clinician knows to watch for recurrence and whether the patient needs cardiology follow-up.
Patients who convert with adenosine often need follow-up with cardiology or electrophysiology. Recurrent AVNRT may warrant catheter ablation, which has a high cure rate. A single episode in a young patient with a clear trigger may need only reassurance and instructions to return if it recurs. That decision belongs to the follow-up clinician, not the emergency department.
Discharge instructions should include what to do if the palpitations return: try the Valsalva, sit down, and call emergency services if there's chest pain, fainting, or shortness of breath. Those instructions are cheap and they matter.
FAQ
How fast does adenosine work after the push? Conversion typically occurs within 30 seconds of the push, often during the saline flush. The drug's half-life is under 10 seconds, so if the rhythm hasn't converted by about a minute, it isn't going to with that dose. That speed is why the push technique — large vein, rapid injection, immediate flush — determines success more than the dose itself.
Can I give adenosine if the patient has asthma? Avoid it in severe asthma or active bronchospasm, because adenosine can trigger bronchoconstriction through mast cell activation. Mild, well-controlled asthma is a judgment call, and many clinicians proceed with monitoring. If the wheeze is active or the patient has needed recent systemic steroids, choose diltiazem or a beta-blocker instead, assuming no contraindication.
What if the first 6 mg dose doesn't convert the rhythm? Wait 1 to 2 minutes, then give 12 mg with the same rapid push and flush. A second failure should prompt reassessment of the diagnosis before a third dose. Atrial flutter and focal atrial tachycardia often slow with adenosine without converting, and that partial response is diagnostic information, not a reason to keep pushing the same drug.
Is adenosine safe in pregnancy? Yes, it's the preferred agent for stable SVT in pregnancy because its half-life is very short and placental transfer is minimal, per AHA guidance. The same 6 mg starting dose applies. Diltiazem and beta-blockers carry more fetal considerations, so adenosine is the practical first choice when the patient is stable and there's no asthma contraindication.
What's the difference between adenosine and diltiazem for SVT? Adenosine acts in seconds and self-terminates; diltiazem takes 2 to 5 minutes and lasts hours. Adenosine is better for diagnosis and for a first attempt; diltiazem is better when the rhythm keeps recurring or when adenosine is contraindicated. Diltiazem is unsafe in heart failure with reduced ejection fraction and in pre-excitation, which limits it in those patients.
Can adenosine cause a dangerous rhythm? It can precipitate atrial fibrillation in a minority of patients, usually transient. The dangerous scenario is pre-excited atrial fibrillation, where adenosine can accelerate conduction down an accessory pathway and trigger ventricular fibrillation. That's why an irregular wide-complex tachycardia is a contraindication, and why confirming the rhythm is narrow and regular comes before the push.
Why do some patients need cardioversion instead of adenosine? Cardioversion is for instability: hypotension, altered mental status, ischemic chest pain, acute heart failure, or shock. In those patients, the seconds adenosine takes to work are seconds the patient doesn't have, and electricity is faster and more reliable. In a stable patient, the drug is preferred because it avoids sedation and the shock itself.
Does a failed adenosine dose mean the diagnosis is wrong? Not necessarily. Poor IV access, a slow push, or a dose that was partially metabolized before reaching the heart can all cause failure with a correct diagnosis. But two failed doses should prompt a fresh look at the rhythm, because atrial flutter, focal atrial tachycardia, and pre-excited rhythms can mimic AVNRT and behave differently under adenosine.
This article is general educational information about a common emergency medication decision. It is not a diagnosis, treatment plan, or substitute for clinical judgment. Drug doses, contraindications, and protocols vary by institution and patient, and a qualified local clinician should make the treating decision. In an emergency, contact local emergency services immediately.