What class of drug is adenosine, how does it act to treat narrow‑complex supraventricular tachycardia, and what key information must a paramedic know for its administration?

Adenosine for Narrow Complex Tachycardia: Paramedic Essential Knowledge

Adenosine is an endogenous nucleoside that acts as an ultra-short-acting antiarrhythmic agent by blocking the atrioventricular (AV) node and decreasing automaticity, making it the first-line pharmacologic treatment for narrow-complex supraventricular tachycardia (SVT) after vagal maneuvers fail. 1

Drug Classification and Mechanism of Action

Adenosine functions as a purine nucleoside that works through specific adenosine receptors in cardiac tissue. Its mechanism is straightforward:

• Blocks AV nodal conduction by hyperpolarizing cardiac cells and slowing calcium channel recovery
• Decreases automaticity in cardiac pacemaker cells
• Terminates reentrant circuits that depend on the AV node (the mechanism in most narrow-complex SVTs)
• Has an extremely short half-life of less than 10 seconds, meaning effects are transient 1, 2

The drug specifically targets rhythms where the AV node is part of the reentrant circuit—including AV nodal reentrant tachycardia (AVNRT) and AV reentrant tachycardia (AVRT)—with conversion rates of 90-98% 1.

Critical Dosing Protocol

The dosing sequence is non-negotiable and must be followed precisely:

1. First dose: 6 mg rapid IV push followed immediately by normal saline flush 1
2. Second dose: 12 mg if no effect within 2 minutes 1
3. Third dose: 12 mg may be repeated once more if needed

Administration Technique (Critical for Success)

• Must be given as rapid IV bolus through a peripheral line as close to the heart as possible (antecubital preferred over hand/wrist)
• Immediate saline flush (5-20 mL) is mandatory to push the drug centrally before it degrades 2
• The short half-life means slow administration or distal IV sites will result in failure 1
• Patients with central lines or post-heart transplant require only 1 mg due to direct cardiac delivery 1

Recent evidence suggests weight-based dosing (≥0.1 mg/kg) may improve first-dose conversion rates, though this is not yet standard protocol 3.

Absolute Contraindications (Do Not Give)

Stop and choose cardioversion instead if the patient has:

• Asthma or active bronchospasm (can cause severe, life-threatening bronchospasm) 1, 2
• Second- or third-degree AV block without a pacemaker 2
• Sick sinus syndrome or symptomatic bradycardia without a pacemaker 2
• Known hypersensitivity to adenosine 2
• Wide-complex tachycardia of uncertain origin (may precipitate ventricular fibrillation or profound hypotension) 1

Expected Patient Experience and Side Effects

Warn the patient before administration that they will experience unpleasant but brief (15-60 seconds) symptoms:

• Chest pressure or discomfort (most common)
• Flushing and feeling of impending doom
• Dyspnea or "can't breathe" sensation
• Headache
• Nausea

These effects are diagnostic of adequate dosing and resolve within 1 minute without treatment 1. If the patient experiences nothing, the dose likely didn't reach the heart effectively.

Life-Threatening Complications (Rare but Critical)

While adenosine is generally safe, have resuscitation equipment immediately available because:

• Ventricular fibrillation can occur (incidence <1%) 2, 4
• Profound hypotension leading to shock 1, 2
• Cardiac arrest (rare but reported) 2
• Seizures (new onset or recurrent) 2
• Atrial fibrillation may be induced, typically lasting 15 seconds to 6 hours 2
• Prolonged asystole (usually 3-5 seconds, but can be longer) 2

Defibrillator must be immediately available before administration 1.

Drug Interactions Paramedics Must Know

Drugs That Block Adenosine (May Cause Failure)

• Methylxanthines (caffeine, theophylline, aminophylline) antagonize adenosine's effects 2
• Recent coffee consumption may require higher doses
• Do not use methylxanthines to treat adenosine-induced seizures 2

Drugs That Potentiate Adenosine (Increase Risk)

• Dipyridamole (Persantine) dramatically increases adenosine effects—use extreme caution or avoid 2
• Carbamazepine may enhance effects 5

Additive Effects

• Beta-blockers, calcium channel blockers, digoxin may cause additive AV nodal blockade 2
• Use with caution but not contraindicated

Diagnostic Value Beyond Treatment

Adenosine serves dual purposes:

1. Therapeutic: Terminates AV node-dependent SVTs
2. Diagnostic: Temporarily blocks the AV node, revealing underlying atrial activity 1

If the rhythm doesn't convert but you see atrial flutter or fibrillation during the brief AV block, you've identified the rhythm and can proceed with appropriate rate control rather than repeated adenosine.

When Adenosine Fails or Is Inappropriate

If adenosine doesn't work or is contraindicated:

• Hemodynamically unstable patients (SBP <80 mmHg, altered mental status, chest pain, acute heart failure): immediate synchronized cardioversion 1
• Stable patients with failed adenosine: Consider diltiazem or verapamil (though these cause more hypotension and must be given slowly) 1
• Recurrent SVT after conversion: Longer-acting agents like diltiazem may be needed 1

Special Populations

Pregnancy

• Adenosine is safe in pregnancy and preferred over other antiarrhythmics 6

Elderly

• No dose adjustment needed, but monitor closely for prolonged AV block 2

Heart Failure

• Use with caution; hypotension risk is higher 2
• Adenosine is still preferred over calcium channel blockers in this population

Common Pitfalls to Avoid

1. Slow push or inadequate flush: Results in drug degradation before reaching the heart
2. Distal IV site: Use antecubital or more proximal access
3. Not warning the patient: Unprepared patients may panic from the chest discomfort
4. Giving to wide-complex tachycardia of unknown origin: Can precipitate VF 1
5. Using in known asthma: Absolute contraindication 2
6. Not having resuscitation equipment ready: Rare but serious complications require immediate intervention
7. Stopping after first 6 mg dose fails: The 12 mg dose has significantly higher success rates

Documentation Requirements

Record on your patient care report:

• Exact dose and time of each administration
• Rhythm before and after (ideally with rhythm strips)
• Patient symptoms during administration
• Any complications or adverse effects
• Total number of doses required for conversion