Is the success rate of intravenous adenosine for terminating supraventricular tachycardia in adults dependent on patient population, and what factors are associated with failure?

Success Rate Variability and Failure Predictors for Adenosine in SVT

Adenosine's success rate for terminating supraventricular tachycardia is highly dependent on patient population, with conversion rates ranging from 57% to 96% depending on specific patient factors, and failure is most commonly associated with inadequate dosing relative to body weight, competitive antagonism from methylxanthines, and recurrence rather than true non-response. 1

Population-Dependent Success Rates

By SVT Mechanism

• AVNRT (AV nodal reentrant tachycardia) achieves the highest conversion rates at 90–95%, making it the most adenosine-responsive population. 1
• AVRT (AV reentrant tachycardia with accessory pathway) demonstrates slightly lower success at 78–96%, still highly effective but with greater variability. 1
• Atrial flutter and atrial tachycardia do not convert with adenosine because the AV node is not part of the reentrant circuit; adenosine only unmasks these rhythms diagnostically by producing transient AV block. 1

By Clinical Setting

• Emergency department populations show 96% initial conversion but experience 57% recurrence within 5 minutes, requiring subsequent therapy with longer-acting agents rather than repeat adenosine doses. 2
• Electrophysiology laboratory studies (controlled induction) report 81% conversion with 12 mg and 94% conversion with 18 mg, demonstrating that higher doses overcome initial failures in most cases. 3
• Hemodynamically stable outpatients achieve 93% mean success across pooled studies of over 600 episodes, representing the most favorable population. 4

Common Factors Associated with Adenosine Failure

Inadequate Weight-Based Dosing

• Patients receiving ≥0.1 mg/kg adenosine have significantly higher first-dose conversion rates (p=0.006) compared to those receiving <0.1 mg/kg, with no increase in adverse effects. 5
• The standard 6 mg initial dose is suboptimal for patients weighing >60 kg; doubling the initial dose to 12 mg increases first-dose conversion likelihood and reduces the need for multiple administrations. 2

Competitive Antagonism from Methylxanthines

• Theophylline, caffeine, and theobromine act as competitive adenosine-receptor antagonists and require larger doses for effective AV-nodal blockade. 1
• Patients with significant blood levels of these substances may require doses up to 18 mg to achieve conversion. 1, 3

Rapid Metabolism Before Reaching the Heart

• Adenosine has a half-life of <10 seconds, requiring rapid bolus administration (1–2 seconds) through the most proximal IV access (antecubital preferred) followed immediately by a 20 mL saline flush. 1
• Distal IV sites or slow injection result in complete drug degradation before reaching the AV node, mimicking true failure. 1

Recurrence Versus True Non-Response

• True non-response (failure to convert with adequate dosing) occurs in only 4–9% of eligible patients when maximum doses (up to 30 mg cumulative) are used. 3, 6
• Immediate recurrence (conversion followed by reinitiation within seconds to minutes) occurs in 57% of emergency department cases and represents electrical instability rather than drug failure. 2
• Recurrence is triggered by premature atrial or ventricular complexes post-conversion and requires a longer-acting AV-nodal blocker (diltiazem or β-blocker) rather than repeat adenosine. 1

Populations Excluded from Adenosine Therapy

Absolute Contraindications

• Active asthma or bronchospasm – risk of severe, life-threatening bronchospasm. 1
• Second- or third-degree AV block or sick-sinus syndrome without a pacemaker – risk of prolonged asystole. 1
• Pre-excited atrial fibrillation (Wolff-Parkinson-White) – adenosine may precipitate rapid ventricular response or ventricular fibrillation by enhancing accessory-pathway conduction. 1, 7

Hemodynamically Unstable Patients

• Patients with hypotension, altered mental status, shock, chest pain, or acute heart failure were excluded from efficacy trials because they require immediate synchronized cardioversion (near-100% success) rather than pharmacologic therapy. 7, 8

Dose-Response Relationship

• 3 mg cumulative: 35.2% conversion 6
• 6 mg cumulative: 57.4–70% conversion 2, 6
• 12 mg cumulative: 81–93.4% conversion 3, 6
• 18 mg cumulative: 94% conversion 3
• 30 mg cumulative (6 + 12 + 12 mg): 91.4% conversion 6

The dose-response curve demonstrates that most failures at lower doses are overcome by escalation to 12–18 mg, with true refractory cases representing <6% of the eligible population. 3, 6

Special Populations Requiring Dose Adjustment

Reduced Initial Dose (3 mg)

• Dipyridamole or carbamazepine therapy – these drugs potentiate adenosine's effects. 1
• Cardiac transplant recipients – denervated hearts are hypersensitive to adenosine. 1
• Central venous access – faster delivery to the heart requires lower doses. 1

Increased Dose Requirements

• Patients on methylxanthines (theophylline, high caffeine intake) may require 18 mg single bolus for effective conversion. 1, 3

Key Clinical Pitfall

The most common "failure" of adenosine is not true drug resistance but rather recurrence within 5 minutes, which occurs in 57% of emergency department cases and should be managed with a longer-acting AV-nodal blocker (IV diltiazem 15–20 mg or IV metoprolol 2.5–5 mg) rather than repeated adenosine doses. 2, 1