What is the role of adenosine in treating supraventricular tachycardia (SVT)?

Adenosine for Supraventricular Tachycardia

Adenosine is the first-line pharmacologic agent for terminating hemodynamically stable supraventricular tachycardia (SVT) after vagal maneuvers fail, with success rates of 78-96% for AVNRT and AVRT. 1

Initial Management Algorithm

Step 1: Assess Hemodynamic Stability

• If hemodynamically unstable (hypotension, altered mental status, shock, chest pain, acute heart failure): proceed immediately to synchronized cardioversion 2
• Adenosine may be considered first only if the tachycardia is regular with narrow QRS complex, but defibrillator must be immediately available 2

Step 2: For Stable Patients

• Attempt vagal maneuvers first 1
• If vagal maneuvers fail, proceed to adenosine administration 1

Administration Protocol

Dosing Regimen

• Initial dose: 6 mg IV push via large proximal vein, followed immediately by 20 mL saline flush 1
• If no conversion within 1-2 minutes: 12 mg IV push with saline flush 1
• If still no conversion: repeat 12 mg IV push one additional time 1
• Administer as rapid bolus—this is critical for efficacy 1

Dose Modifications Required

• Reduce initial dose to 3 mg in patients: 1
  • Taking dipyridamole or carbamazepine
  • With transplanted hearts
  • Receiving central venous administration
• Increase dose in patients with significant blood levels of theophylline, caffeine, or theobromine 1

Mechanism and Effectiveness

Adenosine terminates approximately 95% of AVNRT cases and 78-96% of AVRT cases by causing transient AV nodal block. 1 The drug works by activating A1 receptors in the AV node, producing direct negative chronotropic and dromotropic effects 3. It has an extremely short half-life of less than 10 seconds, as it is rapidly taken up by erythrocytes and vascular endothelial cells 3.

Diagnostic Value

Beyond its therapeutic role, adenosine serves as a diagnostic tool: 1

• Continuous ECG recording during administration helps distinguish drug failure from successful termination with immediate reinitiation
• Can unmask underlying atrial flutter or atrial tachycardia by producing AV block
• Helps differentiate SVT from other narrow-complex tachyarrhythmias

Critical Safety Considerations and Contraindications

Absolute Contraindications

• Asthma or bronchospasm: adenosine can cause severe bronchospasm 1, 3
• High-grade AV block or sinus node dysfunction (unless functioning pacemaker present) 3
• Bronchoconstriction or bronchospastic lung disease 3

Serious Risks Requiring Immediate Defibrillator Availability

• Atrial fibrillation induction (1-15% incidence): can conduct rapidly in patients with accessory pathways (Wolff-Parkinson-White syndrome) and potentially trigger ventricular fibrillation 2, 3
• Fatal and nonfatal cardiac arrest, sustained ventricular tachycardia, and myocardial infarction have occurred 3
• AV block develops in approximately 6% of patients (first-degree 3%, second-degree 3%, third-degree 0.8%) 3

Use with Caution

• Pre-existing first-degree AV block or bundle branch block 3
• Obstructive lung disease without bronchoconstriction (emphysema, bronchitis) 3
• Autonomic dysfunction, hypovolemia, stenotic valvular disease, pericarditis, or stenotic carotid artery disease 3
• Acute myocardial ischemia or unstable angina 3

Common Side Effects

Most side effects are transient, lasting less than 60 seconds: 1

• Flushing (most common)
• Dyspnea
• Chest discomfort
• Headache
• These are dose-dependent and self-limiting

Post-Conversion Management

• Monitor for recurrence after successful conversion 1
• If SVT recurs: treat with adenosine again or consider longer-acting AV nodal blocking agent (diltiazem or β-blocker) 1
• If adenosine unmasks another form of SVT (atrial flutter, atrial tachycardia): consider longer-acting AV nodal blocking agent 1

Special Populations

Pregnancy

• Adenosine is safe and effective during pregnancy 1

Renal and Hepatic Impairment

• No dose adjustment needed—adenosine does not require renal or hepatic function for activation or inactivation 3

Common Pitfalls to Avoid

• Inadequate flush: must use 20 mL saline flush immediately after bolus to ensure drug reaches central circulation 1
• Slow administration: adenosine must be given as rapid IV push; slow administration results in peripheral metabolism before reaching the heart 1
• Wrong initial dose: starting with 12 mg instead of 6 mg increases unnecessary side effects, as 70% of patients convert with 6 mg or less 1
• Failure to have defibrillator available: particularly dangerous in patients with potential accessory pathways 2
• Use in asthmatic patients: absolute contraindication due to bronchospasm risk 1, 3