Mechanism of Action of Adenosine in Treating Supraventricular Tachycardia
Adenosine terminates SVT primarily by activating purine receptors that cause transient atrioventricular (AV) nodal conduction block, interrupting reentrant circuits that involve the AV node. 1
Primary Mechanism
Adenosine works through several key mechanisms:
- Purine receptor activation: Adenosine exerts its pharmacological effects by activating cell-surface A1 and A2 adenosine receptors 1
- AV nodal effects: Through A1 receptor activation, adenosine causes:
- Direct negative chronotropic (heart rate) effect
- Direct negative dromotropic (conduction) effect
- Direct negative inotropic (contractility) effect 1
- Reentry circuit interruption: By blocking the anterograde AV nodal limb of a reentrant circuit, adenosine effectively terminates SVT that involves the AV node in the reentry pathway 2, 3
Pharmacokinetics Supporting Rapid Action
Adenosine's effectiveness in SVT is enhanced by its unique pharmacokinetic properties:
- Ultrashort half-life: Less than 10 seconds in whole blood 1, 2
- Rapid cellular uptake: Primarily by erythrocytes and vascular endothelial cells via a specific transmembrane nucleoside carrier system 1
- Quick metabolism: Once inside cells, adenosine is rapidly phosphorylated by adenosine kinase to adenosine monophosphate or deaminated by adenosine deaminase to inosine 1
Differential Effects on SVT Subtypes
Adenosine's effectiveness varies by SVT mechanism:
- Highly effective (90-95%) for SVTs involving the AV node 4, 2:
- Atrioventricular nodal reentrant tachycardia (AVNRT)
- Atrioventricular reentrant tachycardia (AVRT)
- Variable effectiveness for focal atrial tachycardia (AT):
- Effective for triggered focal AT
- Less effective for reentrant focal AT
- Minimal effect on automatic focal AT 4
- Diagnostic utility: During transient AV block, adenosine can help differentiate between AVNRT/AVRT and other arrhythmias by unmasking atrial activity 4, 2
Clinical Application
The American College of Cardiology/American Heart Association/Heart Rhythm Society guidelines recommend:
- Initial dose: 6 mg rapid IV bolus
- If ineffective: Up to 2 subsequent doses of 12 mg may be administered
- Administration technique: Rapid bolus via proximal IV access, followed immediately by saline flush 4, 5
Practical Considerations and Precautions
- Transient side effects: Chest discomfort, flushing, dyspnea typically lasting less than 1 minute 2, 6
- Potential for arrhythmias: May precipitate atrial fibrillation that could conduct rapidly in patients with accessory pathways 4
- Special populations: Safe in pregnancy due to its short half-life, making it unlikely to reach fetal circulation 4
- Contraindications: Use with caution in patients with severe bronchospastic lung disease or significant heart block 5
Comparison to Other Treatments
Adenosine offers several advantages over alternative treatments:
- Faster onset of action compared to calcium channel blockers and beta blockers 2
- Lower risk of serious adverse effects than verapamil, particularly avoiding risks of cardiac arrest and hypotension 2, 6
- Equivalent efficacy to verapamil in adults for terminating PSVT 2
Adenosine's unique mechanism of action and pharmacokinetic profile make it the preferred first-line pharmacological agent for acute termination of SVT after vagal maneuvers have failed, particularly when the arrhythmia involves the AV node in the reentry circuit.