When to use electrical cardioversion (defibrillation) versus adenosine for supraventricular tachycardia (SVT)?

When to Use Electrical Cardioversion vs Adenosine for SVT

Synchronized electrical cardioversion is indicated immediately for hemodynamically unstable SVT (hypotension, syncope, myocardial ischemia, or severe heart failure), while adenosine is the first-line pharmacologic treatment for hemodynamically stable SVT after vagal maneuvers fail. 1, 2

Hemodynamic Status Determines the Approach

Unstable SVT: Immediate Cardioversion

• Perform synchronized cardioversion immediately if the patient shows signs of hemodynamic instability, including hypotension, altered mental status, signs of shock, ongoing chest pain suggesting ischemia, or acute heart failure 1
• Use initial energy of 50-100 J with a biphasic defibrillator for SVT 3
• However, recent evidence suggests that if the tachycardia is regular with narrow QRS complex, a single adenosine trial may be reasonable even in unstable patients before proceeding to cardioversion, as it may avoid sedation-related risks 4
• A 2025 multicentre study found adenosine may be safely attempted first in unstable patients, with electrical cardioversion having only a modest advantage (weighted OR 2.41) and no complications observed in either group 4

Stable SVT: Stepwise Pharmacologic Approach

• Begin with vagal maneuvers (Valsalva maneuver for 10-30 seconds, carotid massage for 5-10 seconds after confirming no bruit, or ice-cold wet towel to face) as first-line intervention 1, 2
• If vagal maneuvers fail, administer adenosine 6 mg as rapid IV push through a large proximal vein, followed immediately by 20 mL saline flush 1, 2
• If no conversion within 1-2 minutes, give 12 mg IV push; may repeat 12 mg dose once more if needed 2, 5
• Success rates for adenosine in terminating AVNRT and AVRT range from 78-96%, with some studies reporting up to 93% overall success 2, 6

Critical Decision Points

When Adenosine is Preferred Over Cardioversion

• Regular narrow-complex tachycardia in stable patients is the classic indication for adenosine 2
• Pregnancy: adenosine is safe and effective at all stages, with minimal fetal exposure due to its extremely short half-life 1, 2
• Diagnostic uncertainty: adenosine serves dual therapeutic and diagnostic purposes, potentially unmasking atrial flutter or atrial tachycardia 2
• Avoiding sedation risks: in borderline unstable patients, adenosine avoids the need for procedural sedation required for cardioversion 4

When to Skip Adenosine and Proceed to Cardioversion

• Frank hemodynamic collapse or cardiac arrest requires immediate cardioversion 1, 6
• Irregular narrow-complex tachycardia (likely atrial fibrillation) will not respond to adenosine 2
• Known asthma or active bronchospasm is an absolute contraindication to adenosine due to risk of severe bronchospasm 2, 5
• Second or third-degree AV block or sick sinus syndrome contraindicate adenosine 5
• Wide-complex tachycardia of uncertain etiology: while adenosine can be diagnostic, cardioversion is safer if ventricular tachycardia cannot be excluded 6

Important Caveats and Pitfalls

Adenosine Administration Technique

• Must be given as rapid IV push followed immediately by saline flush through a large proximal vein; failure to do so dramatically reduces efficacy 2, 5
• Have a defibrillator immediately available when administering adenosine, especially if Wolff-Parkinson-White syndrome is a consideration, as adenosine can precipitate atrial fibrillation with rapid ventricular rates 2
• Reduce initial dose to 3 mg in patients taking dipyridamole or carbamazepine, those with transplanted hearts, or if giving via central venous access 2
• Increase doses may be needed in patients with significant caffeine, theophylline, or theobromine levels 2

Post-Conversion Management

• Monitor for recurrence for at least 1-2 hours after successful conversion, as SVT commonly recurs due to adenosine's short half-life 2, 5
• If SVT recurs, repeat adenosine or consider longer-acting AV nodal blockers (diltiazem or beta-blockers) rather than immediately proceeding to cardioversion 2, 3
• If adenosine repeatedly fails after appropriate dosing, then proceed to synchronized cardioversion rather than continuing to escalate pharmacologic therapy 5

Expected Transient Effects

• Flushing, dyspnea, and chest discomfort occur in approximately 30% of patients but last less than 60 seconds and require no treatment 5, 6
• Warn patients before administration that they will experience brief but uncomfortable sensations 6

Special Populations

• Pregnant patients: adenosine is the first-line drug after vagal maneuvers fail; if ineffective, use IV metoprolol or propranolol before considering cardioversion 1
• Pediatric patients: initial dose is 0.05 mg/kg IV rapid bolus, increased in 0.05 mg/kg increments up to maximum 0.25 mg/kg 5

The Bottom Line Algorithm

1. Assess hemodynamic stability first
2. If unstable (hypotension, altered mental status, ischemia, heart failure): proceed to synchronized cardioversion 50-100 J, though a single adenosine trial is reasonable if narrow-complex and regular 1, 4
3. If stable: attempt vagal maneuvers first 1, 2
4. If vagal maneuvers fail: give adenosine 6 mg rapid IV push with saline flush (unless contraindicated by asthma, high-grade AV block, or sick sinus syndrome) 2, 5
5. If no conversion in 1-2 minutes: give adenosine 12 mg, may repeat once 2
6. If adenosine fails after appropriate dosing or is contraindicated: proceed to synchronized cardioversion 1, 5