What is the next step in treating Supraventricular Tachycardia (SVT) when vagal maneuvers fail?

Treatment for SVT When Vagal Maneuvers Fail

Administer adenosine 6 mg as a rapid IV push through a large proximal vein (such as antecubital) followed immediately by a 20 mL saline flush. 1, 2

Adenosine Administration Protocol

• Initial dose: Give 6 mg IV push rapidly through a large proximal vein, followed immediately by 20 mL saline flush 1, 2
• Second dose: If no conversion within 1-2 minutes, give 12 mg IV push using the same technique 1, 2
• Third dose: If still no conversion, may give another 12 mg IV push 1, 2
• Success rate: Adenosine terminates approximately 95% of AVNRT and 78-96% of reentrant SVTs involving the AV node 1, 2

Critical Safety Considerations

• Have a defibrillator immediately available when administering adenosine, particularly if Wolff-Parkinson-White syndrome is a consideration, as adenosine can precipitate atrial fibrillation with rapid ventricular rates 1, 2
• Contraindicated in asthma due to risk of bronchospasm 2
• Dose adjustments required: Reduce initial dose to 3 mg in patients taking dipyridamole or carbamazepine, those with transplanted hearts, or if given via central venous access 2
• Higher doses may be needed in patients with significant theophylline, caffeine, or theobromine levels 1, 2

If Adenosine Fails or SVT Recurs

For Hemodynamically Stable Patients:

• Intravenous diltiazem or verapamil are reasonable next options, with conversion rates of 64-98% 1

  • These calcium channel blockers should be infused slowly over up to 20 minutes to minimize hypotension 1
  • Do not use if VT or pre-excited atrial fibrillation is suspected, as these can cause hemodynamic collapse or ventricular fibrillation 1
  • Avoid in systolic heart failure 1

• Intravenous beta blockers (metoprolol or propranolol) are reasonable alternatives 1

  • Beta blockers have an excellent safety profile but may be less effective than calcium channel blockers for acute termination 1
  • In pregnancy, IV metoprolol or propranolol are reasonable when adenosine fails 1

For Hemodynamically Unstable Patients:

• Proceed immediately to synchronized cardioversion if the patient develops hypotension, altered mental status, signs of shock, chest pain, or acute heart failure 1
• Initial energy: 50-100 J for SVT (biphasic waveform) 1
• Increase stepwise if initial shock fails 1
• Cardioversion successfully restores sinus rhythm in essentially all hemodynamically unstable SVT patients who failed pharmacological therapy 1

Important Diagnostic Considerations

• Continuous ECG recording during adenosine administration helps distinguish between drug failure and successful termination with immediate reinitiation 2
• If adenosine unmasks atrial flutter or atrial tachycardia rather than terminating the rhythm, this indicates the arrhythmia does not involve the AV node in a reentrant circuit 1, 2
• Automatic tachycardias (ectopic atrial tachycardia, multifocal atrial tachycardia, junctional tachycardia) will not respond to adenosine or cardioversion and require rate control with AV nodal blocking agents 1

Common Pitfalls to Avoid

• Slow IV push renders adenosine ineffective due to its 0.6-10 second half-life—must be given as rapid bolus 2
• Inadequate saline flush prevents adenosine from reaching central circulation quickly enough 1, 2
• Using peripheral IV instead of proximal vein reduces effectiveness 2
• Administering calcium channel blockers or beta blockers in pre-excited atrial fibrillation (Wolff-Parkinson-White with atrial fibrillation) can cause ventricular fibrillation—these are Class III contraindications 1
• Delaying cardioversion in unstable patients to attempt additional drug therapy worsens outcomes 1

Special Populations

• Pregnancy: Adenosine is safe and effective; it is the first-line drug after vagal maneuvers fail 1, 2
• Pediatrics: Adenosine remains first-line with the same dosing protocol and excellent safety profile 3