What is the initial treatment for a patient presenting with supraventricular tachycardia (SVT)?

Initial Treatment for Supraventricular Tachycardia

Begin with vagal maneuvers immediately in all hemodynamically stable patients with SVT, followed by intravenous adenosine if vagal maneuvers fail; proceed directly to synchronized cardioversion in any hemodynamically unstable patient. 1

Immediate Hemodynamic Assessment

First, determine hemodynamic stability before any intervention. 2

• If the patient shows signs of hemodynamic instability (hypotension, altered mental status, chest pain, acute heart failure), proceed directly to synchronized cardioversion at 50-100 J biphasic energy without attempting vagal maneuvers or medications 1
• Hemodynamically stable patients should follow the stepwise approach below 2

Step 1: Vagal Maneuvers (First-Line for Stable Patients)

The modified Valsalva maneuver is the most effective vagal technique and should be attempted first. 1, 3

Modified Valsalva Maneuver Technique:

• Patient bears down against a closed glottis for 10-30 seconds (generating at least 30-40 mm Hg intrathoracic pressure) while supine 2
• Immediately after bearing down, lay the patient flat with legs elevated 1
• Success rate: 43-54% for rhythm conversion 1, 4
• This technique is significantly more effective than standard carotid sinus massage (SUCRA: 0.9992 vs 0.0613) 3

Alternative Vagal Maneuvers if Modified Valsalva Fails:

• Carotid sinus massage: Apply steady pressure over right or left carotid sinus for 5-10 seconds after confirming absence of bruit by auscultation 2
• Ice-cold wet towel to face (diving reflex) 2
• Combined success rate when switching between techniques: 27.7% 2

Critical pitfall: Never apply pressure to the eyeball—this practice is dangerous and has been abandoned. 2

Step 2: Intravenous Adenosine (If Vagal Maneuvers Fail)

Adenosine is the next intervention with 90-95% effectiveness for terminating SVT. 1, 4

Adenosine Administration Protocol:

• Initial dose: 6 mg rapid IV push through a large peripheral vein, followed immediately by 20 mL saline flush 1
• Administer via proximal IV as rapid bolus 2
• If no response, may give 12 mg dose 1
• Success rate: 78-96% for AVNRT and AVRT 2

Dosing Adjustments:

• Reduce to 3 mg for patients taking dipyridamole, carbamazepine, or with transplanted heart 1
• Larger doses may be needed with theophylline, caffeine, or theobromine 1

Critical Safety Measures:

• Have electrical cardioversion equipment immediately available 1
• Continuous ECG recording during administration helps distinguish drug failure from successful termination with immediate reinitiation 2
• Contraindicated in asthma patients due to risk of severe bronchoconstriction 1
• Side effects (chest discomfort, shortness of breath, flushing) are common but serious adverse effects are rare due to very short half-life 2

Step 3: Synchronized Cardioversion (If Adenosine Fails or Patient Unstable)

Synchronized cardioversion is indicated for hemodynamically unstable patients or after failed pharmacotherapy in stable patients. 1

• Initial biphasic energy: 50-100 J, increase stepwise if initial shock fails 1
• Provide adequate sedation/anesthesia in stable patients before cardioversion 1

Critical Safety Warnings

Avoid AV nodal blocking agents (verapamil, diltiazem, adenosine) in patients with wide-complex tachycardia or known accessory pathways—these can precipitate ventricular fibrillation. 1

Specific Contraindications:

• Never use verapamil or diltiazem in patients with systolic heart failure or wide-complex tachycardia of uncertain etiology due to risk of hemodynamic collapse 1
• Distinguish VT from SVT with aberrant conduction before administering any AV nodal blocking agent 2
• Administration of verapamil or diltiazem for pre-excited atrial fibrillation may accelerate ventricular rate and lead to ventricular fibrillation 2

Special Considerations

Vagal maneuvers and adenosine will not be effective if the rhythm does not involve the AV node as a requisite component of a reentrant circuit. 2

• Automatic tachycardias (ectopic atrial tachycardia, multifocal atrial tachycardia, junctional tachycardia) are not responsive to cardioversion and require rate control with AV nodal blocking agents 1
• Adenosine may be useful diagnostically to unmask atrial flutter or atrial tachycardia, but uncommonly terminates these arrhythmias 2
• Record a 12-lead ECG to differentiate tachycardia mechanisms and guide treatment 2