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

Initial Treatment for Supraventricular Tachycardia (SVT)

Begin with vagal maneuvers in hemodynamically stable patients, followed immediately by adenosine if unsuccessful; proceed directly to synchronized cardioversion in unstable patients without attempting vagal maneuvers or medications. 1, 2, 3

Immediate Assessment: Hemodynamic Stability

First, determine if the patient is hemodynamically unstable, defined by hypotension, acutely altered mental status, signs of shock, chest pain, or acute heart failure symptoms. 1, 2

For Hemodynamically Unstable Patients

Perform immediate synchronized cardioversion without attempting vagal maneuvers or pharmacological therapy. 1, 2 This approach successfully restores sinus rhythm in all hemodynamically unstable SVT patients and avoids delays and complications from drug therapy. 1, 2

For Hemodynamically Stable Patients: Stepwise Algorithm

Step 1: Vagal Maneuvers (First-Line)

Attempt vagal maneuvers immediately with the patient in the supine position. 1

• Modified Valsalva maneuver is 2.8-3.8 times more effective than standard technique and should be attempted first. 2 Have the patient bear down against a closed glottis for 10-30 seconds, equivalent to at least 30-40 mm Hg intrathoracic pressure. 1
• Carotid sinus massage can be performed after confirming absence of bruit by auscultation, applying steady pressure over the right or left carotid sinus for 5-10 seconds. 1
• Ice-cold towel to face or facial immersion in 10°C water activates the diving reflex and can terminate tachycardia. 1
• Overall success rate is approximately 27.7%, with switching between techniques improving outcomes. 1, 3

Critical caveat: Vagal maneuvers only work for SVTs involving the AV node as part of the reentrant circuit (AVNRT, AVRT) and will not terminate automatic atrial tachycardias. 1, 2

Step 2: Adenosine (Second-Line)

If vagal maneuvers fail, adenosine is the next step with Class I, Level B-R recommendation. 1

• Success rates are 78-96% in emergency department settings, and 90-95% specifically for AVNRT and orthodromic AVRT. 1, 2, 4
• Administer as a rapid bolus via proximal IV followed immediately by saline flush. 1
• Standard dosing: 6 mg initial bolus, followed by up to two additional 12 mg boluses if needed at 1-2 minute intervals. 5
• Maintain continuous ECG recording during administration to distinguish drug failure from successful termination with immediate reinitiation. 1

Important safety considerations:

• Side effects (chest discomfort, shortness of breath, flushing) occur in approximately 30% of patients but are brief (<1 minute) due to the drug's half-life of only 0.6-10 seconds. 1, 4, 5
• Electrical cardioversion equipment must be immediately available because adenosine may precipitate atrial fibrillation that could conduct rapidly and potentially cause ventricular fibrillation, particularly in patients with accessory pathways. 1, 6
• While extremely rare, adenosine can induce ventricular fibrillation even in structurally normal hearts without accessory pathways, though this idiosyncratic reaction has been reported in fewer than 2% of cases. 6
• Adenosine is diagnostically useful as it will unmask atrial flutter or atrial tachycardia through transient AV block, though it rarely terminates these rhythms. 1

Step 3: IV Calcium Channel Blockers or Beta Blockers (Third-Line)

If adenosine fails or causes immediate recurrence, consider IV diltiazem, verapamil, or beta blockers. 1, 2

IV diltiazem or verapamil (Class IIa, Level B-R):

• Success rates of 64-98% for terminating SVT. 1, 2
• Administer as slow infusion over up to 20 minutes to minimize hypotension risk. 1, 2
• Absolute contraindications: Do not use if VT is suspected, in pre-excited atrial fibrillation, or in patients with suspected systolic heart failure. 1, 2 Giving these agents to VT patients can cause hemodynamic collapse or ventricular fibrillation. 2

IV beta blockers (Class IIa, Level C-LD):

• Reasonable alternative with excellent safety profile, though limited evidence shows they are less effective than diltiazem. 1, 2
• Options include metoprolol, esmolol, or propranolol. 2

Step 4: Synchronized Cardioversion (Fourth-Line for Stable Patients)

When pharmacological therapy is ineffective or contraindicated in stable patients, perform synchronized cardioversion. 1, 2

• Highly effective with success rates approaching 100% for terminating SVT. 1, 2
• Perform after adequate sedation or anesthesia with initial energy of 50-100J. 2
• Do not perform cardioversion if SVT is terminating and reinitiating spontaneously. 1, 2

Critical Pitfalls to Avoid

• Never administer verapamil, diltiazem, or beta blockers to wide-complex tachycardia of uncertain etiology until VT is definitively excluded, as this can cause catastrophic hemodynamic collapse. 1, 2
• Never perform carotid massage without first auscultating for bruits. 1, 2
• Abandon the outdated practice of applying pressure to the eyeball, as this is potentially dangerous. 1, 2
• Ensure resuscitation and defibrillation equipment is immediately available whenever administering adenosine. 6

Recent Evidence Supporting Adenosine in Unstable Patients

A 2025 multicentre study found that adenosine may be a safe first-line attempt even in unstable SVT patients before proceeding to electrical cardioversion, potentially reducing sedation-related risks, though electrical cardioversion had slightly higher success rates (weighted OR 2.41). 7 However, the ACC/AHA guidelines maintain that immediate cardioversion remains the standard for unstable patients. 1, 2