What is the initial drug of choice for a patient with Supraventricular Tachycardia (SVT)?

Initial Drug of Choice for SVT

Adenosine is the initial drug of choice for hemodynamically stable patients with supraventricular tachycardia, administered as a 6 mg rapid IV push followed by a 20 mL saline flush, with a second dose of 12 mg if the first fails to convert the rhythm within 1-2 minutes. 1

Initial Management Approach

First-Line: Vagal Maneuvers

• Attempt vagal maneuvers (Valsalva or carotid sinus massage) before pharmacologic therapy in stable patients, as these terminate up to 25-27.7% of PSVTs 1
• The Valsalva maneuver involves bearing down against a closed glottis for 10-30 seconds, generating at least 30-40 mm Hg of intrathoracic pressure 1
• Carotid massage should only be performed after confirming absence of carotid bruit, applying steady pressure for 5-10 seconds 1

Second-Line: Adenosine

• Adenosine is the preferred pharmacologic agent due to its rapid onset, short half-life (seconds), and superior safety profile compared to calcium channel blockers 1, 2
• Initial dose: 6 mg rapid IV push through a large (antecubital) vein, followed immediately by 20 mL saline flush 1
• Second dose: 12 mg if no conversion within 1-2 minutes 1
• Success rate: 90-95% for AVNRT and orthodromic AVRT 1, 3, 2

Critical Dosing Adjustments for Adenosine

• Reduce initial dose to 3 mg in patients taking dipyridamole or carbamazepine, those with transplanted hearts, or if given via central venous access 1
• Higher doses may be required in patients taking theophylline, caffeine, or theobromine 1
• Patients with impaired venous return (right heart failure, pulmonary hypertension) may require doses exceeding standard recommendations 4

Important Safety Considerations

• Contraindicated in patients with severe asthma due to risk of bronchospasm 1
• Have a defibrillator available when administering adenosine to patients with possible WPW syndrome, as it may precipitate atrial fibrillation with rapid ventricular rates 1
• Common transient side effects include flushing, dyspnea, and chest discomfort lasting less than 60 seconds 1

Alternative Agents When Adenosine Fails or Is Contraindicated

Calcium Channel Blockers (Second-Line)

• Verapamil 2.5-5 mg IV or diltiazem are reasonable alternatives with 64-98% conversion rates 1
• Administer slowly over up to 20 minutes to minimize hypotension 1
• Never use in suspected ventricular tachycardia or pre-excited atrial fibrillation, as this may precipitate ventricular fibrillation 1, 5
• Avoid in patients with systolic heart failure 1
• Do not combine with beta blockers due to risk of profound bradycardia and hypotension 1

Beta Blockers (Second-Line)

• IV beta blockers (esmolol, metoprolol) are reasonable alternatives with excellent safety profile, though less effective than diltiazem 1
• Particularly useful in patients who cannot tolerate calcium channel blockers or experience recurrence after adenosine conversion 1

Refractory Cases

• For adenosine-refractory SVT, consider procainamide (15 mg/kg over 30-60 minutes) or amiodarone (5 mg/kg over 20-60 minutes) 3, 5
• Consult cardiology/electrophysiology before administering antiarrhythmics beyond adenosine due to potential for serious adverse effects 3

When to Proceed Directly to Cardioversion

• Immediate synchronized cardioversion is indicated for any patient with hemodynamic instability (hypotension, altered mental status, shock, chest pain, acute heart failure) 1
• Cardioversion is also indicated in stable patients when pharmacologic therapy fails or is contraindicated, with essentially 100% success rate 1, 3
• Consider adenosine first even in unstable patients if the tachycardia is regular with narrow QRS complex 1

Common Pitfalls to Avoid

• Do not delay adenosine administration to obtain a 12-lead ECG in symptomatic patients, though recording rhythm during drug administration aids diagnosis 1, 3
• Ensure rapid bolus technique with immediate saline flush, as adenosine's half-life is only seconds 1, 2
• Verify the rhythm is not ventricular tachycardia before administering calcium channel blockers or beta blockers 1
• Do not routinely combine amiodarone and procainamide 3