What is the initial medical management for stable supraventricular tachycardia (SVT)?

Medical Management of Stable SVT

Initial Management: Vagal Maneuvers First

Begin immediately with vagal maneuvers in all hemodynamically stable patients with SVT before any pharmacologic intervention. 1, 2

Vagal Maneuver Technique

• Perform the modified Valsalva maneuver as the most effective vagal technique, achieving success rates up to 43% and superior to carotid sinus massage 2, 3
• Have the patient lie supine and bear down against a closed glottis for 10-30 seconds, generating at least 30-40 mmHg intrathoracic pressure 1, 2
• If the modified Valsalva fails, attempt carotid sinus massage by applying steady pressure over the right or left carotid sinus for 5-10 seconds after confirming absence of bruit by auscultation 1
• Alternative technique: apply an ice-cold wet towel to the face to activate the diving reflex 1
• Vagal maneuvers terminate 25-31% of SVTs overall 2, 4

Critical pitfall: Never apply pressure to the eyeball—this technique is dangerous and has been abandoned 1

Second-Line: Adenosine

If vagal maneuvers fail, immediately administer adenosine 6 mg IV rapid push through a large vein, followed by a 20 mL saline flush. 1, 2

Adenosine Dosing Protocol

• Adenosine terminates 90-95% of reentrant SVTs (AVNRT and orthodromic AVRT) 1, 2
• If no conversion occurs within 1-2 minutes, give 12 mg IV rapid push using the same technique 2
• Recent evidence suggests that starting with 12 mg may be more effective than 6 mg (54.2% vs 40.6% conversion rate), though this contradicts current guidelines 5
• Have a defibrillator immediately available, as adenosine can precipitate atrial fibrillation with rapid ventricular response 2

Important consideration: Adenosine serves dual purposes as both therapeutic and diagnostic agent, unmasking atrial flutter or atrial tachycardia when it fails to terminate the rhythm 1

Third-Line: Longer-Acting AV Nodal Blockers

If adenosine fails or SVT recurs, use intravenous calcium channel blockers or beta-blockers in hemodynamically stable patients. 1, 2

Medication Options (in order of preference based on evidence)

• Diltiazem or verapamil 2.5-5 mg IV are particularly effective for converting AVNRT to sinus rhythm with 80-98% success rates 1, 2
• Intravenous beta-blockers are reasonable alternatives with excellent safety profiles, though diltiazem proved more effective than esmolol in head-to-head comparison 1
• These agents provide more sustained rhythm control and are especially useful when adenosine unmasks atrial fibrillation or flutter requiring rate control 2

Critical Safety Warnings for AV Nodal Blockers

Never administer verapamil or diltiazem until ventricular tachycardia is definitively excluded in wide-complex tachycardia—this can cause hemodynamic collapse or ventricular fibrillation. 1, 2

• Avoid diltiazem or verapamil in patients with suspected systolic heart failure 1
• Do not use AV nodal blockers in pre-excited atrial fibrillation (Wolff-Parkinson-White with AF)—use procainamide or ibutilide instead, or proceed directly to cardioversion 2, 6
• Ensure absence of pre-excited AF before administering these agents, as patients can develop ventricular fibrillation 1

Emergency Intervention: Synchronized Cardioversion

Perform immediate synchronized cardioversion for hemodynamically unstable patients or when pharmacologic therapy fails in stable patients. 1, 2

• Use initial biphasic energy of 50-100 J for SVT 2
• Synchronized cardioversion is highly effective, terminating SVT in the vast majority of cases 1
• This is indicated when vagal maneuvers and adenosine are ineffective or not feasible in unstable patients 1

Alternative Pharmacologic Agents

Intravenous amiodarone may be considered when other therapies are ineffective or contraindicated in hemodynamically stable patients. 1

• Amiodarone was effective in small cohort studies for terminating AVNRT 1
• Long-term toxicity is not seen with short-term intravenous administration 1

Special Considerations for Non-Reentrant Rhythms

Recognize that ectopic atrial tachycardia, multifocal atrial tachycardia, and junctional tachycardia have gradual onset/termination and are not responsive to cardioversion. 2

• These rhythms may be treated with AV nodal blocking agents for rate control rather than rhythm conversion 2
• Adenosine will not terminate these rhythms but can help diagnostically by unmasking atrial activity 1

Documentation and Monitoring

• Record a 12-lead ECG during tachycardia to differentiate mechanisms and guide treatment 1
• Have appropriate resuscitative measures and personnel immediately available, as fatal cardiac events including cardiac arrest, ventricular arrhythmias, and myocardial infarction have occurred with adenosine 7
• Monitor for adverse effects including flushing (most common), chest discomfort, shortness of breath, headache, and throat/neck/jaw discomfort 7