What are the latest guidelines for cardioverting a patient with supraventricular tachycardia (SVT)?

Latest Guidelines for SVT Cardioversion

For hemodynamically unstable SVT patients, perform immediate synchronized cardioversion without delay; for stable patients, use vagal maneuvers first (specifically the modified Valsalva technique), followed by adenosine, then IV calcium channel blockers or beta blockers, reserving cardioversion for refractory cases. 1, 2

Hemodynamically Unstable Patients

Proceed directly to synchronized cardioversion as the definitive first-line treatment. 1, 2

• Hemodynamic instability is defined as hypotension with altered mental status, signs of shock, acute heart failure or pulmonary edema, or ongoing chest pain suggesting ischemia 2
• Do not delay cardioversion to attempt vagal maneuvers or administer medications if the patient is deteriorating 2
• Use initial energy of 50-100J for synchronized cardioversion 3
• Success rate approaches 100% for terminating SVT 4
• Critical pitfall: Avoid IV calcium channel blockers (verapamil, diltiazem) or beta blockers in unstable patients, as these can worsen hypotension and cause further hemodynamic deterioration 2, 4

Special Case: Pre-excited Atrial Fibrillation

• Synchronized cardioversion is mandatory for hemodynamically unstable pre-excited AF 1
• For stable pre-excited AF, use ibutilide or IV procainamide—never use AV nodal blocking agents (adenosine, calcium channel blockers, beta blockers, or digoxin) as these can precipitate ventricular fibrillation 1, 3

Hemodynamically Stable Patients: Stepwise Algorithm

First-Line: Modified Valsalva Maneuver

The modified Valsalva maneuver is 2.8-3.8 times more effective than standard Valsalva and should be attempted first. 3, 5

• Position the patient supine before beginning 3
• Patient bears down against a closed glottis for 10-30 seconds, generating intrathoracic pressure of at least 30-40 mmHg 1, 3
• Alternative vagal maneuvers include carotid sinus massage (after confirming absence of carotid bruits by auscultation, apply steady pressure for 5-10 seconds) or applying an ice-cold wet towel to the face 1
• One study showed Valsalva was more successful than carotid sinus massage, with switching between techniques achieving 27.7% overall success 1
• Important limitation: Vagal maneuvers only work for SVTs involving the AV node as part of the reentrant circuit (AVNRT, orthodromic AVRT) and will not terminate automatic atrial tachycardias 3, 4

Second-Line: Adenosine

If vagal maneuvers fail, adenosine achieves 90-95% success rates in orthodromic AVRT and AVNRT. 1, 2, 3

• Administer 6 mg as rapid IV bolus through a large vein, followed immediately by saline flush 1
• If ineffective, give up to two subsequent doses of 12 mg 1
• Recent evidence suggests an initial 12 mg dose may be more effective than 6 mg (54.2% vs. 40.6% conversion rate, p=0.03), though this contradicts current guideline recommendations 6
• Critical safety measure: Have cardioversion equipment immediately available, as adenosine may precipitate AF that conducts rapidly to the ventricle and can cause ventricular fibrillation 1
• Minor, brief side effects (<1 minute) occur in approximately 30% of patients 1
• Atrial or ventricular premature complexes may occur immediately after conversion, potentially reinitating tachycardia and requiring antiarrhythmic drugs 1, 3

Third-Line: IV Calcium Channel Blockers or Beta Blockers

IV diltiazem, verapamil, or beta blockers achieve 80-98% success rates when adenosine fails. 1, 3

• Administer as slow infusion over up to 20 minutes to minimize hypotension risk 4
• Critical pitfall: Ensure the rhythm is truly SVT before administering these agents—verapamil or diltiazem given to ventricular tachycardia or pre-excited AF can cause hemodynamic collapse or ventricular fibrillation 2, 4
• In cases of diagnostic uncertainty with wide-complex tachycardia, treat as VT until proven otherwise 4

Fourth-Line: Synchronized Cardioversion

When pharmacological therapy fails or is contraindicated in stable patients, proceed to synchronized cardioversion. 1, 3

• Perform after adequate sedation or anesthesia 1
• Use initial energy of 50-100J 3, 4
• Success rate approaches 100% for terminating SVT 4

Post-Conversion Management

• Monitor for atrial or ventricular premature complexes that may reinitiate tachycardia 1, 3
• Consider antiarrhythmic drugs to prevent acute reinitiation if premature complexes occur 1, 3
• Refer all patients to a heart rhythm specialist for evaluation of definitive management with catheter ablation 2, 7

Special Population: Pregnant Patients

• Vagal maneuvers remain first-line 1
• Adenosine is safe and recommended as first-line pharmacologic therapy (unlikely to reach fetal circulation due to short half-life) 1
• Synchronized cardioversion is safe at all stages of pregnancy when pharmacologic therapy fails—apply electrode pads away from the uterus and perform fetal monitoring if time allows 1
• IV metoprolol or propranolol are reasonable alternatives when adenosine fails 1
• IV verapamil may be reasonable but carries higher risk of maternal hypotension than adenosine 1