What are the recommended AV (atrioventricular) nodal blocking agents for managing supraventricular tachycardia?

AV Nodal Blocking Agents for Supraventricular Tachycardia

For acute management of SVT, adenosine is the first-line AV nodal blocking agent with a 90-95% success rate, followed by IV diltiazem or verapamil if adenosine fails, while IV beta-blockers serve as reasonable alternatives in hemodynamically stable patients. 1, 2

Acute Management Algorithm

First-Line: Adenosine

• Adenosine 6 mg IV rapid bolus is the drug of choice after vagal maneuvers fail, terminating AVNRT in approximately 95% of patients within 30 seconds. 1, 2, 3
• If the initial 6 mg dose fails, administer 12 mg IV rapid bolus, which can be repeated once if needed. 4, 5
• Adenosine works by producing transient AV nodal block, interrupting the reentrant circuit at the AV node. 1, 6
• The drug has an exceptionally short half-life of less than 10 seconds, allowing rapid upward dosage titration and making side effects transient (lasting <1 minute). 3, 7
• Critical precaution: Have cardioversion equipment immediately available during adenosine administration, as it can precipitate atrial fibrillation in 1-15% of patients. 1, 4
• Contraindications include severe bronchial asthma, high-grade AV block, and sinus node dysfunction without a pacemaker. 4

Second-Line: Calcium Channel Blockers

• IV diltiazem or verapamil achieve 80-98% conversion rates for hemodynamically stable SVT and are particularly effective for AVNRT. 1, 8
• These agents work by inhibiting calcium influx through slow channels in the AV node, slowing conduction and prolonging the effective refractory period. 6
• Diltiazem is more effective than esmolol for acute conversion. 1
• Never administer calcium channel blockers if ventricular tachycardia or pre-excited atrial fibrillation is suspected, as this can precipitate ventricular fibrillation and hemodynamic collapse. 1, 8
• Avoid in patients with suspected systolic heart failure, as verapamil produces transient reduction in myocardial contractility. 1, 6

Third-Line: Beta-Blockers

• IV beta-blockers (metoprolol, esmolol) are reasonable alternatives with an excellent safety profile, though less effective than calcium channel blockers. 1
• Evidence for beta-blocker effectiveness in terminating AVNRT is more limited compared to adenosine or calcium channel blockers. 1
• Use extreme caution with concomitant IV calcium channel blockers and beta-blockers due to potential potentiation of hypotensive and bradycardic effects. 1

Hemodynamically Unstable Patients

• Immediate synchronized cardioversion is the definitive treatment when adenosine and vagal maneuvers fail or are not feasible in unstable patients. 1, 8
• Look for hypotension, altered mental status, shock, chest pain, or acute heart failure as signs of instability. 8
• Synchronized cardioversion is highly effective at terminating SVT and must be performed promptly to restore sinus rhythm. 1

Long-Term Management

• Oral verapamil or diltiazem is recommended for ongoing management in patients not pursuing catheter ablation. 1, 2
• Oral beta-blockers are equally recommended for ongoing management. 1
• Catheter ablation of the slow pathway achieves 94.3-98.5% success rates and is the most effective long-term therapy, potentially eliminating the need for chronic pharmacological therapy. 8, 2, 9

Common Pitfalls to Avoid

• Do not use calcium channel blockers or beta-blockers in wide-complex tachycardia of unknown etiology until VT is definitively excluded. 1, 8
• Adenosine should be avoided in patients with severe bronchial asthma due to risk of bronchoconstriction. 1, 4
• The practice of applying pressure to the eyeball is potentially dangerous and has been abandoned. 1
• Record a 12-lead ECG during drug administration, as the response aids in diagnosis even if the arrhythmia does not terminate. 1