What is the dosing regimen for diltiazem (calcium channel blocker) in the management of supraventricular tachycardia (SVT)?

Diltiazem Dosing for SVT

For acute SVT management, administer diltiazem 0.25 mg/kg (approximately 15-20 mg for average adults) IV over 2 minutes, followed by a second dose of 0.35 mg/kg (20-25 mg) after 15 minutes if needed, then maintain with continuous infusion at 5-15 mg/hour titrated to heart rate. 1, 2, 3

Initial IV Bolus Dosing

• First dose: 0.25 mg/kg (15-20 mg) administered over 2 minutes 1, 2, 3
• Second dose (if needed): 0.35 mg/kg (20-25 mg) given 15 minutes after the first dose if no therapeutic response occurs 1
• Conversion to sinus rhythm typically occurs within 2-3 minutes of administration, with maximal heart rate reduction in 2-7 minutes 3, 4
• The 0.25 mg/kg dose achieves 100% conversion in AV nodal reentry tachycardia and 81-90% conversion in orthodromic reciprocating tachycardia 5, 4

Maintenance Infusion

• Start continuous infusion at 5 mg/hour, titrate up to 15 mg/hour based on heart rate response 1, 2
• Infusion maintains at least 20% heart rate reduction in 83% of patients over 24 hours 3
• Upon discontinuation, heart rate reduction may persist for 0.5 to >10 hours (median 7 hours) 3

Critical Contraindications

Absolutely avoid diltiazem in these situations:

• Pre-excited atrial fibrillation/flutter (WPW syndrome or accessory pathways conducting to ventricles) - may accelerate ventricular response and cause hemodynamic collapse 1, 2, 3
• Second or third-degree AV block without functioning pacemaker 1, 2, 3
• Decompensated heart failure or severe LV dysfunction - negative inotropic effects worsen cardiac output 1, 2
• Hypotension or cardiogenic shock 2, 3
• Sick sinus syndrome without pacemaker 2

Monitoring Requirements

• Continuous ECG monitoring is mandatory during administration 1, 3
• Frequent blood pressure measurements throughout infusion 1, 3
• Defibrillator and resuscitation equipment must be immediately available 2, 3

Common Adverse Effects

• Hypotension is the most frequent adverse effect (occurring in 7-11% of patients), though usually short-lived lasting 1-3 hours 3, 5
• Only 3.2% of patients require intervention (IV fluids or Trendelenburg positioning) for blood pressure support 3
• Bradycardia occurs in approximately 10-15% of patients 2, 6
• Symptomatic hypotension requiring dose adjustment occurs in only 6% of patients 4

Clinical Pearls and Pitfalls

• Adenosine remains first-line for stable PSVT due to its ultra-short half-life; use diltiazem when adenosine fails or is contraindicated 2
• If the first 0.25 mg/kg bolus fails, the second 0.35 mg/kg bolus succeeds in the majority of remaining patients 7
• Never combine diltiazem with beta-blockers acutely - the overlapping longer half-lives can cause profound bradycardia and heart block 1, 2
• In hemodynamically unstable patients, skip medications entirely and proceed directly to synchronized cardioversion 2
• Diltiazem works by slowing AV nodal conduction and increasing AV nodal refractoriness, making it effective only when the AV node is part of the reentrant circuit 2, 4
• One critical pitfall: In patients with atrial fibrillation and WPW, diltiazem can shorten the RR interval from 240 ms to 180 ms, potentially triggering ventricular fibrillation 8

Mechanism of Action

• Diltiazem prolongs AV nodal refractoriness and slows AV nodal conduction 4
• It has no effect on accessory pathway refractoriness, which is why it works for orthodromic tachycardia (where the AV node is in the circuit) but is dangerous in pre-excited atrial fibrillation (where the accessory pathway bypasses the AV node) 8, 4

Special Populations

• Elderly patients (60-91 years): Use the same 0.25 mg/kg dose, but monitor more carefully for bradycardia and hypotension 6
• In elderly patients, diltiazem achieves sinus rhythm in approximately 50% with atrial fibrillation and slows ventricular rate to <100 bpm in most others 6
• Bradycardia requiring drug cessation occurs in 16% of elderly patients but reverses rapidly upon discontinuation 6