What is the recommended Cardizem (diltiazem) dosing regimen for an adult with supraventricular tachycardia, including the initial intravenous bolus and subsequent infusion rates?

Diltiazem Dosing for SVT

For acute supraventricular tachycardia in hemodynamically stable adults, administer diltiazem 0.25 mg/kg (approximately 15-20 mg for average-weight patients) IV over 2 minutes, followed by a second dose of 0.35 mg/kg (approximately 20-25 mg) after 15 minutes if needed, then start a continuous infusion at 5-15 mg/hour for sustained rate control. 1, 2

Initial IV Bolus Dosing

• First bolus: Administer 0.25 mg/kg actual body weight IV over 2 minutes (20 mg is reasonable for the average 70-80 kg patient). 2, 1

• Second bolus (if needed): If inadequate response after 15 minutes, give 0.35 mg/kg actual body weight IV over 2 minutes (25 mg for the average patient). 2, 1

• This weight-based dosing achieves 86-100% conversion rates for SVT involving the AV node (AVNRT and orthodromic AVRT), with median conversion time of 2-3 minutes. 3, 4

• Patients with low body weight should always be dosed on a mg/kg basis rather than using fixed doses. 2

Maintenance Infusion

• Initial infusion rate: Start at 10 mg/hour immediately following successful bolus administration. 2, 1

• Titration: May reduce to 5 mg/hour if adequate response is maintained, or increase in 5 mg/hour increments up to a maximum of 15 mg/hour if further rate control is needed. 2, 1

• Duration limit: Infusions should not exceed 24 hours, as safety and efficacy beyond this timeframe have not been established. 2

Critical Treatment Algorithm

Step 1: Assess Hemodynamic Stability

• If unstable (hypotension, altered mental status, shock, chest pain, acute heart failure): Skip all medications and proceed directly to synchronized cardioversion. 5, 1

Step 2: Rule Out Absolute Contraindications

• Pre-excited atrial fibrillation/flutter (WPW syndrome): Diltiazem can accelerate ventricular response and cause ventricular fibrillation—this is an absolute contraindication. 1, 6

• Wide-complex tachycardia of uncertain origin: Never give diltiazem if ventricular tachycardia cannot be excluded, as it may cause hemodynamic collapse. 1, 6

• Decompensated heart failure or severe LV dysfunction: Negative inotropic effects can worsen cardiac output. 1, 6

• Second- or third-degree AV block without pacemaker: Risk of complete heart block. 1, 6

• Hypotension or cardiogenic shock: Diltiazem will worsen hemodynamics. 1

Step 3: First-Line Therapy (Before Diltiazem)

• Vagal maneuvers should be attempted first in all stable patients (27-28% success rate). 5, 7

• Adenosine is the preferred first-line pharmacologic agent (90-95% conversion for AVNRT, 78-96% for AVRT), given as 6 mg rapid IV push, then 12 mg × 2 if needed. 1, 7

Step 4: Diltiazem as Second-Line Agent

• Use diltiazem when adenosine fails, is contraindicated (asthma, severe COPD), or when a longer-acting agent is preferred. 5, 1

• Diltiazem is particularly effective for AVNRT (100% conversion) and orthodromic AVRT (81% conversion) at the 0.25 mg/kg dose. 4

Monitoring Requirements

• Continuous ECG monitoring throughout administration is mandatory. 1

• Frequent blood pressure measurements every 2-5 minutes during bolus and titration. 1

• Defibrillator and resuscitation equipment must be immediately available. 1

• Monitor for hypotension (most common adverse effect, occurring in 6-11% of patients), though symptomatic hypotension is less frequent. 3, 4

Critical Safety Warnings

• Never combine with beta-blockers acutely—this combination causes profound bradycardia, heart block, and potential cardiovascular collapse. 1, 6

• Obtain 12-lead ECG during tachycardia to confirm SVT mechanism and exclude ventricular tachycardia or pre-excitation before administering diltiazem. 1, 7

• If diltiazem reveals underlying atrial flutter or atrial fibrillation (by slowing ventricular response without conversion), continue the infusion for rate control rather than attempting rhythm conversion. 7

Rescue Strategy for Diltiazem Failure

• If diltiazem fails to convert stable SVT, proceed to synchronized cardioversion (near-100% termination rate). 1, 7

• In recent comparative data, diltiazem successfully rescued 35% of cases that failed adenosine monotherapy and controlled 77% of conversions to atrial fibrillation without hypotensive events. 8

Special Population Considerations

• Elderly patients (≥60 years): The standard 0.25 mg/kg dose is effective and safe, achieving good results in 86% of elderly patients, though bradycardia occurs in 16% (usually reversible with drug cessation). 9

• Pregnancy: Adenosine remains the preferred agent; diltiazem should be reserved for adenosine-refractory cases. 7