Yes, you can start Cardizem (diltiazem) in a patient on digoxin with atrial fibrillation and rapid ventricular response
The combination of diltiazem and digoxin is explicitly recommended and safe for rate control in atrial fibrillation with rapid ventricular response, provided the patient does not have severely reduced left ventricular ejection fraction (LVEF <40%). 1, 2
Key Decision Point: Assess Left Ventricular Function First
If LVEF is preserved (>40%):
- Proceed with diltiazem addition to digoxin - this combination is reasonable and effective for rate control both at rest and during exercise 1, 2
- The combination provides superior rate control compared to either agent alone, with fewer episodes of rate control loss 3
- Medium-dose diltiazem (240 mg/day) combined with digoxin is both effective and safe, providing better sustained rate control than high-dose diltiazem (360 mg/day) 4, 2
If LVEF is reduced (<40%):
- Use diltiazem with extreme caution or avoid entirely due to negative inotropic effects that can precipitate or worsen heart failure 1, 2
- Beta-blockers are strongly preferred over calcium channel blockers in this population, as they provide mortality benefit that digoxin does not offer 5
- If beta-blockers are contraindicated, consider IV amiodarone instead of diltiazem 1
Practical Administration Strategy
For acute rate control in the emergency setting:
- Administer IV diltiazem bolus at low dose (≤0.2 mg/kg) rather than the standard 0.25 mg/kg to reduce hypotension risk while maintaining efficacy 6
- The combination of IV diltiazem plus digoxin achieves rate control faster (15 minutes vs 22 minutes) with fewer fluctuations than diltiazem alone 3
- Expect successful rate control (heart rate <100 bpm) within 15-22 minutes 3
For chronic oral therapy:
- Start diltiazem at reduced doses: 30 mg slow-release twice daily or 60 mg three times daily 2
- Titrate cautiously up to 240 mg/day for standard formulations 2
- This combination reduces resting heart rate by approximately 26% and exercise heart rate by 24% 7
Critical Safety Monitoring
Check digoxin levels:
- Diltiazem can increase plasma digoxin concentrations, though studies show this effect is minimal and clinically insignificant in most patients 7, 4
- Reduce digoxin dose by 50% if adding amiodarone concurrently, as this combination significantly elevates digoxin levels 8
- Target therapeutic digoxin concentration of 0.5-0.9 ng/mL 8
Monitor for:
- Hypotension (occurs in 18% with low-dose diltiazem vs 35% with standard dose) 6
- Excessive bradycardia, particularly if patient is also on beta-blockers 2
- Signs of digoxin toxicity: confusion, nausea, visual disturbances, arrhythmias 5
- Renal function and electrolytes, especially potassium, as hypokalemia increases digoxin toxicity risk 8, 5
Absolute Contraindications to This Combination
Do NOT use diltiazem if:
- Severe left ventricular dysfunction (LVEF <40%) is present 1, 2
- Patient is already on a beta-blocker - the combination of diltiazem with beta-blockers carries a Class III (harm) recommendation due to excessive bradycardia risk 2
- Wolff-Parkinson-White syndrome is present, as digoxin may facilitate conduction through accessory pathways 2
- Second- or third-degree heart block without a pacemaker 8
Common Pitfall to Avoid
The most critical error is using diltiazem in patients with reduced ejection fraction heart failure. Always verify LVEF before initiating this combination. If LVEF is unknown in the acute setting and the patient shows any signs of heart failure, choose IV amiodarone or optimize beta-blocker therapy instead 1.