What is the recommended approach for converting a patient with chronic, severe pain from intravenous (IV) fentanyl to intrathecal fentanyl?

Converting IV to Intrathecal Fentanyl

For patients with chronic severe pain requiring conversion from IV to intrathecal (IT) fentanyl, use a conservative 100:1 conversion ratio (100 mcg IV fentanyl = 1 mcg IT fentanyl per hour), reduce the calculated dose by 25-50% to account for incomplete cross-tolerance, and titrate cautiously with close monitoring over the first 48-72 hours. 1

Conversion Algorithm

Step 1: Calculate Total Daily IV Fentanyl Dose

• Multiply the hourly IV fentanyl infusion rate by 24 to determine the total daily dose in micrograms 2
• Example: If receiving 200 mcg/hour IV, the total daily dose is 4,800 mcg (4.8 mg) 2

Step 2: Apply the IV to IT Conversion Ratio

• Use the 100:1 ratio as your starting point (100 mcg IV = 1 mcg IT per hour), which is the most commonly cited conversion among pain specialists 1
• Divide the total daily IV dose by 100 to get the initial IT hourly rate 1
• Example: 4,800 mcg IV daily ÷ 100 = 48 mcg IT per hour

Step 3: Apply Dose Reduction for Safety

• Reduce the calculated IT dose by 25-50% to account for incomplete cross-tolerance 2, 3
• If pain was well-controlled on IV fentanyl, use the 50% reduction (more conservative) 2
• If pain was poorly controlled, consider using only a 25% reduction or the full calculated dose 2
• Example: 48 mcg/hour × 0.5 (50% reduction) = 24 mcg/hour IT fentanyl as starting dose

Step 4: Initiate IT Therapy with Monitoring

• Start the IT infusion at the reduced calculated dose 1
• Monitor pain scores, sedation levels, and respiratory status every 4-6 hours for the first 24-48 hours 2, 3
• Provide breakthrough medication (typically short-acting IV opioid at 10-15% of total daily dose) during the transition period 2, 3

Step 5: Titrate Based on Response

• If the patient requires more than 3-4 breakthrough doses per day, increase the baseline IT infusion by 25-50% 2
• Reassess pain control and side effects daily during the first week 2
• High-dose IT fentanyl (up to 20 times typical doses) can be used safely in highly selected patients who fail other approaches 4

Critical Clinical Considerations

Wide Practice Variation Exists

• Survey data reveals substantial variation in how pain specialists perform this conversion, with the 100:1 ratio being most common but not universal 1
• Fentanyl has less spinal selectivity than morphine, leading some practitioners to use more aggressive (lower) conversion ratios 1
• Given this uncertainty, a conservative approach is warranted 1

Advantages of IT Fentanyl

• IT fentanyl may resolve peripheral edema that occurs with IT morphine or hydromorphone, typically within 50 days of rotation 5
• Fentanyl is lipophilic and has different pharmacokinetics compared to hydrophilic opioids like morphine 4, 5
• IT fentanyl can be safely dose-escalated in refractory cases where other IT opioids have failed 4

Common Pitfalls to Avoid

• Do not use transdermal fentanyl conversion ratios (which are entirely different) when converting from IV to IT routes 2, 6
• Do not forget the 25-50% dose reduction for incomplete cross-tolerance, as this significantly increases the risk of respiratory depression 2, 1
• Do not assume the 100:1 ratio is universally accepted—recognize that expert opinion varies and err on the side of caution 1
• Avoid rapid titration without adequate monitoring during the first 48-72 hours 2
• Do not use mixed agonist-antagonist opioids during or after this conversion 2

Special Populations

Renal Impairment

• Fentanyl is preferred over morphine in severe renal impairment (CrCl <30 mL/min) because it lacks neurotoxic metabolites 3
• No specific dose adjustment is required for IT fentanyl in renal dysfunction 3

Hepatic Impairment

• Both IV and IT fentanyl undergo hepatic metabolism, so dose adjustments may be necessary in significant hepatic dysfunction 3
• Start with lower doses and titrate more cautiously 3