What is the recommended intrathecal (in the spinal canal) to intravenous (IV) conversion ratio for hydromorphone?

Intrathecal to IV Hydromorphone Conversion Ratio

The intrathecal to IV conversion ratio for hydromorphone is approximately 1:100, which is the same conservative ratio used for morphine and represents the safest starting point for conversion. 1, 2

Evidence-Based Conversion Approach

The most recent guideline evidence supports using a 1:100 ratio (intrathecal:IV) as the standard conversion for both morphine and hydromorphone when transitioning from intrathecal to intravenous routes. 1 This conservative approach:

• Reflects the 100-300 times greater potency of intrathecal delivery due to direct spinal cord administration bypassing systemic circulation 1
• Minimizes risk of under-dosing and withdrawal in opioid-tolerant patients while maintaining safety margins 1
• Is supported by expert consensus, with the 100:1 ratio being the most commonly applied conversion by pain specialists surveyed for both morphine and hydromorphone 2

Clinical Application Algorithm

Step 1: Calculate the initial IV dose

• Multiply the daily intrathecal hydromorphone dose (in mg) by 100 to get the starting IV dose 1
• Example: 0.5 mg/day intrathecal = 50 mg/day IV hydromorphone

Step 2: Apply incomplete cross-tolerance reduction

• If pain was well-controlled on intrathecal therapy, reduce the calculated IV dose by 25-50% 3
• If pain was poorly controlled, may use 100% of calculated dose or increase by 25% 3

Step 3: Provide breakthrough dosing

• Order IV hydromorphone boluses every 15 minutes as needed for breakthrough pain 4
• Breakthrough doses should be 10-20% of the total 24-hour dose 4

Step 4: Monitor and titrate

• Assess pain control and side effects every 60 minutes initially 4
• If inadequate control after 24 hours, increase total daily dose by 25-50% 1
• If patient requires >3 breakthrough doses per day, increase the scheduled dose 4

Important Caveats and Safety Considerations

Wide inter-patient variability exists:

• Some sources cite conversion ratios ranging from 1:100 to 1:300 for intrathecal to IV morphine 1
• Survey data shows pain specialists apply varied ratios, though 1:100 remains most common 2
• Hydromorphone may require more aggressive conversion than morphine due to less spinal selectivity 2

Critical safety points:

• Never use standard oral-to-IV ratios (typically 5:1 for hydromorphone) when converting from intrathecal routes 1
• Patients on intrathecal opioids are highly opioid-tolerant and risk withdrawal if under-dosed 1
• No serious adverse events (respiratory depression, excessive sedation) were reported using 1:100 ratio in cancer patients 5
• The 1:100 ratio facilitated rapid elimination of systemic opioids in 96% of patients 5

Common pitfall to avoid:

• Do not assume linear dose equivalence between routes—the conversion reflects pharmacokinetic differences in delivery, not just potency 1

Supporting Evidence Quality

The 1:100 conversion ratio is supported by:

• Most recent guideline recommendations from the National Comprehensive Cancer Network 1
• Prospective research demonstrating safety in 275 cancer patients with median conversion ratio of 105.5:1 (IQR 90-120) 5
• Expert consensus survey showing this as the most common practice pattern among pain specialists 2