Mr. Gateway’s pain is managed with a PCA and he is medically stabilized. He then undergoes repeat palliative surgical resection. His post-operative course has been complicated by difficult to manage pain and the patient’s resistance to transition to oral therapy.
He is now post-operative day 3 and has been using a hydromorphone PCA and reports satisfactory pain control. His renal function has normalized. He is now able to eat soft solids. You spend time talking with him about his concerns and discuss a possible oral regimen with him.
His PCA settings are:
- 0.4mg IV hydromorphone available every 7 minutes, no limit, no continuous infusion
How will you help determine an oral opioid regimen for Mr. Gateway?
Equianalgesic Dosing
Converting from intravenous to oral opioid regimens can be guided by equianalgesic tables. Data within equianalgesic tables has been derived from single-dose cross-over studies conducted in the 1950s and 1960s in opioid-naïve patients with acute post-operative pain and chronic cancer pain. Dose conversions have not been adequately studied beyond these initial studies and there are limitations to the tables.
For instance, factors that can influence pharmacokinetics and pharmacodynamics are not accounted for in these tables. Patient-specific factors that should be considered include organ function, pain level, interacting medications, and comorbidities.
Nevertheless, equianalgesic tables can serve as a guide when calculating opioid conversions and comparing opioid potencies.
An “opioid conversion calculator,” such as the one accessed on the following website, can be used to help calculate opioid conversions: http://www.globalrph.com/narcoticonv.htm
Click on the "Opioid Risk and Dosing" button below to learn more about relative strengths of different opioids, conversions from oral morphine equivalent to transdermal fentanyl, and conversions from oral morphine equivalent to oral methadone.
Reference
Knotkova H, Fine PG, Portenoy RK. Opioid rotation: the science and the limitations of the equianalgesic dose table. J Pain Symptom Manage. 2009 Sep;38(3):426-39 http://www.ncbi.nlm.nih.gov/pubmed?term=19735903.
Incomplete Cross-Tolerance
After assessing Mr. Gateway's current pain level and how to transition him to an oral therapy, you will also need to account for incomplete cross-tolerance.
Definition
- Tolerance is a state of adaptation to a drug that results in reduced effects at a given dosage.
- Cross-Tolerance is tolerance to a second drug (i.e. opioid) developed as a result of exposure to a first drug (i.e. opioid).
- Incomplete cross-tolerance accounts for differences in opioid characteristics including receptor activity; therefore when patients are rotated to a different opioid, they are more likely to be sensitive to both the analgesic and adverse effects of the new opioid.
Incomplete Cross-Tolerance Scenarios
Consider one of the following scenarios to account for incomplete cross-tolerance in Mr. Gateway:
- Pain that is well controlled and there is a need for a different opioid formulation AND/OR the patient is experiencing intractable adverse effects.
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- Decrease the dose by 25-57% (the extent of the reduction is commonly 25%, 33%, or 75%)
- Pain is not well-controlled
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- Use the calculated equianalgesic dose (click the "Opioid Risk and Dosing" button to learn more) or in the presence of severe pain, consider increasing the calculated dose by a small percentage.
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- This is not common practice and should be only implemented under close monitoring.
Equanalgesic Question 1
You access Mr. Gateway's PCA record and see he's used 15.6 mg IV hydromorphone as recorded by his PCA in the last 24 hours.
Question: How much total daily oral oxycodone does this equal?
Access the opioid conversion calculator here.
To manually calculate, convert Mr. Gateway's daily dose (hyrdomorphone 15.6 mg) to oral oxycodone by referring to an equianalgesic table and setting up a mathematical proportion using cross-multiplication.
Equianalgesic Question 2
Question: What reduction of the total dose of 208 mg oral oxycodone per 24 hours for Mr. Gateway accounts for incomplete cross-tolerance?
Choose Formulation and Divide Dose
An immediate-release oxycodone formulation dosed PRN may be more beneficial at this time – Mr. Gateway's pain requirements are likely to change as his postoperative/acute pain improves. Short-acting opioids are not typically dosed around the clock and can be adjusted as indicated.
Long-acting opioids which are administered “around the clock” could be considered in the future depending on his current pain control needs. This could also be initiated as an outpatient.
Divide the total daily dose of oxycodone by 6 since it will be administered every 4 hours.
- around 25 mg every 4 hours (about a 25% reduction)
- around 20 mg every 4 hours (about a 33% reduction)
- around 15 mg every 4 hours (about a 50% reduction)
Providing a range can allow for more flexibility in dosing. E.G.: Oxycodone 15-20 mg PO q4h PRN pain.
