Opioids often are categorized as low potency (i.e. codeine, hydrocodone) and high-potency (i.e. oxycodone, morphine, hydromorphone, fentanyl). When given in “equianalgesic doses,” the analgesic effect and common side effects (nausea/vomiting, constipation, sedation, confusion, pruritis) of different opioids can vary in different patients. Due to differences in levels of expressed subtypes of opioid receptors, a given patient might be more sensitive to the analgesic effect or side effects of a specific medication. Therefore, if dose escalation of one opioid is inadequate to control pain and further increases in dose are limited by intolerable side effects, rotation to another opioid is recommended.4 Tables documenting equianalgesic doses of different opioids are based on only moderate evidence from equivalency trials performed in healthy volunteers.6 Due to interpatient differences in responses, it is recommended that the equianalgesic dose of the new medication be decreased by 25% to 50% for initial dosing.5
Certain treatments are indicated for specific pain syndromes. Bony metastases respond to NSAIDs, bisphosphonates, and radiation therapy in addition to opioid medications. As focal back pain is the first symptom of spinal cord compression, clinicians should have a high index of suspicion for compression in any patient with malignancy and new back pain. Steroids and radiation therapy are considered emergent treatments for pain control and to prevent paralysis in this circumstance. Pain due to bowel obstruction is usually colicky in nature and responds well to octreotide as discussed in the section on nausea and vomiting. Steroids (such as dexamethasone 4 mg PO bid-tid) might be an effective adjuvant medication in bone pain, tumor pain, or inflammation.
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Table 2. Starting doses and equianalgesic doses of common opioids
*Half this dose should be used in renal or liver dysfunction and in the elderly.
Preferred in renal dysfunction.
SOURCES: Adapted from Assessment and treatment of physical pain associated with life-limiting illness. Hospice and Palliative Care Training for Physicians: UNIPAC. Vol 3. 3rd ed. Glenview, IL: American Academy of Hospice and Palliative Medicine; 2008, and Evidence-based standards for cancer pain management. J Clin Oncol. 2008;26(23):3879-3885.
Back to the Case
At home, the patient was taking 60 mg of extended-release morphine twice daily and six doses per day of 15-mg immediate-release morphine for breakthrough pain. This is the equivalent of 210 mg of oral morphine in 24 hours. His pain is severe on this regimen, but it is unclear how much of this medication he is absorbing due to his vomiting. Using the IV route of administration and a patient-controlled analgesia (PCA) system will enable rapid dose titration and pain control. The equivalent of the 24-hour dose of 210 mg oral morphine is 70 mg IV morphine, which is equivalent to a drip basal rate of approximately 3 mg IV morphine per hour. This basal rate with a bolus dose of 7 mg (10% of the 24-hour dose) IV morphine q1 hour prn is reasonable as a starting point.
Review of the Data: Nausea and Vomiting
Nausea and vomiting affect 40% to 70% of patients in a palliative setting.7 A thorough history and physical exam can enable one to determine the most likely causes, pathways, and receptors involved in the process of nausea and vomiting. It is important to review the timing, frequency, and triggers of vomiting. The oral, abdominal, neurologic, and rectal exams, in addition to a complete chemistry panel, offer helpful information. The most common etiologies and recommended medications are included in Table 3. It is worthwhile to note that serotonin-antagonists (i.e. ondansetron) are first-line therapies only for chemotherapy and radiation-therapy-induced emesis. If a 24-hour trial of one antiemetic therapy is ineffective, one should reassess the etiology and escalate the antiemetic dose, or add a second therapy with a different (pertinent) mechanism of action. Although most studies of antiemetic therapy are case series, there is good evidence for this mechanistic approach.8