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Other Concerns
PCA complications: The most well-studied adverse effects of PCAs are nausea and respiratory depression. There is no difference between PCAs and conventional analgesia in rates of nausea or respiratory depression.17
Nausea is the most common side effect in postoperative patients on PCAs. Patients rapidly develop tolerance to nausea over a period of days. However, many clinicians are concerned about respiratory depression and the risk of death. The overall incidence of respiratory depression with PCAs is less than 1% (range from 0.1 to 0.8%), similar to conventional analgesia. However, the incidence is significantly higher when basal rates are used, rising to 1.1 to 3.9%. Other factors predisposing a patient to increased risk of respiratory depression are older age, obstructive sleep apnea, hypovolemia, renal failure, and the concurrent use of other sedating medications.18
Medication errors are also common. The overall incidence of medication mishaps with PCAs is 1.2%.19 More than 50% of these occur because of operator-related errors (e.g., improper loading, programming errors, and documentation errors). Equipment malfunction is the next most common error.
Opioid equianalgesic dosing conversions: The equianalgesic dose ratio is the ratio of the dose of two opioids required to produce the same analgesic effect. (See Table 1, right.) For example, IV morphine is three times as potent as oral morphine, with an equianalgesic dose ratio of 1:3. Equianalgesic dose tables vary somewhat in their values, which have been largely determined by single-dose administration studies.20 The generalizability of these tables to chronic opioid administration is not well studied.
Incomplete cross tolerance: When switching from one opioid to another, lower doses can be used to control pain.21, 22 Tolerance to one opioid does not completely transfer to the new opioid. Starting at half to two-thirds of the new opioid dose is generally recommended to avoid opioid-specific tolerance and inter-individual variability.23,24
Back to the Case
Opioids are the mainstay of pharmacological management of moderate-to-severe cancer pain. Evaluation of the patient reveals that her acute increase in pain is likely due to progression of her cancer. She had been taking morphine (sustained-release, 90 mg oral) twice daily for her pain and had been using approximately five doses per day of immediate-release oral morphine 20 mg for breakthrough pain. This is equivalent to a total 24-hour opioid requirement of 280 mg oral morphine.