PCA Opioid Choices

There does not appear to be a clearly superior opioid for use in PCA devices. Because morphine has an active metabolite, there can be an accumulation of effects especially in patients with renal failure and/or the elderly, so it is generally best to avoid in these patients. Fentanyl has a shorter duration of action than morphine, which is good if one is concerned about accumulating effects, but "bad" in that patients must frequently activate the PCA. The pharmacology of hydromorphone makes it an excellent choice for postoperative PCA use. Meperidine should only be used in the very, very rare case where a patient has documented intolerance to all other opioid choices and then it is recommended to limit the total dose to 600 mg in a 24-h period for no more than 3 days. Methadone is especially useful in patients who take methadone chronically whereas it is not appropriate as a first-line agent in others given the accumulation of effects over time and since the conversion from methadone to another agent when the patient is taking oral analgesics is difficult given the dose-dependent potency of methadone.

Table 18.5 lists the recommended starting demand doses to be used in opioid-na'ive patients. In the opioid-tolerant patient these doses will need to be individualized based on the amount of opioid the patient takes per day leading to higher initial demand doses and possibly the initial use of continuous infusions (see above). High-risk patients, identified as elderly, morbidly obese, or those with a history of obstructive sleep apnea should have lower initial demand doses (e.g., one-halfthe usual demand dose).

Table 18.5 Initial patient-controlled analgesia demand doses in opioid-na'ive patients.

Opioid agonist

Demand dose

Morphine

1-2 mg

Hydromorphone

0.2-0.3mg

Fentanyl

20-30 mcg

Methadone

1-2 mg

Meperidine

10-20mg

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