2.4. Patient-controlled analgesia

PCA is a technique which allows patients to self-administer small doses of an analgesic drug within preset limits. The first experiences of this method in pain management were described in the late 1960’s (Sechzer 1968). At first this technique was used as a research tool to accurately measure pain, while later it has also become a method for improving the treatment of postoperative pain. In addition, PCA can be used in diverse clinical settings including analgesia during labour (Frank et al. 1987) and in the treatment of cancer pain (Citron et al. 1986, Kalso & Vainio 1990).

A PCA device delivers a prescribed amount of drug (a bolus dose), when the patient presses a button connected to the device. To help prevent excessive drug administration, the device ignores further patient demands until a lockout period has passed. This lockout interval is programmed into the pump and is usually set at between 5 and 10 minutes. In addition to the bolus dose, a background infusion is also programmable in the device. The purpose of this background infusion is to maintain therapeutic drug levels in the blood while the patients are not self-initiating boluses, such as during sleep. As it is impossible to predict any patient’s absolute requirement for the analgesic drug, however, the use of a continuous background infusion may lead to an increased risk of side effects (Notcutt & Morgan 1990). On most pumps it is possible to set a maximum dose over a period of time (usually 4 hours).

The literature concerning the efficacy of PCA is contradictory. There are studies which indicate that PCA is superior to i.m. supplementation of the same opioid (Wheatley et al. 1992, Boulanger et al. 1993), while in other studies no difference in postoperative analgesia between the PCA and i.m. groups have been detected (Kleiman et al. 1988, McGrath et al. 1989). A meta-analysis of randomised controlled trials comparing PCA and i.m. injections indicated that analgesic efficacy was better with PCA and the patients preferred PCA over conventional analgesia (Ballantyne et al. 1993). The results from a more recent meta-analysis show also that PCA provides a slightly better analgesia than conventional opioid treatment (Walder et al. 2001).

Morphine has been the opioid used in many studies for PCA-delivered rescue analgesic. Recent studies have suggested, however, that NSAIDs may reduce morphine requirements by reducing the excretion of the active metabolite, morphine-6-glucuronide. A morphine sparing effect may not result in parallel reductions in opioid-related adverse effects (Tighe et al. 1999). Fentanyl has inactive metabolites and minimal renal excretion, and because of these properties it may be a better rescue analgesic than morphine for studies assessing the analgesic and adverse effects of NSAIDs. Fentanyl has been administered for postoperative pain as PCA bolus doses with and without background infusion (Lehmann et al. 1988, Rowbotham et al. 1989, Welchew & Breen 1991, Laitinen & Nuutinen 1992). In the PCA bolus mode, the bolus doses of fentanyl administered range from 20–50 µg with lockout intervals up to 10 minutes (Laitinen & Nuutinen 1992, Cooper et al. 1995, Howell et al. 1995). No published study has directly assessed the advantages of adding a background infusion to PCA fentanyl bolus dose. Oxycodone, 14-hydroxy-7,8-dihydrocodeinone, is a strong opioid agonist with properties similar to morphine (Pöyhiä & Kalso 1992). However, hallucinations may occur less frequently with oxycodone than with morphine (Kalso & Vainio 1990). In postoperative patient-controlled analgesia, oxycodone appears to be equipotent to morphine with no differences in side effects, such as nausea, vomiting, pruritus and urinary retention (Silvasti et al. 1998). At present, local anesthetics are also being infused using the PCA technique, e.g. into the intrascalenic plexus (Borgeat et al. 1997).