Understanding oxymoronic oxycodone cesarean analgesia

Analgesia after caesarean section has been somewhat eccentric in Melbourne; a rectal dose of oxycodone and diclofenac, sometimes with the extra packing of rectal paracetamol has been popular. Curiously, there is little evidence or acknowledgement of this type of analgesia in the literature or in authoritative guidelines, and there seems to be very little pharmacological information about the pharmacokinetics of rectal diclofenac and oxycodone. What is known about the effects of 1 g of rectal paracetamol does little reassure about the potential benefits of the other rectal drugs.

The analgesic efficacy of NSIADS after CS is well studied and confirmed, although I have expressed concerns about the potential harm of NSAIDS in some cases.
Opioids of course remain the mainstay of postcesarean analgesia but there is a debate about the best route of administration.

Oral oxycodone has become increasing popular for postcesarean analgesia, and this does have a fair bit of published support. Recently, Davis et al in American Journal of O&G (April 2006) compared oral oxycodone and paracetamol with IVPCA morphine and found better analgesia and satisfaction in the oral group. Both groups had NSAID. The oral group received 10 mg of oral oxycodone 3 hourly for 12 hours from the end of surgery and then as required. Accumulative doses were not published, but a perusal of other caesarean analgesia studies indicate that the IV morphine consumption for the first 24 hours averages around 40 mg, although there is a wide spread.

Oxycodone seems to have similar potency to morphine, but a bioavailability of 60%, twice that of morphine. Thus 40 mg of IV morphine should equal about 60 mg of oral oxycodone. Thus to deliver the average analgesia, 7.5 mg 3 hourly should be prescribed, with paracetamol and NSAID (if not contraindicated). Of course there is a range of analgesic requirements, and the demands diminish over the hours, thus a 5 – 15 mg dose range, starting at 10 mg, would seem to cover most, with a parenteral rescue (Davis et al used 50 mg pethidine in 10% of patients on their rigid oral regimen) available.

Controlled release oxycodone has also become popular although its use for postoperative pain is somewhat, if not absolutely off-licence. Its use for post caesarean analgesia is unstudied, although it seems to have some benefit after orthopaedic surgery when compared to standard therapies. The calculations above suggest a 20 mg CR 12 hourly dose with 10 mg IR extra as required may be the appropriate regimen after caesarean, but this is unstudied.

All the studies on oral analgesia have been after elective surgery. The oral route may not be appropriate after the emergency caesarean after a long labour or other condtions which may inhibit gastric emptying. Old fashioned IM morphine, as long as it is made available and given seems to give as much satisfaction as IVPCA and even neuraxial morphine. The analgesic benefits of neuraxial opioid may be negated by pruritis and nausea (Lim, Singapore Med J, Aug 2005). This debate is bound to carry on for some time!