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with antiemetic prophylaxis, P6 acupoint stimulation seems to reduce the risk of nausea but
not vomiting (Lee & Done, 2004 Level I).
Supplemental oxygen (inspired oxygen concentration 80%) did not reduce PONV (Orhan‐Sungur
et al, 2008 Level I).
Impairment of gastrointestinal motility
Opioids impair return of bowel function after surgery. The peripheral acting opioid antagonists
alvimopan and methylnaltrexone were effective in reversing opioid‐induced slowing of GI
transit time and constipation, and alvimopan was an effective treatment for postoperative
ileus; insufficient evidence exists about the efficacy or safety of naloxone or nalbuphine
(McNicol et al, 2008 Level I).
A combined formulation of controlled‐release (CR) oxycodone and naloxone has been studied.
Compared with CR oxycodone alone in patients with chronic non‐malignant pain, the
combination formulation resulted in similar analgesic efficacy but less bowel dysfunction
(Vondrackova et al, 2008 Level II).
Urinary retention
Opioids cause urinary retention due to presumed central and peripheral mechanisms. Opioid
antagonists reverse this effect; naloxone reversed opioid‐induced urinary retention in 100% of
patients, while the peripheral opioid antagonist methylnaltrexone was effective in 42% of
study participants (Rosow et al, 2007 Level II). These data suggest that at least part of the
bladder dysfunction caused by opioids is peripherally mediated. CHAPTER 4
Premedication with gabapentin reduced urinary retention caused by opioids (NNT 7) (Tiippana
et al, 2007 Level I). This effect is most likely related to the opioid‐sparing effect of gabapentin.
Pruritus
The mechanism of opioid‐induced pruritus, which is particularly common after neuraxial
opioid administration, is not fully understood, but central mu‐opioid receptor‐mediated
mechanisms are thought to be the primary cause (Ganesh & Maxwell, 2007). Naloxone,
naltrexone, nalbuphine and droperidol are effective in the treatment of opioid‐induced
pruritus, although minimum effective doses remain unknown (Kjellberg & Tramer, 2001 Level I).
Prophylactic 5HT3 antagonists reduce the incidence of pruritus after neuraxial opioids (Bonnet
et al, 2008 Level I); however the authors of this meta‐analysis were concerned about possible
publication bias.
Premedication with gabapentin (Sheen, Ho, Lee, Tsung & Chang, 2008 Level II) and mirtazepine
(Sheen, Ho, Lee, Tsung, Chang et al, 2008 Level II) also reduced the incidence and severity of
pruritus associated with intrathecal opioids. Evidence of any benefit with propofol is mixed
(Ganesh & Maxwell, 2007). Promethazine was not effective for the treatment of spinal
morphine‐induced pruritus but was associated with more sedation; droperidol and, to a lesser
extent propofol, were of use (Horta et al, 2006 Level II).
Cognitive function and confusion
The risk of delirium and/or changes in cognitive function has been compared in patients
receiving different PCA opioids. There was no statistically significant difference in the rates of
confusion between morphine and fentanyl but the rates were 4.3% for patients given fentanyl
and 14.3% for those receiving morphine; there was less depression of cognitive function with
fentanyl (Herrick et al, 1996 Level II). Compared with morphine, no differences in cognitive
function were reported in patients receiving tramadol (Silvasti et al, 2000 Level II; Ng et al, 2006
Level II), but cognitive function was poorer in patients given hydromorphone (Rapp et al, 1996
Level II). Studies of lower quality reported a significantly greater incidence of delirium with
Acute pain management: scientific evidence 67
