Study designs of evaluations included in the review
Randomised controlled trials (RCTs) were eligible for inclusion in the review.
Specific interventions included in the review
Studies that compared acetaminophen (oral or i.v.) with placebo, given in addition to PCA morphine, were eligible for inclusion. The exclusion criteria were: the use of continuous morphine infusion or continuous regional anaesthesia in addition to PCA morphine; the use of PCA with an alternative opioid (not morphine); the use of non-steroidal anti-inflammatory agents in the control group, or the use of other analgesics in either the intervention or the control group; and rectal administration of acetaminophen. Most of the included studies involved comparisons of i.v. acetaminophen and i.v. saline (placebo); other studies evaluated oral acetaminophen compared with placebo, PCA i.v. morphine plus acetaminophen compared with PCA i.v. morphine without placebo, and i.v. acetaminophen or propacetamol compared with placebo. Treatment began either during or after surgery and lasted between 24 and 48 hours.
Participants included in the review
Studies that included patients aged over 18 years who had undergone major surgery and required PCA morphine analgesia were eligible for inclusion. Most of the included studies were in patients undergoing orthopaedic surgery (including herniated disk surgery, spinal stabilisation, hip or knee arthroplasty, and acute limb fracture reduction and fixation); other studies were in patients undergoing elective hepatic resection and elective Caesarean section. No further details of the participants in the included studies were provided.
Outcomes assessed in the review
Studies that reported opioid-related side-effects (primarily nausea, vomiting) or patient satisfaction were eligible for inclusion. The primary outcome for analysis was post-operative nausea and/or vomiting (PONV). Other outcomes of interest were post-operative pain, sedation, pruritus, urinary retention, respiratory depression, and morphine use in first 24 hours. Definitions of each outcome were listed in the review. The included studies used different scales to measure post-operative pain, including visual analogue scales (VAS).
How were decisions on the relevance of primary studies made?
The authors did not state how the papers were selected for the review, or how many reviewers performed the selection.