Five of the studies analyzed had opioid reduction as
the primary end point. However, this end point has
several limitations including inter-individual variability
in sensitivity to opioids and variability in the level of
tolerable pain.8 For opioid delivered by patientcontrolled
analgesia, other confounders include an
initial learning curve for proper pump use, and patients
may worry that they are pushing the button too often,
creating a self-imposed maximum. In addition, when
morphine consumption is used as an end point, the
sample sizes required for 80% power are approximately
twice that predicted unless one adjusts for age