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2.3 OUTCOME MEASURES IN ACUTE PAIN
MANAGEMENT
What follows is a brief guide to some of the outcome measures used particularly in the acute
pain literature. A comprehensive review is beyond the scope of this document and more
detail may be found elsewhere (Breivik et al, 2008).
2.3.1 Outcome measures
Pain
The aim of many clinical trials is to determine whether a drug or intervention provides
adequate pain relief for the majority of participants or is equivalent or non‐inferior to an
existing accepted treatment. This can be achieved by repeated single measures at fixed time
points, which may encompass only a proportion of the total illness. When comparison is made
CHAPTER 2 with a placebo, a statistically significant result can be achieved with a relatively small number
of patients (eg n=40) (Collins et al, 2001). The primary outcome is chosen by the researcher and
may not be of direct importance to the individual patient, particularly if it relates to only a
proportion of the total time he/she was in pain. It is also important to consider that
statistically significant differences in pain scores may not reflect clinically significant
differences, although these are harder to define (see above).
Data derived from categorical and visual analogue scales of pain intensity or relief produce a
range of summary outcomes that can be used to assess (Moore et al, 2003):
• the degree of analgesic effect:
− difference between the baseline and postintervention score of pain intensity or pain
relief (Summed pain intensity difference [SPID]);
− the area under the time‐analgesic effect curve for a given time (total pain relief
[TOTPAR]);
− dose of rescue analgesic consumption required in a given time period (eg PCA use);
• the time to analgesic effect:
− the time to onset of analgesic effect;
− mean time to maximum reduction in pain intensity or to peak relief;
• the duration of effect:
− time for pain to return to at least 50% of baseline;
− time for pain intensity to return to baseline or for pain relief to fall to zero;
− time to remedication/rescue analgesia.
A widely used method of describing the effectiveness of an analgesic intervention is the
number‐needed‐to‐treat (NNT). In this setting it is commonly defined as the number of
patients that need to be treated to achieve at least 50% pain relief (eg at least 50% maximum
TOTPAR) in one patient compared with a placebo over a 4 to 6 hour treatment period (Moore
et al, 2003). Analysis at other cut‐off points (30% to 70% max TOTPAR) has shown the same
relative efficacy of different treatments (McQuay et al, 2003).
The validity of this approach as a true method of comparison may be questioned as there is
no standardisation of the acute pain model or patient and only single doses of the analgesic
agents are used. However, it may sometimes be reasonable to extrapolate estimates of
analgesic efficacy from one pain model to another (Barden et al, 2004 Level I).
40 Acute Pain Management: Scientific Evidence
