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performed because of the marked differences in methodology and reporting between trials
(Duedahl et al, 2006 2006). However, a systematic review reported that dextromethorphan had
‘preventive analgesia’ effects (Katz & Clarke, 2008 Level I). A later study looking at the effect of
four oral doses of dextromethorphan given over 24 hours to patients after abdominal
hysterectomy showed better pain relief immediately after surgery but not later at 6 hours and
24 hours (Chau‐In et al, 2007 Level II).
Magnesium
Systematic reviews of perioperative magnesium, failed to find convincing evidence of
improved analgesia (Lysakowski et al, 2007 Level I) or any ‘preventive analgesic’ effects
(McCartney et al, 2004 Level I). Magnesium added to morphine for PCA was opioid‐sparing and
led to better pain relief (Unlugenc et al, 2003 Level II); added to tramadol it was opioid‐sparing
but only provided better pain relief for the first 2 hours (Unlugenc et al, 2002 Level II).
IV magnesium may be useful in the treatment of migraine, however the studies are
contradictory (see Section 9.6.5 for details).
Amantadine and memantine
A bolus dose of IV amantadine had no effect on postoperative analgesia after abdominal
hysterectomy (Gottschalk et al, 2001 Level II). However perioperative oral amantadine reduced
morphine consumption, wound pain on palpation and bladder spasms, after radical
CHAPTER 4 prostatectomy (Snijdelaar et al, 2004 Level II).
Oral memantine reduced the number of demands for bolus doses of ropivacaine for analgesia
via a brachial plexus catheter and, in combination with a continuous ropivacaine infusion, led
to a reduction in the incidence of phantom limb pain at 6 months but not 12 months, following
traumatic upper limb amputation (Schley et al, 2007 Level II). It was not effective in reducing the
incidence of postmastectomy pain syndrome (Eisenberg et al, 2007 Level II).
Key messages
1. Perioperative low‐dose ketamine used in conjunction with patient‐controlled analgesia
morphine is opioid‐sparing and reduces the incidence of nausea and vomiting (N) (Level I
[Cochrane Review]).
2. In general, a perioperative low‐dose ketamine infusion is opioid‐sparing, but does not
produce a clinically significant reduction in pain scores or opioid‐related adverse effects (S)
(Level I).
3. Ketamine is a safe and effective analgesic for painful procedures in children (N) (Level I).
4. Ketamine and dextromethorphan have preventive (U) but not pre‐emptive analgesic
effects (N) (Level I).
5. Magnesium does not reduce postoperative pain scores or opioid consumption and has no
preventive analgesic effect (N) (Level I).
6. Ketamine may improve analgesia in patients with severe acute pain that is poorly
responsive to opioids, although evidence is conflicting (W) (Level II).
7. Ketamine reduces postoperative pain in opioid‐tolerant patients (N) (Level II).
The following tick box represents conclusions based on clinical experience and expert
opinion.
The primary role of low dose ketamine is as an ‘antihyperalgesic’, ‘antiallodynic’,
‘tolerance‐protective’ and preventive analgesic, rather than as an analgesic per se (N).
86 Acute Pain Management: Scientific Evidence
