Page 24 Acute Pain Management

 The information about the complications of nitrous oxide is from case reports only. There
are no controlled studies that evaluate the safety of repeated intermittent exposure to
nitrous oxide in humans and no data to guide the appropriate maximum duration or
number of times a patient can safely be exposed to nitrous oxide. The suggestions for the
use of nitrous oxide are extrapolations only from the information above. Consideration
should be given to duration of exposure and supplementation with vitamin B12,
methionine, and folic or folinic acid (U).
 If nitrous oxide is used with other sedative or analgesic agents, appropriate clinical
monitoring should be used (U).
SUMMARY NMDA‐receptor antagonists

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

2. [Cochrane Review]).
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 primary role of low dose ketamine is as an ‘antihyperalgesic’, ‘antiallodynic’,
‘tolerance‐protective’ and preventive analgesic, rather than as an analgesic per se (N).
Antidepressant drugs
1. In neuropathic pain, tricyclic antidepressants are more effective than selective
serotonergic re‐uptake inhibitors (S) (Level I [Cochrane Review]).
2. Duloxetine is effective in painful diabetic neuropathy and fibromyalgia (N) (Level I
[Cochrane Review]).
3. There is no good evidence that antidepressants are effective in the treatment of chronic
low back pain (R) (Level I [Cochrane Review]).
4. Tricyclic antidepressants are effective in the treatment of chronic headaches (U) and
fibromyalgia (N) (Level I).
5. Antidepressants reduce the incidence of chronic neuropathic pain after herpes zoster (U)
(Level II).
Note: withdrawal of previous key message:
Antidepressants reduce the incidence of chronic neuropathic pain after breast surgery
This has been deleted as the information and evidence supporting it has been withdrawn.

xxiv Acute Pain Management: Scientific Evidence