Pre-emptive analgesia: a systematic review of clinical studies.

Basic scientific evidence suggests that an analgesic intervention made before surgery will produce a better outcome than the same intervention made after surgery. The evidence from randomized controlled trials (RCTs) which tested this hypothesis in patients is reviewed. Four studies with paracetamol or NSAIDs did not show any pre-emptive effect. Of seven studies with local anaesthetic six did not show a pre-emptive effect. In the four studies with opioids there was weak evidence of a pre-emptive effect in three. There are few perfect RCTs, and unfortunately this rule applies in the pre-emptive analgesia field. Many of the studies which did not show a pre-emptive effect lacked power. The opioid studies which did show a pre-emptive effect had other technical weaknesses. One way to combat lack of power would be to combine data (meta-analysis). This is very difficult in this field because of the outcome measures which investigators are using.