- The immune system is an important mediator of sensory hypersensitivity, which manifests as hyperalgesia. The inflammatory reaction to tissue damage during surgery may provide postoperatively a source of sensory signals that could induce central sensitization. Experimental data show that central sensitization depends on N-methyl D-aspartate (NMDA) receptor activation, which may be blocked by ketamine, even in subanaesthetic doses. To test whether this effect is clinically important, we compared the effect of preoperative ketamine with placebo on serum interleukin-6 (IL-6) levels (IL-6 is one of the chief mediators of inflammatory response to injury). ASA classes I and II women scheduled for abdominal hysterectomy received either ketamine 0.15 mg/kg or placebo before skin incision. IL-6 levels were measured before and during the operation, and 4, 24, 48 and 72 h later. The ketamine group had lower operative heart rates and blood pressures, and IL-6 levels were significantly lower at 4 h (9.3 ± 12.6 versus 43.8 ± 9.5 pg/ml, P < 0.005), 24 and 48 h postoperatively (24.2 ± 11.9 versus 33.6 ± 5.6 and 6.0 ± 5.5 versus 22.2 ± 8.5, respectively, P < 0.05). Adding low-dose ketamine before incision reduces serum IL-6 response to surgery.