Synergism between the antinociceptive effects of intrathecal midazolam and fentanyl on both A delta and C somatosympathetic reflexes.

The interaction between the antinociceptive effects of fentanyl and midazolam, administered intrathecally (i.t.), was examined in dogs. Midazolam 1 mg (i.t) depressed the A delta and C fibre mediated somatosympathetic reflexes to 68.3 and 85.2% of control values and then reduced the subsequent doses of fentanyl (i.t.) required to abolish these reflexes by 50%. After midazolam (1 mg, i.t.) the ED values for fentanyl were markedly less than the theoretical predicted additive values. This indicates synergism between the effects of fentanyl and midazolam.