HYPERALGESIA DURING SEDATION - EFFECTS OF BARBITURATES AND PROPOFOL IN THE RAT

Subhypnotic doses of thiopentone are considered to possess antianalges ic or hyperalgesic properties In this study, we have tested the hypoth esis that the coincidence of sedation and hyperalgesia is a property o f both barbiturate and non-barbiturate anaesthetic agents. In a random ized, prospective blinded study the effects of slow (20 min) iv infusi ons of thiopentone, pentobarbitone, methohexitone or propofol on nocic eptive threshold were measured in rats by tail pressure analgesimetry and compared with saline-infused control animals. Nociceptive threshol ds were correlated with measurements of plasma drug concentrations and behavioural assessments, Comparison of pre-infusion nociceptive thres hold with the lowest threshold obtained during drug infusion revealed decreases in all four treatment groups. As a percentage of the pre-inf usion values, the decreases were: thiopentone: 42.5% (P < 0.001), pent obarbitone: 27.8% (P = 0.014), methohexitone: 24.9% (P = 0.013), propo fol: 21.6% (P = 0.006). There were no changer in nociceptive threshold in the control groups. The relationship between nociceptive threshold and plasma drug concentration was usually characterized by an initial decline followed by a rise in nociceptive threshold as the plasma con centration and degree of sedation increased The results support the hy pothesis that hyperalgesia is a property of different anaesthetic agen ts when administered at sub-hynotic concentrations.