Quantitative and qualitative effects of isoflurane on movement occurring after noxious stimulation

Background Anesthetic potency is assessed by determination of the anestheti c concentration that prevents gross, purposeful movement in response to nox ious stimulation. It is unclear whether anesthetics cause a progressive dec rease in the number and force of limb movements evoked by noxious stimulati on, or a step decrease (consistent with an all-or-none effect at the site o f action). The authors hypothesized that isoflurane and halothane would pro gressively depress the movement response. Methods: Isoflurane minimum alveolar concentration (MAC) was determined in rats (N = 14) using a clamp applied to a hind paw. Lateral head movements a nd flexions of the forelimbs and hindlimbs were measured with force transdu cers. Isoflurane was adjusted to 0.6, 0.9, 1.1, and 1.4 MAC, the noxious st imulus applied, and the force and number of Limb and head movements determi ned. Force and movement determinations were made in seven additional haloth ane-anesthetized rats. Results: Isoflurane MAC was 1.3 +/- 0.1%. In general, if movement occurred after application of the noxious clamp, the head and all limbs were involve d. At 0.6 MAC, the median number of extremity and head movements was 3.5 (1 0th-30th percentile, 2.0-11.4) with force generated per movement (force/mov ement) = 6.4 (2.0-13.2) N-s. Movement number decreased to 2.1 (0.25-4.2) at 0.9 MAC (P < 0.05), but force/movement was unchanged at 4.5 (0.4-15.1) N-s (Newton-second). At 1.1 MAC, movement number and force/movement decreased to 0.2 CO.O1.5) and 0.1(0.0-3.2) N-s, respectively(P < 0.005). No significa nt movement occurred at 1.4 MAC. The halothane-anesthetized rats had simila r findings, although at 0.6 MAC they generated more movements (10.5 [5.2-19 .8]) than the rats receiving isoflurane (P < 0.05). Conclusions: The results indicate that increasing anesthetic concentration horn 0.6 to 0.9 MAC had little effect on the motor system controlling the f orce of limb movements, and the neural system generating repented limb, mov ements was depressed, consistent with a differential anesthetic effect at s eparate sites.