POTENCY (MINIMUM ALVEOLAR ANESTHETIC CONCENTRATION) OF ISOFLURANE IS INDEPENDENT OF PERIPHERAL ANESTHETIC EFFECTS

The spinal cord is an important site where inhaled anesthetics suppres s movement in response to noxious stimuli. Inhaled anesthetics also ac t in peripheral tissues, although it is unclear whether these actions influence anesthetic requirements. In six isoflurane-anesthetized mong rel dogs, we placed Y cannulas in the lower aorta and vena cava, allow ing us to divert blood to, and infuse blood from, a bubble oxygenator/ roller pump system or to maintain normal blood flow. This technique pe rmits a greatly diminished isoflurane concentration at the site of the noxious stimulus (tail), while maintaining isoflurane in the remainde r of the body. After baseline minimum alveolar anesthetic concentratio n (MAC1) was determined, venous blood from the lower body was diverted to the bubble oxygenator and reinfused into the lower body via the ao rtic cannula; MAC2 was determined with isoflurane in the lower body at approximate to 0.2%, and MAC3 was determined with isoflurane in the l ower body matched to the end-tidal isoflurane. Bypass was terminated, the native circulation established, and MAC4 determined. MAC1, 2, 3, a nd 4 were (mean +/- SD) 1.3 +/- 0.3%, 1.2 +/- 0.1%, 1.2 +/- 0.2%, and 1.1 +/- 0.2%, respectively (P > 0.05). We conclude that the peripheral effects of isoflurane do not influence the response to a noxious stim ulus.