LASER-DOPPLER MEASUREMENT OF THE EFFECTS OF HALOTHANE AND ISOFLURANE ON THE CEREBROVASCULAR CO2 RESPONSE IN THE RAT

We used laser-Doppler flowmetry to compare the effects of the volatile anesthetics, isoflurane and halothane, on the cerebrovascular response to CO2 inhalation in male Sprague-Dawley rats. The effects of 0.5 and 1.5 minimum alveolar anesthetic concentrations (MAC) of halothane and isoflurane on the microcirculatory response to CO2 were compared at 22, 36, and 66 mm Hg end-tidal partial pressure of carbon dioxide (ETco(2)). An additional group of animals was anesthetized by continuous barbiturate infusion (10-20 mg.kg(-1).h(-1)). Arterial blood pressure was maintained at control levels throughout the experiment using an infusion of phenylephrine (0.5-5 mu g.kg(-1).min(-1)). Laser-Doppler flow (LDF) was greater at 1.5 MAC than at 0.5 MAC at each ETco(2) for both anesthetics. The CO2 reactivity (percent LDF change/mm Hg change ETco(2)) from hypocapnia to normocapnia was similar to that from normocapnia to hypercapnia. CO2 reactivity with barbiturate infusion and 0.5 MAC isoflurane were 1.78 +/- 0.19 and 2.28 +/- 0.22 (no difference), respectively, both being greater than that with 0.5 MAC halothane at 1.19 +/- 0.14 (P < 0.05). A similar difference was suggested at 1.5 MAC halothane and 1.5 MAC isoflurane (1.99 +/- 0.25 and 2.67 +/- 0.35, respectively). The CO2 reactivity was greater at 1.5 MAC halothane compared to 0.5 MAC halothane. The results of this study suggest that an increase in arterial CO2 may increase cerebrocortical red cell flow more with isoflurane than with halothane, at least at moderate anesthetic concentrations.