The response of the feline cerebral circulation to Pa(CO₂) during anesthesia with isoflurane and halothane and during sedation with nitrous oxide

The reduction in cerebral blood flow (CBF) caused by hypocapnia is an important element of neuroanesthetic techniques. While it has been demonstrated previously that the CO₂ response of the cerebral circulation (CO₂.R) is enhanced (i.e., greater ΔCBF/ΔPa(CO₂)) during halothane administration, the effect of isoflurane on CO₂.R has not been evaluated completely. Accordingly, the authors examined CO₂.R in cats during anesthesia with 1.0 MAC isoflurane (with 75% N₂O) and compared it with CO₂.R during anesthesia with 1.0 MAC halothane (with 75% N₂O) and with CO₂.R during the administration of 75% N₂O alone. CO₂.R during anesthesia with isoflurane-N₂O was enhanced relative to that observed during administration of both halothane-N₂O (P < 0.025) and N₂O alone (P < .001). CO₂.R during anesthesia with halothane-N₂O was, in turn, greater than that observed during the administration of N₂O alone (P < 0.025). Furthermore, at similar levels of hypocapnia (Pa(CO₂) 18-20 mmHg), CBF was significantly lower (P < 0.01) during administration of isoflurane-N₂O (29.0 ± 4.5 ml.100 g^-1.min^-1) than during administration of either N₂O (40.6 ± 5.5 ml.100 g^-1.min^-1) or halothane-N₂O (39.6 ± 7.8 ml.100 g^-1.min^-1). CBF values during administration of the N₂O alone and halothane-N₂O were not different during hypocapnia. The results of this study indicate that CO₂.R in cats not only is preserved during administration of 1.0 MAC isoflurane (with 75% N₂O) but is enhanced relative to that observed during anesthesia with 1.0 MAC halothane (with 75% N₂O) and during sedation with N₂O alone. In addition, the induction hypocapnia (Pa(CO₂) 18-20 mmHg) resulted in a reduction of CBF to lower levels during the administration of isoflurane-N₂O than during administration of halothane-N₂O or N₂O alone. If the cerebral circulation of anesthetized humans responds similarly to that of the cat, these results suggest that the induction of hypocapnia during the administration of isoflurane (with N₂O) may facilitate a greater reduction in CBF, and therefore perhaps ICP, than will occur at a comparable Pa(CO₂) during anesthesia with halothane (with N₂O) or during the administration of N₂O alone.