IN-SITU ARTERIAL AND BRAIN-TISSUE PCO2 RESPONSES TO ACETAZOLAMIDE IN CATS

We have studied how in situ arterial (Pa-CO2) and brain tissue P-CO2 ( Pb-CO2) responses to acetazolamide (AZ) are affected by respiratory pa tterns. Sixteen cats were anesthetized with ketamine. Group 1 cats (n = 7) breathed air spontaneously. Group 2 cats (n = 6) were paralyzed a nd ventilated mechanically to maintain Pa-CO2 at 37 +/- 1 Torr before AZ administration; the respiratory rate and depth did not change durin g the course of measurements. Two CO2 sensors to measure in situ Pa-CO 2 and Pb-CO2 continuously were used. One was placed through a burr hol e into the cerebral white matter 15 mm in depth, and another was inser ted into the femoral artery. After intravenous administration of AZ (2 0 mg/ kg), Pa-CO2 decreased, after a significant transient rise, and t hen returned gradually to the baseline in group 1, but it increased gr adually and reached a new steady state in group 2. Ph(CO2) and two gro ups Immediately after administration. We conclude that AZ resulted in a large increase in both Pb-CO2 and the Pb-CO2-Pa-CO2 gradient and tha t there are two distinct In situ Pa-CO2 responses to AZ in spontaneous ly breathing vs. mechanically ventilated animals. The mechanisms for t hese observations are discussed.