Cortical impedance and extracellular volume changes following middle cerebral artery occlusion in cats

In 30 adult cats, anesthetized with nitrous oxide and halothane, the middle cerebral artery was occluded using a transorbital approach. Extracellular volume changes were assessed by recording cortical impedance, and correlated with blood flow, tissue osmolality, and water and electrolyte content of brain tissue. Following middle cerebral artery occlusion, cortical impedance, after a free interval of about 1 min, sharply increased and after 30 to 60 min gradually stabilized between 180 and 200% of control. Calculated extracellular fluid volume decreased from 23.8 +/- 1.2 to 1.3 +/- 1.0% after 1 h and to 12.5 +/- 1.0% after 2 h of ischemia. Shortly after middle cerebral artery occlusion, extracellular volume shifts correlated with blood flow over a range from 3 to 50 ml/100 g/min. Two hours later, a threshold-like dependency existed: below 25 ml/100 g/min extracellular space was reduced to about 50% of control; above 32 ml/100 g/min extracellular space was normal. Non-threshold correlations existed between extracellular space, tissue osmolality, and the electroencephalogram. Final water content of brain tissue correlated with the size of the extracellular space after 15 min, but ont after 2 h of ischemia. This indicates that the narrowing of the extracellular compartment and ischemic brain edema are relatively independent consequences of cerebral ischemia.