Experimental brain infarcts in cats. I. Pathophysiological observations.

In 48 cases the left middle cerebral artery was occluded under light barbiturate anaesthesia using a transorbital approach. The animals were kept alive for 1, 2, and 4 hours after vascular occlusion. Regional cerebral blood flow was measured by the intracardiac microsphere injection technique before ischemia, 15 min after the onset of ischemia, and at the end of experiments. The density of regional ischemia was correlated with EEG changes and with the electrolyte, water and metabolite content of the same tissue samples in which blood flow was assessed. In the territory of the occluded middle cerebral artery, cortical blood flow decreased from 41.4 +/- 3.8 to 21.3 +/- 4.0 ml/100 g/min (means +/- SE), the actual flow rate depending on the individual efficacy of collateral blood supply. At flow rates below 10--15 ml/100 g/min, ischemia involved more than 50% of the middle cerebral artery territory, water and electrolyte homeostasis was severely disturbed and ischemic brain edema developed. Adenosine triphosphate decreased to about 60% of the control value at flow rates below 40 ml/100 g/min, but it remained at this level down to flow rates as low as 5 ml/100 g/min. EEG intensity -- but not EEG frequency -- decreased in parallel with blood flow, indicating that with increasing density of ischemia an increasing portion of the excitable neuropil was inhibited. The development of ischemic brain edema determined the further progression of ischemia. When blood flow decreased below the threshold for water and ion disturbance, ischemia was progressive (critical ischemia), but an amelioration of flow occurred in animals in which flow remained above this level (non-critical ischemia). In the contralateral hemisphere the EEG, blood flow, water and electrolyte content did not change significantly during the initial few hours of ischemia. Diaschisis, in consequence, was not a prominent feature during the early phase of infarct development.