Reversibility of ischemia evaluated by ischemic duration and residual cerebral blood flow

We first investigated the time course of CBF thresholds of ischemic cortices by a retrospective review of 19 patients with MCA occlusion who had clearly defined ischemic duration from onset to angiographic complete recanalization. Secondly, CBF thresholds of ischemic cortices salvageable with intravenous low dose native t- PA infusion therapy (7.2 mg of tisokinase) were examined by a retrospective review of 20 patients with distal embolic occlusions of MCA divisions or branches. All patients underwent pretreatment CBF measurement by SPECT using 99mTc-ECD. Pretreatment SPECT and 3 months post-treatment CT images were compared using computerized coregistration. The degree of hypoperfusion was analyzed by an asymmetry index (AI), which was a count-density ratio for the ischemic lesion to the contralateral corresponding area. Ischemic cortices on SPECT were divided into reversible and irreversible lesions. Judging from the regression lines with 95% confidence interval between the ischemic duration and AI, the infarcted CBF thresholds at 3 and 6 hours after onset may be about 30 and 50% of contralateral presumed normal CBF, respectively. On the other hand, to save the ischemic tissue with our intravenous t-PA infusion therapy, residual CBF might be needed at least 45% of contralateral presumed normal CBF. It is likely that CBF threshold of ischemia surely salvageable with our intravenous t-PA infusion therapy may be approximately 50-55% of contralateral presumed normal CBF.