âą Background and Purpose We tested the hypothesis that cerebral hypoperfusion after experimental global cerebral ischemia is caused by plugging of the microcirculation with activated leukocytes using in vivo microscopic observation of the behavior of leukocytes in the cortical microcirculation during the transition from postischemic hyperperfusion to hypoperfusion.Methods Anesthetized and ventilated rats (n=24) were equipped with a closed cranial window. Physiological variables and cortical regional cerebral blood flow (laser-Doppler flowmetry) were measured continuously. Leukocytes were labeled intravitally with rhodamine 6G and visualized in the microcirculation of the brain surface and outer layers of the cortex with confocal laser scanning microscopy from preischemia to 4 hours after reperfusion that followed 10 minutes of global cerebral ischemia (rCBF<10% of control).Results In controls (n=8), there were no signs of leukocyte activation over the 4-hour observation period. In ischemic rats (n=16), during the transition from hyperperfusion to hypoper-S evere transient global forebrain ischemia leads to a characteristic pattern of cerebral blood flow (CBF) changes in several species: After recirculation a short period of hyperperfusion develops, followed by prolonged hypoperfusion.
-2 The factors causing this blood flow pattern and its consequences for ischemic damage are not fully understood.Generally, hyperperfusion is attributed to maximal vasodilation caused by the accumulation of vasodilator metabolic products, such as H + ,K + , nitric oxide, adenosine, etc, 2 -4 although neurogenic mechanisms may be involved as well.5 It is less clear what causes hypoperfusion. The concept of a mechanical obstruction of the postischemic microcirculation with subsequent hypoperfusion 6 or "no-reflow" 7 by leukocytes ("leukocyte plugging" 8 ) has received much attention. 910 Leukocytes may be activated during ischemia, and activated leukocytes interact with the vascular endothelium, possibly resulting in microvascular obstruction. 10 In skeletal muscle, Harris and Skalak" demonstrated that even a few activated leukocytes may have a profound impact on the resistance of microvascular networks and therefore blood flow.Received April 5, 1993; final revision received October 28, 1993; accepted January 5, 1994.From the Department of Neurology, Humboldt University, Berlin, Germany.Correspondence to Dr Ulrich Dirnagl, Department of Neurology, Humboldt University, Schumannstr 20/21, 10098 Berlin, Germany.fusion there was no change in the behavior of leukocytes. Most notably, no capillary pluggers were seen. In the postischemic period only a slight increase of the number of leukocytes rolling along or sticking to the venular endothelium was seen, and very few capillaries were plugged by leukocytes. Extravasation of leukocytes into the brain tissue was observed in 8 rats beginning 2 hours after ischemia with a variable degree between animals.Conclusions Because there was only mild activation of leukocyte-endothelium interaction w...