Dynamic in vivo measurement of erythrocyte velocity and flow in capillaries and of microvessel diameter in the rat brain by confocal laser microscopy

A new method for studying brain microcirculation is described. Both fluores cently labeled erythrocytes and plasma were visualized on-line through a cl osed cranial window in anesthetized rats, using laser-scanning two-dimensio n confocal microscopy. Video images of capillaries, arterioles, and venules were digitized off-line to measure microvessel diameter and labeled erythr ocyte flow and velocity in parenchymal capillaries up to 200 mu m beneath t he brain surface. The method was used to analyze the rapid adaptation of mi crocirculation to a brief decrease in perfusion pressure. Twenty second per iods of forebrain ischemia were induced using the four-vessel occlusion mod el in eight rats. EEG, arterial blood pressure, and body temperature were c ontinuously controlled. In all conditions, labeled erythrocyte flow and vel ocity were both very heterogeneous in capillaries. During ischemia, capilla ry perfusion was close to 0, but a low blood flow persisted in arterioles a nd venules, while EEG was flattening. The arteriole and venule diameter did not significantly change. At the unclamping of carotid arteries, there was an instantaneous increase (by about 150%) of arteriole diameter. Capillary erythrocyte flow and velocity increased within 5 seconds, up to, respectiv ely, 346 +/- 229% and 233 +/- 156% of their basal value. No capillary recru itment of erythrocytes was detected. All variables returned to their basal levels within less than 100 seconds after declamping. The data are discusse d in terms of a possible involvement of shear stress in the reperfusion per iod.