Automated measurement of fluorescently labeled erythrocyte flux in cerebrocortical capillaries

Effect of hemodilution on RBC velocity, supply rate, and hematocrit in the cerebral capillary network

The effect of isovolemic hemodilution on the circulation of red blood cells (RBCs) in the cerebrocortical capillary network was studied by intravital videomicroscopy with use of a closed-cranial-window technique in the rat. Velocity and supply rate of RBCs were measured by tracking the movement and counting the number of fluorescently labeled cells. Arterial blood was withdrawn in increments of 2 ml and replaced by serum albumin. Arterial blood pressure was maintained constant with an infusion of methoxamine. Both velocity and supply rate of RBCs increased, by approximately equal amounts, as arterial hematocrit was reduced from 44 to 15%. The maximum increase in RBC velocity was 4.6 and in RBC supply rate was 5.2 times the baseline value. Calculated lineal density of RBC, an index of capillary hematocrit, did not change with hemodilution. The results suggest that RBC flow and oxygen supply in the cerebral capillary network are maintained during isovolemic hemodilution. The "optimal hematocrit" is as low as 15%.

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 fluorescently labeled erythrocytes and plasma were visualized on-line through a closed cranial window in anesthetized rats, using laser-scanning two-dimension confocal microscopy. Video images of capillaries, arterioles, and venules were digitized off-line to measure microvessel diameter and labeled erythrocyte flow and velocity in parenchymal capillaries up to 200 microm beneath the brain surface. The method was used to analyze the rapid adaptation of microcirculation to a brief decrease in perfusion pressure. Twenty-second periods of forebrain ischemia were induced using the tour-vessel occlusion model in eight rats. EEG, arterial blood pressure, and body temperature were continuously controlled. In all conditions, labeled erythrocyte flow and velocity were both very heterogeneous in capillaries. During ischemia, capillary perfusion was close to 0, but a low blood flow persisted in arterioles and 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, respectively, 346 +/- 229% and 233 +/- 156% of their basal value. No capillary recruitment of erythrocytes was detected. All variables returned to their basal levels within less than 100 seconds after declamping. The data are discussed in terms of a possible involvement of shear stress in the reperfusion period.

Regulation of oxygen supply in the cerebral circulation

The dynamics and regulation of red blood cell flow in the cerebral microcirculation was studied by intravital fluorescence video-microscopy in a closed cranial window preparation in the rat. The studies revealed that capillary perfusion in the brain is essentially continuous but a stationary difference from capillary to capillary within the same microvascular network exists. The main mechanism of an increase in flow in cerebral capillaries is an increase in linear velocity with no or minor role for classical capillary recruitment. While cyclic opening and closing of capillaries is not evident, low frequency oscillations in capillary flow velocity are present when perfusion or oxygen supply to tissue is challenged. In hypoxic hypoxia and moderate hypercapnia, RBC velocity increases in all capillaries while in severe hypercapnia, redistribution of RBC velocity in the capillary network occurs. Both systemic hypotension and severe hypercapnia are accompanied by an increase in the homogeneity of capillary flow; this change involves the redistribution of RBC flow between thoroughfare channels and exchange capillaries. Thoroughfare channels may thus provide a recruitable flow reserve in the cerebral microcirculation. The capillary flow response to hypoxic and anemic hypoxia depends on the activity neuronal nitric oxide synthase. These findings suggest the presence of a physiological regulatory mechanism of cerebral capillary red blood cell flow and oxygen supply which may involve neuronal nitric oxide as a mediator.