Video microscopy of cerebrocortical capillary flow: response to hypotension and intracranial hypertension

Abstract

Although autoregulation of cerebral blood flow is well established, the response of cerebral capillary circulation to reduced cerebral perfusion pressure (CPP) is unclear. The objective of this study was to determine whether red cell flow velocity in individual capillaries of the cerebral cortex is maintained during acute decreases in CPP. Microcirculation of the superficial parietal cerebral cortex of adult barbiturate-anesthetized artificially ventilated rats was visualized using a new design of closed-perfused cranial window and epifluorescent-intensified video microscopy. Fluorescein-isothiocyanate-labeled red blood cells (FRBC) injected intravenously were used as markers of capillary flow. CPP, defined as mean arterial pressure minus intracranial pressure, was reduced by controlled hemorrhage or by stepwise elevation of local intracranial pressure. The movement of FRBC in the parenchymal capillary network was video recorded at each pressure level, and FRBC velocity in each capillary was measured off-line with use of the dual-window digital cross-correlation technique. FRBC flux in the capillaries was measured by automated cell counting. FRBC velocity at normal perfusion pressure was 1.47 +/- 0.58 (SD) mm/s and changed little in the perfusion pressure range of 70-120 mmHg. The autoregulatory index in this pressure range was 0.0049 mm.s-1.mmHg-1. Opening of previously unperfused capillaries was not observed. FRBC flux correlated with FRBC velocity, but the latter was maintained in a narrower range than FRBC flux, suggesting a decrease in capillary diameter or hematocrit with decreasing perfusion pressure. The results suggest that flow autoregulation is associated with the maintenance of capillary flow velocity and that capillary recruitment does not contribute to flow autoregulation in the rat cerebral cortex.

20-HETE contributes to the acute fall in cerebral blood flow after subarachnoid hemorrhage in the rat

“…These animals were anesthetized and surgically prepared as described in Surgical preparation. In addition, a closed cranial window was prepared over the right parietal cortex with techniques described previously (20). The rats were then transferred to the stage of an intravital microscope, paralyzed with 80 mg of gallamine, and artificially ventilated with a 30% O 2-70% N2 mixture.…”

Hemodynamic scaling of fMRI-BOLD signal: validation of low-frequency spectral amplitude as a scalability factor

“…Variability in vascular compliance may lead to variation in blood flow velocity. In one study for instance, blood flow velocity was measured by video microscopy using an injected fluorescent compound that labeled red blood cells [15]. Using video microscopy and frame-by-frame analysis, the authors observed that, during 5% CO 2 ventilation (i.e., hypercapnia, capillary flow velocity increased relative to normal breathing (i.e., normocapnia).…”

Can dynamic susceptibility contrast magnetic resonance imaging perfusion data be analyzed using a model based on directional flow?

“…These studies show that the capillary bed in gray matter consists of arteriolar capillaries that feed into approximately twice the number of true capillaries, which are drained in turn by a reduced number of venous capillaries. True capillaries have heterogeneous flow lengths between 150 and 500 microns and RBC transit times of 100 -300 msec (36). Flows in arteriolar and venous capillaries are at least twice this speed (36), and "non-auto regulating thoroughfare capillaries," which represent nonexchange thoroughfare routes, have also been described (37).…”

“…True capillaries have heterogeneous flow lengths between 150 and 500 microns and RBC transit times of 100 -300 msec (36). Flows in arteriolar and venous capillaries are at least twice this speed (36), and "non-auto regulating thoroughfare capillaries," which represent nonexchange thoroughfare routes, have also been described (37). This structure supports the rapid delivery of blood to the exchange capillaries and is entirely in keeping with the behavior of the contrast bolus on DSCE-MRI.…”

“…Plasma flow is approximately 2.5 times slower than RBCs, giving a typical mean bolus transit time through the true capillaries of approximately 0.5-0.7 seconds, with an estimated mean flow rate of approximately 0.4 mm/second (38). The flow rates in arteriolar and venous capillaries and in larger vessels is at least double this, and flow in small draining veins a few millimeters distant can exceed 30 mm/ second (26,36,37). Using these values, a contrast bolus might transit a 1-mm voxel in as little as half a second, which is in keeping with the estimates calculated here.…”