Changes of flow velocity and RBC tracking in single capillaries and capillary densities during severe hypotension in rat brain

Abstract

Cerebral blood flow is autoregulated in a certain range of blood pressure (BP), but below the lower threshold, flow starts to decrease and cerebral metabolism and function become impaired. However, little is known about changes of cerebral blood flow outside the autoregulation range, e.g., in severe hypotension. The purpose of this communication is to examine intraparenchymal capillary blood flow during severe hypotension (BP = 40–50 mmHg) by employing a new high-speed (125–500 frames per sec) confocal fluorescence microscopy and image analysis system. Urethane-anesthetized Wistar rats (n=12) with a cranial window were given fluorescein isothiocyanate (FITC)-dextran (MW = 70 kDa) intravenously as a bolus to detect capillaries and obtain dye dilution curves, and FITC-labeled red blood cells (RBCs) to track RBCs in the capillaries. A video recording of a region of interest in the somatosensory area was taken at a speed of 250 frames per sec for 10 sec before and during hypotension induced by 8–10 ml exanguination and after reinfusion of the blood. The video clip was analyzed with KEIO-IS1 software for plasma flow and KEIO-IS2 (see Schiszler et al., Brain'05) to document tracking of all RBCs, which were individually numbered, and to calculate automatically individual RBC velocities. We found that capillary density was decreased during exanguination. i.e., some capillaries were collapsed and less dense, but increased just after reinfusion, and each capillary reached maximal dilation at 10 minutes after reinfusion. Consequently, the distribution of capillaries changed slightly during exanguination and reinfusion of the blood. The hypotension did not cause much change in plasma flow or RBC tracking. The control velocity of RBC in the capillaries was 2.21.1 mm/sec. Capillaries had different numbers of RBC passages per 10 sec, e.g., some had one RBC passage and others had up to three passages per 10 sec. The hypotension produced a decrease of RBC velocity which was maximum just after the end of blood exanguination (RBC velocity decreased to 4018% of the control), but recovered slightly to 7740% of the control at 10 minutes after blood loss. The RBC velocity showed an overshoot to 13490% just after the reinfusion of the blood, and the increase of RBC velocity persisted for 15 minutes thereafter (14815%). It is concluded that decreases in capillary density and RBC perfusion aggravate the brain tissue damage due to severe hypotension, although plasma flow in the remaining capillaries is unchanged (See Figure 1).