Hypoxemia alters erythrocyte perfusion pattern in the cerebral capillary network

The effect of acute hypoxemia on erythrocyte perfusion rates in individual capillaries of the rat cerebral cortex was studied by intravital video microscopy. The motion of erythrocytes in subsurface capillaries of the frontoparietal cortex was visualized through a closed cranial window using fluorescently labeled red blood cells (FRBC) as markers of flow. FRBC velocity and FRBC supply rate were measured in each capillary at rest, moderate hypoxemia (PaO(2) = 40 mm Hg), and severe hypoxemia (PaO(2) = 26 mm Hg). Lineal density of FRBC in the capillaries was calculated as the ratio of supply rate and velocity. Hypoxemia increased erythrocyte perfusion in virtually all capillaries. Average FRBC supply rate increased by 104% in moderate hypoxemia and by 281% in severe hypoxemia. Average FRBC velocity increased by 66 and 173%, respectively. During severe hypoxemia, FRBC supply rate increased significantly more in capillaries with low resting supply rate compared to those with high resting supply rate. Changes in FRBC velocity exhibited a similar pattern. Lineal density of FRBC increased by 28% in moderate hypoxemia and by 48% in severe hypoxemia. The results suggest that acute hypoxemia promotes perfusion homogeneity and recruitment of erythrocytes in the cerebral capillary network.