Increased leukocyte transit times through capillaries contributes to maldistribution of flow

Sepsis has been shown to result in a decrease in functional capillary density (FCD) in animal models and in septic patients. We have previously shown that increased numbers of stopped flow capillaries result in a fall in capillary oxygen saturation [1, 2]. The goal of this study was to determine whether leukocytes played a role in capillary occlusion. Our objective was to examine leukocyte traffic within the microcirculation in the extensor digitorum longus (EDL) muscle using fluorescently (rhodamine 6G) labeled leukocytes and to correlate this with the loss of FCD seen in a 5 hour rat cecal ligation and perforation (CLP) model of sepsis. Ten rats were randomized to sham (n = 5) or CLP (n = 5). The capillary bed in the EDL muscle was observed using intravital video microscopy. Functional capillary density was determined prior to the loss of FCD at 1.5 hours after induction of sepsis or the sham procedure. The location of all capillaries within a single capillary network from arteriole to venule was mapped. At 4 hours rhodamine 6G was injected and the same capillary network was observed using fluorescence microscopy to determine which capillaries contained flowing or stopped leukocytes and to measure leukocyte transit times. Leukocytes were found to preferentially traverse a subpopulation of capillaries in both sham (40% of total capillary paths) and CLP animals (60%). In the sham animals, 80% of the leukocytes traversed the capillary bed in <1 s and only 7 ± 4% of all capillaries had stopped flowing by 5 hours. In CLP animals, leukocyte transit times increased significantly with only 15% traversing in <1 s and 40% taking longer than 10 s (of which half remained stopped for > 30 s). In CLP animals 45 ± 5% of capillaries had stopped flowing by 5 hours; 63 ± 2% of these capillaries were preferential flow paths for leukocytes. The slowing of leukocytes through the capillary bed suggested the possible role of selectins. Six CLP animals were treated with fucoidin, a nonselective general inhibitor of selectins. Fucoidin prevented the increase in leukocyte transit times and the loss of FCD at 5 hours, indicating that selectins were involved in the slowing of leukocytes in sepsis. Leukocytes play a significant role in the maldistribution of capillary blood flow through the loss of perfused capillaries and increased leukocyte transit times. Both effects probably cause a concomitant fall in oxygen saturation in neighboring capillaries, leading to periods of tissue hypoxia.