Sepsis impairs microvascular autoregulation and delays capillary response within hypoxic capillaries

INTRODUCTION The microcirculation supplies oxygen (O2) and nutrients to all cells with the red blood cell (RBC) acting as both a deliverer and sensor of O2. In sepsis, a proinflammatory disease with microvascular complications, small blood vessel alterations are associated with multi-organ dysfunction and poor septic patient outcome. We hypothesized that microvascular autoregulation-existing at three levels: over the entire capillary network, within a capillary and within the erythrocyte-was impaired during onset of sepsis. This study had three objectives: 1) measure capillary response time within hypoxic capillaries, 2) test the null hypothesis that RBC O2-dependent adenosine triphosphate (ATP) efflux was not altered by sepsis and 3) develop a framework of a pathophysiological model. METHODS This was an animal study, comparing sepsis with control, set in a university laboratory. Acute hypotensive sepsis was studied using cecal ligation and perforation (CLP) with a 6-hour end-point. Rat hindlimb skeletal muscle microcirculation was imaged, and capillary RBC supply rate (SR = RBC/s), RBC hemoglobin O2 saturation (SO2) and O2 supply rate (qO2 = pLO2/s) were quantified. Arterial NOx (nitrite + nitrate) and RBC O2-dependent ATP efflux were measured using a nitric oxide (NO) analyzer and gas exchanger, respectively. RESULTS Sepsis increased capillary stopped-flow (p = 0.001) and increased plasma lactate (p < 0.001). Increased plasma NOx (p < 0.001) was related to increased capillary RBC supply rate (p = 0.027). Analysis of 30-second SR-SO2-qO2 profiles revealed a shift towards decreased (p < 0.05) O2 supply rates in some capillaries. Moreover, we detected a three- to fourfold increase (p < 0.05) in capillary response time within hypoxic capillaries (capillary flow states where RBC SO2 < 20 %). Additionally, sepsis decreased the erythrocyte's ability to respond to hypoxic environments, as normalized RBC O2-dependent ATP efflux decreased by 62.5 % (p < 0.001). CONCLUSIONS Sepsis impaired microvascular autoregulation at both the individual capillary and erythrocyte level, seemingly uncoupling the RBC acting as an "O2 sensor" from microvascular autoregulation. Impaired microvascular autoregulation was manifested by increased capillary stopped-flow, increased capillary response time within hypoxic capillaries, decreased capillary O2 supply rate and decreased RBC O2-dependent ATP efflux. This loss of local microvascular control was partially off-set by increased capillary RBC supply rate, which correlated with increased plasma NOx.