Effect of decreased O2 supply on skeletal muscle oxygenation and O2 consumption during sepsis: role of heterogeneous capillary spacing and blood flow.

One of the main aspects of the initial phase of the septic inflammatory response to a bacterial infection is abnormal microvascular perfusion, including decreased functional capillary density (FCD) and increased blood flow heterogeneity. On the other hand, one of the most important phenomena observed in the later stages of sepsis is an increased dependence of tissue O2 utilization on the convective O2 supply. This “pathological supply dependency” is associated with organ failure and poor clinical outcomes. Here, a detailed theoretical model of capillary-to-tissue O2 transport during sepsis is used to examine the origins of abnormal supply dependency. With use of three-dimensional arrays of capillaries with heterogeneous spacing and blood flow, steady-state O2 transport is simulated numerically during reductions in the O2 supply. Increased supply dependency is shown to occur in sepsis for hypoxic (decreased hemoglobin O2 saturation) and stagnant (decreased blood flow) hypoxia. For stagnant hypoxia, a reduction in FCD with decreasing blood flow is necessary to obtain the observed increase in supply dependency. Our results imply that supply dependency observed under normal conditions does not have its origin at the level of individual capillaries. In sepsis, however, diffusion limitation and shunting of O2 by individual capillaries occur to a degree that is dependent on the heterogeneity of septic injury and the arrangement of capillary networks. Thus heterogeneous stoppage of individual capillaries is a likely factor in pathological supply dependency.