Mathematical Model of Mixed Venous SO2 Transients at Onset of Exercise in Discrete Capillary Networks

Abstract

Increases in muscle O2 consumption (VO2) result in higher blood flow to accommodate changing metabolic demand. The time required for diffusion of O2 within a muscle volume to support increased VO2 causes tissue PO2 and mixed venous SO2 to lag behind VO2. The purpose of this study was to determine how diffusive transport affects time transients in venous SO2 following increases in blood flow and VO2. A finite difference model was used to simulate O2 transport in a discrete 3D microvascular network mapped from rat skeletal muscle using intravital video microscopy. Measurements were made in vivo to determine baseline simulation parameters for red blood cell supply rate (RBC SR), capillary inlet SO2, and VO2. Using the baseline solution as a starting point, exercise was simulated using simultaneous 6X step increases in VO2 and RBC SR. Tissue PO2 and capillary venous outflow SO2 (cvSO2) were recorded at 0.2s intervals until steady‐state (SS) was reached. SS mean tissue PO2 decreased from 37.2 ± 2.7 at baseline to 18.2 ± 5.9 mmHg in simulated exercise. Figure I shows the cvSO2 profile of blood as it exits the volume and the relative time course of the step change. This model demonstrates that despite instantaneous step increases in muscle VO2 and microvascular blood flow, diffusive transport of O2 in skeletal muscle imposes an observable time transient to cvSO2 following the onset of exercise.Supported by CIHR MOP 102504 & NIH HL089125