Noninvasive Tissue Oxygen Saturation Measurements Identify Supply Dependency

Background: Hemorrhagic shock can lead to multiple organ failure and death. We have previously shown that noninvasive measurement of tissue oxygen saturation (StO₂) has predictive value for outcomes in patients suffering hemorrhagic shock. Our study objectives were twofold: (1) to compare invasive and noninvasive measurements of local and systemic tissue hemoglobin oxygenation and (2) to compare the effects of various physiologic conditions seen in patients in hemorrhagic shock on tissue hemoglobin oxygenation. Materials and Methods: We studied pigs in controlled conditions mimicking shock induced by one of the following: hypothermia, isovolemic hemodilution, or manipulations of vascular tone. We obtained both invasive and noninvasive measurements in a hind limb of StO₂, tissue hemoglobin index, femoral artery and venous flows, blood pressures, temperature, pH, pO₂, pCO₂, oxygen saturation, lactate, hemoglobin, and base excess. In all cases, we measured baseline values in both experimental and control hind limbs. Results: We found that tissue hemoglobin oxygenation did not vary significantly over relevant physiologic temperatures. Under all physiologic conditions tested, we found supply-dependent oxygen consumption at oxygen levels less than 7 mL O₂/min/kg. Similarly, we found that local oxygen delivery in animals subjected to varying degrees of isovolemic hemodilution or altered vascular tone was correlated with supply-dependent oxygen consumption, as measured by local noninvasive StO₂. Conclusions: Noninvasive StO₂ measurements are valid and durable over a wide range of physiologic conditions and correlate with invasively-measured oxygen delivery.