Oxygen Saturation Determined from Deep Muscle, Not Thenar Tissue, Is an Early Indicator of Central Hypovolemia in Humans

Objective To compare the responses of noninvasively mea- sured tissue oxygen saturation StO2 and calculated muscle oxygen tension PmO2 to standard hemodynamic variables for early detection of imminent hemodynamic instability during progressive central hypovolemia in humans. Design Prospective study. Setting Research laboratory. Subjects Sixteen healthy human volunteers. Interventions Progressive lower body negative pressure LBNP to onset of cardiovascular collapse. Measurements and Main Results Noninvasive measurements of blood pressures, heart rate, and stroke volume were obtained during progressive LBNP with simultaneous assessments of StO2 , PmO2 , and muscle oxygen saturation SmO2. Forearm SmO2 and PmO2 were determined with a novel near infrared spectroscopic measurement device UMMS and compared with thenar StO2 measured by a commercial device HT. All values were normalized to the duration of LBNP exposure required for cardiovascular collapse in each subject i.e., LBNP maximum. Stroke volume was significantly decreased at 25 of LBNP maximum, whereas blood pressure was a late indicator of imminent cardiovascular collapse. PmO2 UMMS was significantly decreased at 50 of maximum LBNP while SmO2 UMMS decreased at 75 of maximum LBNP. Thenar StO2 HT showed no statistical change throughout the entire LBNP protocol. Conclusions Spectroscopic assessment of forearm muscle PO2 and SmO2 provides noninvasive and continuous measures that are early indicators of impending cardiovascular collapse resulting from progressive reductions in central blood volume.