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 measured tissue oxygen saturation (Sto(2)) and calculated muscle oxygen tension (Pmo(2)) 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 Sto(2), Pmo(2), and muscle oxygen saturation (Smo(2)). Forearm Smo(2) and Pmo(2) were determined with a novel near infrared spectroscopic measurement device (UMMS) and compared with thenar Sto(2) 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. Pmo(2) (UMMS) was significantly decreased at 50% of maximum LBNP while Smo(2) (UMMS) decreased at 75% of maximum LBNP. Thenar Sto(2) (HT) showed no statistical change throughout the entire LBNP protocol. Conclusions. Spectroscopic assessment of forearm muscle Po-2 and Smo(2) provides noninvasive and continuous measures that are early indicators of impending cardiovascular collapse resulting from progressive reductions in central blood volume.