We are developing a robust, minimally invasive device for detecting progression toward hemorrhagic shock in trauma patients. To accomplish this, oxygen micro-sensors are being developed that contain a solution of oxygen sensitive phosphorescent probes within gas permeable tubing attached to optical fibers. These micro-sensors can be inserted into peripheral tissue to accurately measure tissue oxygenation. As the blood volume decreases (hemorrhage), physiological mechanisms progressively restrict blood flow to “non essential” peripheral tissues, redirecting that flow to the essential internal organs. It is hypothesized that the sensors will detect the shut down of peripheral blood flow well before the decreasing blood volume reaches the threshold where multi-organ failure begins. Proactive treatment with volume expanders or blood, guided by peripheral oxygen measurements, would significantly reduce multi-organ failure and other complications in trauma cases.