Abstract: A method and apparatus for non-invasive measure of venous oxygen saturation, which can be useful for early diagnosis of microcirculatory dysfunction and treatment of medical conditions, such as septic shock, is disclosed. The method applies external stimulus to a patient to improve the signals to an oximeter yielding more reliable measurements. The device implementing the method uses superficial low frequency and low pressure pulse to the patient near the site of the oximeter as well as optionally controlling the patient's temperature near the site of the oximeter. The results also include non-invasive measurements of SpaO2, SpvO2 and O2E.