The InSpectra tissue spectrometer relies on continuous wave near infrared (NIR) technology to estimate non invasively local tissue hemoglobin oxygen saturation in tissue (% StO2). This technology had been tested in a variety of systems: standard theoretical models of light transport, isolated blood, isolated blood-perfused animal organs and healthy human volunteers with induced limb ischemia. In critical-care medicine, NIRS has also been used to evaluate muscle oxygenation in trauma resuscitation and in lower extremity and abdominal compartment syndrome. However, NIRS has been rarely utilised to measure tissue blood flow and oxygen uptake in critically ill patients.