Near-infrared spectroscopy (NIRS) is non-invasive method to assess tissue oxygenation (StO2) and estimate tissue haemoglobin (THb) levels . The investigators have studied skeletal muscle StO2 in critically ill patients with preserved oxygen (i.e., cardiogenic shock) and with impaired oxygen extraction (i.e., septic shock) , , . In addition to measuring resting StO2, the investigators performed vascular occlusion tests to stop arterial blood flow, to estimate oxygen consumption, and at the end of the occlusion, it was also possible to estimate vascular reactivity and maximal reperfusion capability .