Near-infrared spectroscopy (NIRS) is a modern technique for cardiac surgical patient monitoring. The NIRS method is based on Lambert-Beer's law, which defines the relationship between the concentration of a substance and reduction of the intensity of electro-magnetic radiation. This reduction results from photon absorption by chromatophores, including haemoglobin, myoglobin and cytochrome P1. Absorption at the given wave-length depends on the degree of chromatophore oxygenation, and hence allows for its assessment. NIRS is currently commonly used for the assessment of brain oximetry in thoracic aorta surgery and, increasingly often, in open-heart surgery. Algorithms for sustaining adequate brain saturation were reported to decrease the incidence of neurological complications following cardiac surgery. In recent years, NIRS was proposed as a tool for the assessment of muscle saturation under short-term ischaemia and reperfusion, referred to as the vascular occlusion test (VOT). VOT is a provocative test in which tissue saturation (StO2) is measured at a peripheral site, such as the thenar eminence, while blood flow is transiently occluded with sphygmomanometer. After reaching a predefined ischaemia time or minimal StO2 threshold, the sphygmomanometer tourniquet is rapidly deflated and blood flow in the muscle is restored. Tissue saturation measurement during VOT can identify early disturbances in tissue metabolism and perfusion. The speed of the decrease in tissue saturation on ischaemia was proposed to reflect the local metabolic rate, while the lowest tissue saturation was proposed to reflect the extent of ischaemia. The speed of tissue saturation recovery on reperfusion in shock patients was proposed as a measure of microvascular capacity to increase blood flow after transient ischaemia. The results of recent studies indicated that the speed of the fall of muscle saturation during VOT is reduced in septic shock, while a decreased speed of saturation recovery on reperfusion is related to disturbed microcirculation, for example in hypovolemic shock. An understanding of anaesthetics and of the influence of cardiac surgical procedures on tissue saturation profile during VOT is essential for the future use of this tool for the assessment of the condition of patients during CPB cardiac surgery.