Abstract : Over the last years, tissue microcirculatory and regional perfusion and oxygenation have made an important entry into the functional hemodynamic monitoring of critically ill patients (1). Clinical assessment of these parameters has become possible by the introduction of optical spectroscopic technologies such as near-infrared spectroscopy. This technique is based on the oxygen-dependent optical absorption of blood in the near-infrared spectrum. There are three prime factors which distinguish the currently available devices: 1) the algorithm used to calculate regional/ microcirculatory hemoglobin saturation; 2) the spatial separation of the illumination and detection fiber, which determines the measurement depth; and 3) the location of the probe. In a recent issue of Critical Care Medicine, Soller et al (2) presented a study in volunteers where a decrease in venous return (as a model of hypovolemia) was induced by lower limb negative pressure. In the model, they compared a self-developed near-infrared spectroscopy device, which measures a parameter called muscle oxygen tension, to a commercially available device from Hutchinson Technology (HT), which measures a parameter called tissue oxygen saturation and has been studied clinically (3 5). The authors concluded that their device has superior sensitivity to detect acute hypovolemia when compared with the HT device. However, in their study design, two of the above-mentioned factors were not under experimental control resulting in a faulty conclusion. This may lead to confusion in the field of those using the near-infrared spectroscopy technique and needs to be addressed.