In low flow shock, almost 30% of the circulating volume may be lost before hypotension occurs. Thus, shock should be early recognized prior to the development of hypotension. An earlier sign to look for is vasoconstriction in peripheral tissues due to neurohumoral response to the low circulating volume. The perfusion index (PI) derived from the pulse oximetry signal permits a quantitative analysis of variations of peripheral circulation. However, its ability to detect peripheral vasoconstriction due to neurohumoral response in central hypovolemia induced by lower body negative pressure (LBNP) has never been studied.