1. Method for estimating blood constituent concentration of a user under low perfusion conditions using a spectrophotometry-based monitoring device comprising a multichannel sensor formed from a plurality of channels, and a cardiovascular sensor; comprising: measuring a plurality of photoplethysmographic (PPG) signals, each PPG signal being measured by one of the channels; measuring a cardio-synchronous(CV) signal by using the cardiovascular sensor; detecting an instantaneous heart rate and determining a heart rate variability from the CV signal; for each detected heart rate, projecting the measured PPG signals multiplied by a window function on a principal frequency component, and selecting reliable projected PPG signals; and estimating a value of said blood constituent concentration from the magnitude of said reliable projected PPG signals selected over a predetermined time period; said window function having a window length which is adjusted at each detected heart rate according to the determined heart rate variability; and the frequency of said principal frequency component being estimated by averaging the heart rate over the window length.