The present disclosure relates to a method for estimating blood constituent concentration of a user under low perfusion conditions using a spectrophotometry-based monitoring device the method comprising: measuring a plurality of photoplethysmographic (PPG) signals measuring a cardio-synchronous (CV) signal detecting an instantaneous heart rate and determining a heart rate variability from the CV signal selecting reliable projected PPG signals estimating a value of said blood constituent concentration from the magnitude of said reliable projected PPG signals. The disclosed method requires diminished computational load compared to conventional methods based on frequency domain approach as FFT or DCT. The disclosure also pertains to a monitoring device for estimating blood constituent concentration in tissue under low perfusion of a user.
