Provided according to embodiments of the invention are methods for monitoring the application of positive end expiratory pressure (PEEP) to an individual. Such methods include isolating a pulsatile arterial component (PAC) signal and a venous impedance component (VIC) signal of a PPG signal obtained from a central source site of the individual； monitoring the PAC signal, the VIC signal, or both, over time； and adjusting the level of PEEP based on the PAC signal, the VIC signal, or both.
