Method and system for estimating the efficiency of the lungs of a patient

A method for estimating an efficiency of lungs of a patient receiving respiratory care, comprising:

• a) causing a delivery of a first gas in a first breath to the patient at a perturbative concentration;
• b) after a), determining an end-tidal concentration of the patient based on measurements obtained during exhalation of the first breath of the patient;
• c) recalculating a concentration of the first gas for delivery to the patient in a following breath based at least in part on a variation of the end-tidal concentration of the patient between a preceding breath and a current breath;
• d) determining an updated end-tidal concentration of the patient based on measurements obtained during exhalation after c);
• e) repeating c) and d) until at least one of:
• i) a predetermined time duration after a) elapses,
• ii) a predetermined number of breaths have passed after a), and
• iii) at least two successive updated end-tidal concentration values are substantially equal;
• and
• f) after e), estimating a functional residual capacity (FRC) of the patient.