Abstract: There is provided a method that includes receiving pulse-oximetry measurements (SpO2) of a patient's peripheral arterial blood oxygen saturation during a first time period, and receiving breathing samples of the patient. The method further includes determining, using breathing samples of the patient, oxygen partial pressure measurements (PAO2) and carbon dioxide partial pressure measurements (PACO2) from exhaled air of the patient during a steady-state breathing of the patient during the first time period.