The present invention relates to a device (30) and a related method for determining a blood gas partial pressure in blood in a circulatory system of a subject (12) of interest. Also a system for ventilating a patient is disclosed. Temperature-related measurement values indicative of given blood temperature levels are detected. Oxygen saturation measurements of the blood of said subject (12) under consideration of said temperature-related measurement values are derived. A blood gas partial pressure of a monitored subject (12) from said derived oxygen saturation measurements is determined under consideration of present blood temperature levels of said subject (12). Preferably, pH- representative values attributable to present blood pH-values of the subject (12) are determined. More preferably, the pH-representative values are derived from an oxygen dissociation curve under consideration of changes in said derived oxygen saturation measurements of the blood attributable to detected temperature-related measurement values of the subject's blood.
