Abstract: The invention relates to a method and an apparatus for estimating a PaO2 value for a patient subject to extra corporeal circulation by means of an oxygenator. The method comprises the steps of measuring a PexO2 value in the exhaust gas of the oxygenator and calculating the estimated PaO2 based on the PexO2 value. Advantageously, the estimated PaO2 value pressure is calculated as the difference between the PexO2 value and a correction value, which is calculated in dependency of at least one measurement parameter selected from the following group: PexO2, an arterial temperature value, the blood flow through the oxygenator, venous oxygene saturation, arterial haemoglobin concentration, the fraction (FiO2) of oxygen in the gas supplied to the oxygenator, and the gas flow supplied to the oxygenator.