Abstract: The invention relates to a method and an apparatus for estimating a PaCO2 value for a patient subject to extracorporeal circulation by means of an oxygenator. The method comprises the steps of measuring a PexCO2 value in the exhaust gas of the oxygenator and the patient's arterial blood temperature value Ta, using a temperature sensor arranged in the oxygenator. The estimated PaCO2 value is then calculated, based on the measured PexCO2 value and the arterial temperature measurement. Advantageously, an average value determined from a predetermined number of recent PexCO2 values is used in the calculation. The calculation is performed by adding a correction term to the PexCO2 value, which is composed of a temperature dependent component and an offset component. The correction term may be adjusted by a user. The estimated PaCO2 value is presented on a display. A switch provides easy alteration between pH-stat mode and alpha-stat mode.