A method for evaluating CO2 level in the blood of a patient, comprising detecting in the patients body at least one haemodynamic signal from at least one tissue or part thereof, processing the at least one haemodynamic signal to derive a value related to the CO2 level of the patient and determining an evaluation of CO2 level of the patient based on a relation of the derived value to the CO2 level of the patient, and an apparatus to carry out the same.