Abstract: In a CO2-based method for estimating shunt of a subject, a first value related to alveolar CO2 of the subject is obtained from CO2 measurements on expiration gas exhaled by said subject, a second value is obtained related to arterial CO2 of the subject, a third value is obtained related to cardiac output [QT] or effective pulmonary perfusion [EPP] of the subject, a fourth value is obtained related to CO2 elimination [VCO2] of the subject, the shunt of the subject is calculated based on said first, second, third and fourth values. The method allows the shunt of the subject to be determined in a non-invasive or minimally-invasive way without requiring determination of the venous or capillary CO2 contents of the subject, which in turn allows the method to be carried out at the bedside, enabling reliable monitoring of shunt in clinical practice.