The present invention provides a method for continuous and non-invasive determination of the effective lung volume, the cardiac output, and/or the carbon dioxide content of venous blood of a subject (3). The method comprises the steps of, during a sequence of respiratory cycles, measuring the inspiratory and expiratory flow to and from the subject, and the carbon dioxide content of at least the expiration gas. In each respiratory cycle, a first parameter (ΔFACO2) related to the fraction of alveolar carbon dioxide [FACO2] of the subject, a second parameter (CaCO2) related to the carbon dioxide content of the arterial blood [CaCO2] of the subject, and a third parameter (VTCO2) related to carbon dioxide elimination [VCO2] of the subject are determined based on the measured inspiratory flow, expiratory flow and carbon dioxide content. The effective lung volume, the cardiac output, and/or the carbon dioxide content of venous blood of the subject is then determined based on the correlation of the first, second and third parameters in the sequence of respiratory cycles, preferably by solving an overdetermined system of capnodynamic equations.
