The measurement of O2 and CO2 transport in the arterial and venous mixed blood constitutes the intersection between haemodynamic data and expiratory-inspiratory gases. The arterial-venous difference in O2 (a-v-DO2) and the venous arterial difference in CO2 (v-a-DCO2) calculated from the data of PA, PO2, PCO2 (read by an ABL4 Radiometer) revealed a close correlation with a a-v-DO2 measured directly by means of the LEXO2 (r = 0.99) or with VCO2 measured in expired gases by a capnograph (r = 0.99). In heart surgery the "on-line" monitoring of VCO2, a-v-DO2 and related parameters (REE = Resting Energy Expenditure) in the operating room and in ICU by means of a PDMS (Patients Data Management System) provides very useful information which, when integrated with haemodynamic parameters, allows a better understanding and better care of critical patients.