A SYSTEM FOR THE CONTINUOUS MEASUREMENT OF OXYGEN UPTAKE AND CARBON DIOXIDE OUTPUT IN ARTIFICIALLY VENTILATED PATIENTS

A continuous, non-invasive system is described for measuring oxygen uptake (Voi) and carbon dioxide output (t'CO ^ in mechanically ventilated patients. Inspiratory and mixed expiratory gas samples were pumped through fine-bore tubing to a remote mass spectrometer for analysis. The expiratory flow transducer of a Siemens Servo 900B ventilator was used for expiratory flow measurement and inspiratory flow was calculated from this using the Haldane transformation. A desk-top computer calculated V'Oj, Vcx>2 and respiratory quotient. The system has been validated against standard methods of gas analysis and flow measurement (V02 mean difference —lOmlmin"1: SD9.13; Vco ^ mean difference S.lZmlmin"1: SD4.66). Comparison with Douglas bag measurements in patients has been made (V02 mean difference 10.7mlmin"1: SD9.8; Vcoj mean difference-1.07mlmin"1: SD4.7). Oxygen uptake (VO2) is related to the arterio-venous oxygen content difference by the Fick equation: Oxygen uptake = cardiac output x arterio- venous oxygen content difference (1) Arterial oxygen tension (Pao2) is monitored fre