Blood Gas Analysis and Acid–base Disturbances

V . O2 is identical to tissue V . O2. This is extremely diffi cult to achieve clinically, particularly in mechanically ventilated subjects. Also determination of V . O2 becomes progressively less accurate as FIO2 increases. Various alternative means of measuring V . O2 which overcome some of these diffi culties are available and may be more appropriate for clinical use. For example, a pneumotachnograph can be used to measure inspired and expired volumes continuously, as described previously. This can be combined with continuous determination of FIO2 and FEO2 to obtain V . O2. V . CO2 can also be determined if FECO2 is measured (FICO2 can be assumed to be zero). This allows calculation of the respiratory quotient (RQ). A suitable device for use in critically ill patients is the Deltatrac (Datex/Instrumentarium, Helsinki, Finland). This system measures the difference between FIO2 and FEO2 using a fast-response, paramagnetic differential oxygen sensor; FECO2 is measured with an infrared carbon dioxide analyser. Expired air is collected through an inbuilt mixing chamber and fl ow is measured by gas dilution. Spontaneously breathing patients can be studied using a canopy system (Takala et al., 1989).