Oxygen uptake and static lung compliance during automatic ventilation

Oxygen uptake is an important parameter to control proper tissue oxygen delivery. Oxygen uptake is dependent on adequate lung function and easily disturbed by changes in lung compliance and related parameters such as the tidal volume controlling pressure support. Simultaneous on-line registration of oxygen uptake and lung compliance together with ventilatory pressures applied to achieve preset tidal volumes has been made possible using the computer feedback controlled closed circuit ventilatory system Physio-Flex (Physio Co, Hoofdorp, The Netherlands). The system guarantees for leakage free functioning (maximal leakage 7 mL gas loss/minute) and, therefore, patient oxygen consumption measurements with an accuracy of more than 95%. A specially developed membrane ventilation mode registers on-line flow and displaced volume automatically corrected for temperature, pressure and compressible volume. The current investigation has shown: Decrease oxygen consumption versus oxygen delivery supply dependency may be induced by reflectory decreases of heart rate and cardiac output; in this case a reactive pulmonary parameter change is preceding the event in form of a drop in compliance and corresponding changes in ventilatory pressures necessary to maintain the preset tidal volume. In contrast, decrease of oxygen uptake following changes in cardiac output due to acute hypovolemia has no effect on pulmonary function parameters. This can be diagnostically used as moderate changes of tidal volume also have no significant influence on pulmonary parameters. However, changes due to reduction in depth of anesthesia and relaxation have some influence and need to be excluded.