Models of a PaO2 Course during a Stepwise Change of Continuous Distending Pressure in HFOV

Acute respiratory distress syndrome (ARDS) is an acute severe lung disease commonly encountered in intensive care units. High-frequency oscillatory ventilation (HFOV) could offer effective lung protective ventilation by delivering very low tidal volumes around constant relatively higher continuous distending pressure (CDP) at frequencies of 3 to 15 Hz. Optimization of CDP is not an easy task and it is titrated empirically in the clinical practice. The aim of this study is to investigate if the level of CDP affects the shape of the partial pressure of oxygen (PaO2) response of the organism to the CDP stepwise changes. Ten pigs were used in this study. In order to mimic ARDS, surfactant deficiency was induced by a double or triple lung lavage. Every 10 minutes, CDP was stepwise increased by 2 cmH2O. Increase in CDP was stopped when severe signs of hemodynamics deterioration were observed and then, CDP was stepwise decreased by 2 cmH2O to its initial value. For each CDP step performed, we fitted PaO2 with a one-term power model as follows: y=a·x b, where x is the time, a is the amplitude of the model and exponent b reflects the shape of the model. For values of PaO2200mmHg, PaO2 course follows a shape modelled exclusively by a root function. It is not possible to describe a relationship between the shape of the PaO2 course and the values of CDP. When alveoli are not recruited at all, oxygenation is more sensitive to changes in lung volume and aeration and thus, PaO2 grows or drops rapidly following linear and/or quadratic functions. Instead of, when alveoli are open and recruited changes in PaO2 are less sensitive to minor changes in lung aeration and thus, PaO2 grows and drops slower following only root function. The CDP level does not affect the response of organism in terms of shape change of PaO2, probably due to the fact that the recruitment occurs at different values in each pig.