Continuous Distending Pressure Effects on Variables Contributing to Oxygenation in Healthy and ARDS Model Pigs during HFOV.

Introduction. High frequency oscillatory ventilation (HFOV) is an alternative mode of mechanical ventilation. HFOV has been shown to provide adequate ventilation and oxygenation in acute respiratory distress syndrome (ARDS) patients and may represent an effective lung-protective ventilation in patients where conventional ventilation is failing. The aim of this study is to evaluate effects of continuous distending pressure (CDP) on variables that contribute to the oxygenation in healthy and ARDS lung model pigs. Methods. In order to simulate a lung disease, lung injury was induced by lavage with normal saline with detergent in three pigs. HFOV ventilation was applied before and after the lung lavage. CDP was stepwise increased by 2 cmH₂O, until the maximum CDP (before the lung lavage 32 cmH₂O and after the lung lavage 42 cmH₂O) and then it was stepwise decreased by 2 cmH₂O to the initial value. In this paper we analyzed the following parameters acquired during our experiments: partial pressure of oxygen in arterial blood (PaO₂), cardiac output (CO) and mixed venous blood oxygen saturation (SvO₂). In order to find how both PaO₂ and CO affected SvO₂ during the increase of CDP before and after lavage, a nonlinear regression fitting of the response in SvO₂ on the predictors (PaO₂ and CO) was implemented. Results. Before the lavage, with increasing of CDP, PaO₂ remained constant, CO strongly decreased and SvO₂ slightly decreased. After the lavage, with increasing of CDP, PaO₂ strongly increased, CO decreased and SvO₂ increased. So, development of SvO₂ followed the PaO₂ and CO trends. Changes in PaO₂ and CO occur at decisive CDP step and it was much higher after the lung lavage compared to the healthy lungs. The implemented nonlinear model gives a good goodness of fitting in all three pigs. The values of PaO₂ and CO estimated coefficients changed at the same decisive step of CDP identified by the trends. Also the algorithm identified a CDP step much higher after the lung lavage. Conclusions. The novelty of this study consists of the implementing of a model that allows to predict how PaO₂ and CO affect SvO₂. It is possible to identify a certain level of CDP (higher in ARDS model pigs) at which the contribution of PaO₂ and CO to SvO₂ course changes their weights. Above this value, PaO₂ plays a major role in SvO₂ developments. This is in concordance with the clinical experience that HFOV is suitable for patient with more severe lung diseases when much higher CDP levels are required to assure an adequate oxygenation. [ABSTRACT FROM AUTHOR]