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

摘要

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 cmH_2O, until the maximum CDP (before the lung lavage 32 cmH_2O and after the lung lavage 42 cmH_2O) and then it was stepwise decreased by 2 cmH_2O to the initial value. In this paper we analyzed the following parameters acquired during our experiments: partial pressure of oxygen in arterial blood (PaO_2), cardiac output (CO) and mixed venous blood oxygen saturation (SvO_2). In order to find how both PaO_2 and CO affected SvO_2 during the increase of CDP before and after lavage, a nonlinear regression fitting of the response in SvO_2 on the predictors (PaO_2 and CO) was implemented. Results. Before the lavage, with increasing of CDP, PaO_2 remained constant, CO strongly decreased and SvO_2 slightly decreased. After the lavage, with increasing of CDP, PaO_2 strongly increased, CO decreased and SvO_2 increased. So, development of SvO_2 followed the PaO_2 and CO trends. Changes in PaO_2 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 PaO_2 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 PaO_2 and CO affect SvO_2. It is possible to identify a certain level of CDP (higher in ARDS model pigs) at which the contribution of PaO_2 and CO to SvO_2 course changes their weights. Above this value, PaO_2 plays a major role in SvO_2 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.