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 form 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
... 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 2 O, until the maximum CDP (before the lung lavage 32 cmH 2 O and after the lung lavage 42 cmH 2 O) and then it was stepwise deceased by 2 cmH 2 O 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. From these preliminary results, 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.