Résumé :
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NIV is often provided by small portable ventilators. These devices are capable of meeting high ventilatory demands and even outperforming the most complex devices used in a critical care setting. However, even though additional O2 is often needed during NIV, most of the available portable ventilators do not have controls to set FiO2. Supplemental O2 is then provided by an additional admission in the ventilator tube, so that ensuing FiO2 is influenced by different conditions that make it difficult to estimate the level achieved. Our goal was to evaluate in an experimental model, factors that may influence achieved FiO2 under NIV. Methods : In a lung model, we increased O2 flow from 1 to 6 l/min into the ventilator circuit and we evaluated the influence on achieved FiO2 of modifying ventilator settings, varying lung mechanics and generating different level of leaks. A schema of the model is shown in Fig 1. Statistical analysis : ANOVA was used to analyze the way in which FiO2 was affected by different combinations of factors.We also used multiple regression to estimate the influence of different variables on achieved FiO2 Results : For a given O2 flowwe noted :1) a progressive and significant reduction of achieved FiO2, as leaks increased; it was impossible to achieve FiO2 > 35% when they exceeded 26 l/min (Fig 2); 2) a progressive rise in FiO2 as IPAP increased, except in the model with reduced compliance. Multiple regression analysis to evaluate the impact on FiO2 of all of the factors taken simultaneously showed an r2 = 0.963. This model showed the major effect of leaks on achieved FiO2 and, the lesser impact of the “ventilator settings” factor whereas the “lung mechanics” factor did not seem to modify achieved FiO2. Conclusion : Achieved FiO2 when O2 flow is added to a ventilator circuit varies and strongly depends on the amount of leaks. These results confirm the interest of consider a ventilator with adjustable FiO2 when O2 needs are substantial.
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