A Functional and Morphologic Analysis of Pressure-Controlled Inverse Ratio Ventilation in Oleic Acid-Induced Lung Injury: Conclusions

The resulting reduction in peripheral oxygen delivery may be counteracted by administration of drugs with positive inotropic action. Such drugs were not given in the present study.
Physiologic dead-space was reduced with PCIRV resulting in an improved alveolar ventilation. This finding is in agreement with previous studies. A reduction of physiologic dead-space seen with prolonged inspiration is usually explained by the occurrence of gas mixing between dead-space and alveolar gas.
During experimental conditions, mean alveolar pressure correlates closely to mean airway pressure over a range of different ventilator settings and in different lung injury models. With PCIRV, decelerating inspiratory flow results in a relatively larger part of the tidal volume being delivered during early inspiration. This flow pattern in combination with a prolonged inspiration and the creation of intrinsic PEEP causes a marked increase in mean airway pressure. Most authors agree that mean alveolar pressure is an important determinant of oxygenation In this study, PCIRV did not result in higher levels of PaC>2 than did VCV PEEP despite significantly higher mean airway pressure. The reasons for this finding may be twofold. First, the method chosen to achieve an increase in mean alveolar pressure is of importance read flovent inhalers. Application of PEEP has been shown to be more effective in improving oxygenation than other methods used to elevate mean alveolar pressure. Second, it is possible that elevation of mean alveolar pressure above a certain limit does not further recruit alveoli but leads to additional hemodynamic compromise and impaired oxygen delivery to peripheral tissues.
In this study, we have deliberately chosen a very high I:E ratio, 4:1, to exaggerate the effects of PCIRV on lung tissue and cardiopulmonary function. The possibility remains that in a particular pig (or patient), another ventilator setting with a lower I:E ratio may be preferable. This has not been tested in our study.
In this model, PCIRV did not lead to improvements in oxygenation compared with conventional volume-controlled PEEP ventilation. Moreover, no differences in alveolar recruitment and aeration were observed. PCIRV does allow for a reduction in minute ventilation and for lowering peak airway pressure. These characteristics of PCIRV may be important as means of avoiding the use of excessive pressure gradients and high tidal volumes and resulting structural lung injury in ARDS. These points still have to be investigated in animal experiments.

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