Rationale, Benefits, and Risks of Using NIV in ARDS
Invasive mechanical ventilation increases oxygenation, decreases the work of breathing, helps to reopen or to keep open collapsed alveoli, and improves respiratory mechanics. However, mechanical ventilation per se can aggravate lung injury, a process referred to as ventilator-induced lung injury (VILI) . Moreover, mechanical ventilation can also lead to other complications such as ventilator-induced diaphragm dysfunction  and ventilator-acquired pneumonia . In this context, NIV has been used to ensure ventilatory support instead of invasive mechanical ventilation. From a physiological point of view, the application of positive airway pressure can on the one hand reduce the work of breathing  and, on the other hand, opens under or non-ventilated alveoli and increases functional residual capacity, thus decreasing right-to-left intrapulmonary shunt and improving lung mechanics. In patients with the mild form of ARDS, NIV has been shown to increase oxygenation, reduce dyspnea, and unload the respiratory muscles . In those patients, continuous positive airway pressure alone improves gas exchange but does not unload the respiratory muscles efficiently, whereas NIV with two levels of pressure proves more efficient, unloading the respiratory muscles and relieving dyspnea, compared to continuous positive airway pressure . Finding the good combination of end-expiratory and inspiratory pressure is not easy, however, since the maximal applied pressure is generally limited to 20 or 25 cmH2O, because of leaks and discomfort. An individual titration on both oxygenation (PEEP) and comfort and work of breathing (pressure support) is therefore necessary.