Deleterious Effects of Spontaneous Breathing in the Acute Phase of ARDS
The beneficial effects of NMBAs in the most hypoxaemic patients in the initial phase of ARDS may also be due to the prevention of the deleterious role of spontaneous breathing (SB). In the early phase of ARDS, the high respiratory drive that results from hypoxaemia may result in elevated and uncontrolled transpulmonary pressure (TPP) with over-distension, particularly in the dependant vertebro-diaphragmatic zones, which results in major ventilator-induced lung injury (VILI) . In these cases, the maintenance of a plateau pressure below 30 cmH2O might not be sufficient to ensure protective ventilation, particularly in patients with the most severe forms of ARDS, as suggested by recent experimental work from Yoshida et al. . In a rabbit model of ARDS, these authors demonstrated that the preservation of ventilatory effort lacked the same effects on oxygenation, pulmonary ventilation and lung injury depending on the severity of the ARDS (i.e. mild or severe). The preservation of SB induced improvements in pulmonary ventilation and oxygenation in mild ARDS subjects but resulted in increased TPP and VILI in a group of animals with severe ARDS. These deteriorations of the ventilatory and histological parameters were prevented by abolishing SB via the administration of NMBAs in the severe ARDS group.
In summary and in contrast to what occurs in patients with mild to moderate ARDS, in the initial phase of severe ARDS, the prevention of VILI and the optimization of alveolar recruitment appear to be based on controlled protective ventilation and the use of NMBAs and the consequent abolition of spontaneous ventilatory effort.