Movement to Spontaneous Breathing
In severe ARDS, the value of neuromuscular blockade and prone positioning mean that spontaneous breathing cannot be recommended in early severe ARDS [58, 59].
Fig. 9.1 Flow diagram for weaning in ARDS; derived from [26, 27, 34, 47-50]
After this initial period, spontaneous breathing supported by modes such as ASV, APRV, BIPAP, or pressure support has appealed . In less severe ARDS and as lungs recover, spontaneous ventilation has potential to improve lung ventilation at lower levels of airway pressure and traditionally has been encouraged as it requires less sedation and improves cardiopulmonary function, presumably by recruiting non-ventilated lung units, requiring a shorter duration of ventilatory support [57, 30]. In animals with severe lung injury, spontaneous breathing could worsen lung injury, and muscle paralysis might be more protective for injured lungs by preventing injuriously high trans-pulmonary pressure and high driving pressure . Inspiratory pressure support provided to match the demand for a higher minute ventilation can lead to large trans-pulmonary pressure swings and TV that exceed the recommended maximums, both of which are major contributing factors to ventilator-induced lung injury and possibly to weaning failure [31, 61].
Strong spontaneous efforts in animals increase lung injury . In injured lungs, the lungs’ normal fluidlike behavior is not universally present. With spontaneous breathing, the negative pleural pressures generated by diaphragmatic excursion are not uniformly transmitted across lung surfaces but concentrated in dependent regions. In the supine position, these dependent posterior regions are affected, but following prolonged prone positioning, this may also affect the ventral regions . Locally increased pleural pressure variations cause underdiagnosed regional overstretching in dorsal (or dependent) lung regions, accompanying alveolar air shift from ventral to dorsal parts of the lung (i.e., pendelluft) . Excessive spontaneous efforts may cause local overstretch because of a significant pendelluft effect .
Of concern is the cryptic damage due to high trans-pulmonary pressures, and regional overdistension will postpone successful weaning and increase both morbidity and mortality. Limiting the trans-alveolar pressure by strictly limiting driving pressures and tidal volumes generated is important .