Fig. 2
a Typical airway pressure release ventilation (APRV) airway Pressure and Flow curves. Correctly set APRV has a very brief duration at expiration (time at low pressure (T Low )) and extended inspiratory duration (time at high pressure (T High )) [17]. The THigh is ~90% of each breath. The two other APRV settings are the pressure at inspiration (P High ) and at expiration (P Low ). PHigh is set sufficiently high to recruit and open alveoli, and PLow is always set at 0 cmH2O to facilitate expiratory flow. However, TLow is sufficiently short such that end-expiratory pressure (PLow) never reaches 0 cmH2O identified by the tracheal pressure (green line) maintaining a level of PEEP. b This figure summarizes our novel method to maintain alveolar stability by adaptively adjusting the expiratory duration as directed by the expiratory flow curve. The rate of lung collapse is seen in the Normal (slope 45°) and acutely injured lung (ARDS, slope 30°). ARDS causes a more rapid lung collapse due to decreased lung compliance. Our preliminary studies have shown that if the ratio of the peak expiratory flow (PEF, 60 L/min) to the end-expiratory flow (EEF, 45 L/min) (EEF/PEF) is equal to 75%, this expiratory duration (0.5 s) is sufficiently brief to stabilize alveoli [14, 38]. The lung with ARDS collapses more rapidly such that the EEF/PEF of 75% identifies a shorter expiratory duration of 0.45s is necessary to stabilize alveoli. Although the EEF/PEF is fixed, the expiratory duration is not, but rather is adaptive and will stabilize alveoli regardless of lung injury severity. Thus, this method of setting expiratory duration is adaptive to changes in lung pathophysiology and personalizes the mechanical breath to each individual patient. The values presented in this legend are just an example and may not reflect the actual values obtained in real life situations. (Reprinted with permission) [18]