Fig. 2.

Two representative patients recovering from acute hypoxemic respiratory failure tested over a range of pressure support equal to 12 cmH₂O, from a recent study [8]. Patient A (left) gets under-assisted (incapable of keeping Vt_target) when required to exert a Pmus greater than what is sustainable over time (Pmus_lim) (i.e. greater than 40% of the maximal inspiratory pressure, MIP). Accordingly, P0.1 and respiratory rate (RR) increase keeping minute ventilation (MV). Conversely, patient B (right), whose strength and endurance of the respiratory muscles are preserved, can keep Vt_target over time (i.e. Pmus less than 40% of MIP with P0.1 and RR constant) even when PS is nearly zeroed. Both patients are at risk for over-assistance whenever PS is raised above the level needed to get Vt_target given their Crs (PS_over); however, the increase of Vt over Vt_target is limited by lower Crs (Patient B). In this condition, even decreasing RR, MV raises and both patients get hyperventilated. Blood gas analysis at baseline: Patient A (pH 7.47, PaCO₂ 39 mmHg, HCO₃- 28 mmol/L, PaO₂ 77 mmHg, FiO₂ 0.5); Patient B (pH 7.51, PaCO₂ 45 mmHg, HCO₃- 35 mmol/L, PaO₂ 116 mmHg, FiO₂ 0.4)