Fig. 1

Physiologic rationale behind the use of the inspiratory flow curve to detect patient inspiratory effort. The upper panels show the pressure–time curve of airway pressure (P_aw, full line), esophageal pressure (P_es, dotted line) and alveolar pressure (P_alv, dashed line) during pressure support ventilation. The lower panels show the flow-time curve during pressure support ventilation. Left Panels: In absence of patient inspiratory effort, the inspiratory flow is maximal at the beginning of inspiration and the pressure gradient between the airway opening and the alveoli (area with diagonal lines) is progressively reduced. Thus, the inspiratory flow parallels the exponential decay of the airway opening-alveoli pressure difference. Right Panels: In presence of patient inspiratory effort, the fall in pleural pressure reduces alveolar pressure. The greater the inspiratory effort, the more the airway-alveolar pressure gradient is sustained (area with diagonal lines). The inspiratory flow curve moves further away from the exponential decay towards a downward concavity shape, proportionally to the level of activation of inspiratory muscles