Mask mechanics and leak dynamics during noninvasive pressure support ventilation: a bench study
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Objective: To study the mask mechanics and air leak dynamics during noninvasive pressure support ventilation. Setting: Laboratory of a university hospital. Design: A facial mask was connected to a mannequin head that was part of a mechanical respiratory system model. The mask fit pressure (Pmask-fit) measured inside the mask's pneumatic cushion was adjusted to 25 cmH2O using elastic straps. Pressure support (PS) was set to ensure a maximal tidal volume distal to the mask (VTdistal) but avoiding failure to cycle to exhalation. Measurements: Airway pressure (Paw), Pmask-fit, mask occlusion pressure (Pmask-occl=Pmask-fit–Paw), VT proximal (VTprox), distal to the mask (VTdistal), air leak volume (Leak=VTprox–VTdistal), and inspiratory air leak flow rate (difference between inspiratory flow proximal and distal to the mask) were recorded. Results: PS 15 cmH2O was the highest level that could be used without failure to cycle to exhalation (VTdistal of 585±4 ml, leak of 32±1 ml or 5.2±0.2% of VTprox, and a minimum Pmask-occl of 1.7±0.1 cmH2O). During PS 16 cmH2O the Pmask-occl dropped to 1.1±0.1 cmH2O, and at this point all flow delivered by the ventilator leaked around the mask, preventing the inspiratory flow delivered by the ventilator from reaching the expiratory trigger threshold. Conclusion: Pmask-fit and Pmask-occl can be easily measured in pneumatic cushioned masks and the data obtained may be useful to guide mask fit and inspiratory pressure set during noninvasive positive pressure ventilation.
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