Pressure/Volume (PV) Loop can be used to determine the lower inflection point (LIP) and thus the appropriate PEEP level to prevent atelectrauma in patients with ARDS. Similarly, the upper inflection point (UIP) can be used to determine the plateau pressure and prevent over expansion of the alveoli.
However, these measurements on the PV loop represent the global values. In the above example of a patient with ARDS, the regional PV loops are depicted in the most dorsal (red), and ventral regions (black) of the lungs along with the middle regions (blue). Notice that the UIPs and LIPs are different among these regions. If global LIP is used to determine PEEP, it may not be able to open the alveoli of the most dorsal regions of the lungs as their LIP is higher than the set PEEP. Similarly, using the UIP to determine the plateau pressure, may result in over expansion of the alveoli in the most ventral region of the lungs as their UIP is lower than the global UIP.
This is why, yoy may want to set the PEEP a bit higher than the LIP and the plateau lower than the UIP. Better yet if you use end-expiratory transpulmonary pressure and inspiratory transpulmonary pressures as guidance for PEEP and plateau levels in ARDS patients.
References:
Scaramuzzo, G., et al., Heterogeneity of regional inflection points from pressure-volume curves assessed by electrical impedance tomography. Critical Care, 2019. 23(1): p. 119
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