The Pressure–Volume Curve Is Greatly Modified by Recruitment

TLDR

A mathematical ARDS lung model incorporating gravity simulated how varying alveolar threshold opening pressures, PEEP, and peak inspiratory pressure altered the static pressure‑volume curve, revealing that recruitment of collapsed units extends the linear portion beyond the total compliance of aerated alveoli. The model demonstrated that the lower inflection point was unreliable for setting PEEP, recruitment could shift the upper inflection point, yet overall pressure‑volume curve changes were inconsistent and generally underestimated the PEEP required to prevent end‑expiratory collapse, rendering the curve an unreliable predictor of optimal ventilator settings.

Abstract

A mathematical model of the ARDS lung, with simulated gravitational superimposed pressure, evaluated the effect of varying alveolar threshold opening pressures (TOP), PEEP and peak inspiratory pressure (PIP) on the static pressure-volume (PV) curve. The lower inflection point (Pflex) was affected by SP and TOP, and did not accurately indicate PEEP required to prevent end-expiratory collapse. Reinflation of collapsed lung units (recruitment) continued on the linear portion of the PV curve, which had a slope at any volume greater than the total compliance of aerated alveoli. As recruitment diminished, the reduced PV slope could produce an upper Pflex at 20 to 30 cm H2O pressure. An upper Pflex caused by alveolar overdistension could be modified or eliminated by recruitment with high TOP. With constant PIP as PEEP increased, and TOP range of 5 to 60 cm H2O, PEEP to prevent end-expiratory collapse was indicated by minimum PV slope above 20 cm H2O, minimum hysteresis, and maximum volume at a pressure of 20 cm H2O. With constant inflation volume as PEEP increased, the effect on PV slope was unpredictable. Although increased PV slope indicated recruitment, maximum PV slope usually underestimated PEEP required to prevent end-expiratory collapse. Therefore, with this model the PV curve did not reliably predict optimal ventilator settings.