The Science Journal of the American Association for Respiratory Care
Introduction: Static inspiratory pressure volume (SP-V) curves of the respiratory system have been advocated to guide ventilator settings (PEEP and tidal volume) with the goal of optimizing stable recruitment. It is generally well known the DP-V curves are shifted rightward by an amount dependent on auto-PEEP and flow resistive pressure. It is less generally understood that recruitment may occur as a function of VT and PEEP. Because increasing tidal volume may aid recruitment, the relevance of SP-V curves to the dynamic condition of tidal ventilation remains obscure. Our objective was to compare the pressure-volume relationships under static and dynamic conditions. We compared functional residual capacity (FRC) under dynamic conditions to the volume from the corresponding SP-V curve under a range of ventilatory conditions in an oleic acid lung injury model.
Methods: Five mongrel dogs were anesthetized, paralyzed and monitored to assure a stable preparation. Acute lung injury was induced by oleic acid infusion and the injury was well established after 4 hours of ventilation. FRC at ZEEP was measured by helium dilution and flow and pressure by standard methods. By dropping PEEP to ZEEP for 20 seconds we measured end expiratory lung volume change (
Results: Mean total FRC was increased above the SP-V FRC by 267±86, 425±129, 494±199 ml for PEEP = 8, 16, 24 cmH2O (arrows on representative figure). Increasing the flow setting shifted DP-V curves rightward.
Conclusions:. We conclude that substantial recruitment occurs under dynamic conditions that is not reflected by the classical SP-V curve and that the contours of the SP-V curve may not necessarily reflect the events of tidal ventilation. Dynamic P-V curves are shifted rightward in proportion to flow and autoPEEP detracting from their clinical utility.
(See Original for Figure)