The Relationship Between Gas Delivery Patterns and the Lower Inflection Point of the Pressure-Volume Curve During Partial Liquid Ventilation

To determine whether a positiveend-expiratory pressure (PEEP) level equivalent to the lower inflectionpoint (LIP) could be identified by evaluation of the airway pressure, flow (▪), and volume vs time waveforms during partial liquidventilation (PLV). Prospective application of PEEP during PLV in a healthy animal model. University hospital animal laboratory. Five healthy sheep weighing 30 kg each. The sequential application of 0 to 20 cm H2O PEEP in 2.5-cmH2O steps during PLV with both pressure and volumeventilation. Analysis of the pressure, volume, and ▪ waveforms as PEEP is sequentially increased. At 0 cm H2O PEEP, Vt wasmarkedly reduced compared with PEEP Vt at ≥ 7.5 cmH2O (p < 0.05) in pressure control ventilation (PCV), and peak inspiratory pressure minus PEEP was markedly increasedcompared with PEEP at ≥ 5.0 cm H2O (p < 0.05) involume control ventilation. At 10 cm H2O PEEP, allwaveforms began to stabilize, and no significant differences in anyvariable assessed were measured at > 12.5 cm H2OPEEP. The application of PEEP during PLVmarkedly alters airway waveforms. Low PEEP decreases Vt inPCV and increases airway pressure in VCV. The PEEP level equal to the LIP during PLV can be grossly estimated from airway waveforms. PEEP at≥10 cm H2O is needed to normalize gas delivery tofunctional residual capacity in the uninjured lung that is partiallyfilled with perfluorocarbon.