The relationship between gas delivery patterns and the lower inflection point of the pressure-volume curve during partial liquid ventilation

Study question: To determine whether a positive end-expiratory pressure (PEEP) level equivalent to the lower inflection point (LIP) could be identified by evaluation of the airway pressure, flow (V), and volume vs time waveforms during partial liquid ventilation (PLV). Design: Prospective application of PEEP during PLV: in a healthy animal model. Setting: University hospital animal laboratory. Participants: Five healthy sheep weighing 30 kg each. Interventions: The sequential application of 0 to 20 cm H2O PEEP in 2.5-cm H2O steps during PLV with both pressure and volume ventilation, Measurements: Analysis of the pressure, volume, and V waveforms as PEEP is sequentially increased. Results: At 0 cm H2O PEEP, VT was markedly reduced compared with PEEP VT at greater than or equal to 7.5 cm H2O (p < 0.05) in pressure control ventilation (PCV), and peak inspiratory pressure minus PEEP was markedly increased compared with PEEP at greater than or equal to 5.0 cm H2O (p < 0.05) in volume control ventilation, At 10 cm H2O PEEP, all waveforms began to stabilize, and no significant differences in any variable assessed were measured at > 12.5 cm H2O PEEP, Conclusions: The application of PEEP during PLV markedly alters airway waveforms. Low PEEP decreases VT in PCV and increases airway pressure in VCV, The PEEP level equal to the LIP during PLV can be grossly estimated from airway waveforms. PEEP at greater than or equal to 10 cm H2O is needed to normalize gas delivery to functional residual capacity in the uninjured lung that is partially filled with perfluorocarbon.