Abstract Rationale Optimizing positive end-expiratory pressure (PEEP) during surgery is critical for preventing lung collapse and overdistension, both of which can cause injury. Methodologies to individualize PEEP yield inconsistent values, making it difficult to determine the optimal setting. Factors such as body habitus, underlying pulmonary conditions, and duration on a ventilator make this a dynamic and patient-specific problem. We hypothesize that optimizing PEEP requires understanding how it affects individual pulmonary mechanics. To employ the forced oscillation technique, we built an oscillometer compatible with mechanical ventilation and determine if it accurately measure properties of a surrogate lung. Methods A 3-inch haptic speaker was sealed on one side with an elastic membrane by a cone compatible with ventilator tubing, while the other side remained open to atmosphere. A 1000W amplifier powered the speaker and transmitted a composite signal with frequencies 6, 10, 14, and 22Hz. Together with flow and pressure sensors, the set-up functioned as our oscillometer. The compressible volume of the oscillator is 50 mL of air. The oscillometer was put in circuit with a Dräger Savina 300 ventilator and attached to a surrogate lung with fixed compliance and resistance (IngMar Medical Quicklung). The ventilator was set to pressure-control ventilation with driving pressure at 15 cmH2O and respiratory rate of 10 breaths/min. We ran our oscillometer for one minute of ventilation on the surrogate lung at PEEP set to 5, 10, 15, and 20 cmH2O. This was repeated using every combination of surrogate resistance (R = 5, 20, 50 cmH2O*sec/L) and compliance (C = 10, 20, 50 mL/cmH2O). We calculated respiratory impedance with the pressure and flow signals for every experimental condition. We fit these to the equation-of-motion for the lung to get measured resistance. Results Neither compliance nor PEEP affected measured resistance (Figure 1a). The mean measured resistances of all tests at Rp values of 5, 20, and 50 were 8.02, 19.68, and 44.30, respectively. Coherence for the input frequencies was 0.9. Our input frequencies were isolated from ventilator frequencies in the flow power spectrum (Figure 1b). Conclusions This study demonstrates the feasibility of measuring respiratory system mechanics during mechanical ventilation using oscillometry over a range of physiological PEEPs, compliances, and resistances. Measured resistance values were high compared to the surrogate when R = 5, and were low when R = 50, but were consistent across different PEEP and compliance values. We conclude that our oscillometry system provides a means of monitoring pulmonary mechanics during surgery. This abstract is funded by: UVM LCOM Department of Anesthesiology internal funds
Casey et al. (Fri,) studied this question.