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Ventilator Gas Delivery Waveform Substantially Impacts Plateau Pressure and Peak to Plateau Pressure Gradient Determination
*Dirk C Johnson, *Adrian A Maung, Kevin M Schuster, *Felix Y Lui, *Gina M Luckianow, Kimberly A Davis, Lewis J Kaplan Yale University School of Medicine, New Haven, CT
Objective: To determine whether plateau pressure (Pplat) measurement is lowered and peak airway pressure (Pawpeak) to Plat gradient is increased by measurement on a decelerating gas delivery waveform Design: Prospective before and after Setting: University adult surgical ICU Patients: Mechanically ventilated injured and critically ill patients Interventions: Pplat and Pawpeak measurement on decelerating and square gas delivery waveforms Main Outcome Measure: Pplat, and Pawpeak to Pplat gradient; data are means + standard deviations and were compared by paired t-test with significance for p < 0.05 Results: Pplat and other routine ventilator parameters were measured in 82 (47 trauma, 35 emergency general surgery) consecutive convenience sampled adult ICU mechanically ventilated patients on decelerating and then square gas delivery waveforms; peak gas flow was fixed at 40 L/minute; all other parameters (rate, tidal volume, PEEP) were held constant. All patients were managed on AC volume cycled ventilation using fentanyl and midazolam or propofol; no neuromuscular blockade was used. Patients with Pawpeak above 35 cm H2O were excluded. Comparing decelerating to square gas delivery, mean Pawpeak was lower (25.1+2.3 v 33.1+2.1 cm H2O; p < 0.0001) and mean Pplat was lower (21.3+1.9 v 24.8+2.5 cm H2O; p < 0.0001) resulting in a decreased Pawpeak to Pplat gradient (3.8+2.1 v 8.3+2.3; p< 0.0001). Conclusions: Changing from a decelerating to a square gas delivery waveform significantly increases Pplat, and Pawpeak thereby increasing the Pawpeak to Pplat gradient. This increase may prompt unwarranted therapy aimed at reducing the gradient to its normal value of < 4 cm H2O pressure. Conversely, patients with high Pawpeak on a square waveform may benefit from transitioning to a decelerating waveform prior to changing ventilation parameters.
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