The Science Journal of the American Association for Respiratory Care

2010 OPEN FORUM Abstracts


Robert L. Chatburn, Sherry Babic; Respiratory Institute, Cleveland Clinic, Cleveland, OH

BACKGROUND: The Dräger Evita Infinity V500 ventilator has an AutoRelease feature in Airway Pressure Release Ventilation (APRV) mode that allows the operator to set an inspiratory trigger threshold for mandatory breaths (transition to Phigh) as a percent of peak expiratory flow (% PEF). This setting determines expiratory time (Tlow or release time) and facilitates adjustments of APRV when autoPEEP is intended to replace set PEEP. The purpose of this study was to determine the effects of % PEF on end expiratory lung pressure (EELP), exhaled tidal volume (VT) and mean airway pressure (mPaw). METHODS: A passive patient with ARDS was modeled using an Ingmar ASL 5000 lung simulator. Simulator settings were: compliance = 30 mL/cmH20, resistance = 10 cm H20/L/s. Ventilator settings: mode = APRV AutoRelease, inspiratory pressure (Phigh) = 30 cm H20, expiratory pressure (Plow) = 0 cm H20, frequency 10 breaths/min. % PEF settings were: 0, 10, 20, 30, 40, 50, 60, 70 and 80%. Mean values for EELP, VT and mPaw were calculated from at least 5 breaths using the Post Run Analysis feature of the simulator. RESULTS: As % PEF was increased, expiratory time decreased and EELP increased. The increase in EELP caused a decrease in the inspiratory pressure gradient between the airway opening and the lung, thus decreasing VT. The change in expiratory time was a small fraction of the total cycle time, so the increase in mPaw was minimal. PEEP at the airway opening increased above zero as expiratory time decreased and ranged from 20-50% of EELP for PEF% > 0. CONCLUSION: Appropriate use of the APRV requires careful consideration of the complex interactions of ventilation parameters. APRV setting recommendations (Crit Care Med 2005;33:S228-S240) suggest inspiratory trigger thresholds of 50-75% PEF or expiratory times from 0.2-0.8s. In this model, 50-75% PEF resulted in VT ranging from 345-510 mL and EELP from 15-20 cm H20. Expiratory times from 0.27-0.84s (associated with the %PEF settings) resulted in VT ranging from 348-846 mL and EELP from 3-20 cm H20. These ranges are expected to vary with changes in patient resistance and compliance, further complicating ventilator management. Additionally, inadvertent PEEP at the airway greatly underestimates EELP. Nevertheless, the %PEF trigger feature seems to provide more precise control during APRV than setting an arbitrary Tlow. Sponsored Research - Ventilator was loaned for period of study.