The Science Journal of the American Association for Respiratory Care

2000 OPEN FORUM Abstracts

EVALUATION OF TUBE COMPENSATION AND PRESSURE SUPPORT ON REDUCING IMPOSED WOB IN RESPONSE TO VARIATIONS IN INSPIRATORY DRIVE

Robert S. Campbell RRT, FAARC, James J. Lawson RRT, Sandra L. Miller MD, Jay A. Johannigman MD, Fred A. Luchette MD, Kenneth Davis Jr. MD, Paul Austin CRNA MS (LtCol USAF NC), Richard D. Branson BA, RRT. Division of Trauma/Critical Care, University of Cincinnati College of Medicine

Introduction: Resistance (R) of artificial airways (AA) is recognized as a cause of increased WOB in mechanically ventilated pts. Pressure support (PS) is commonly used to reduce the WOB imposed by AAs. Tube Compensation (TC) is a newer feature of two ventilators that may offer advantages over PS in overcoming imposed WOB due to the R of AA. We evaluated the imposed WOB and ventilator output during TC and PS in a spontaneously breathing lung model with varying inspiratory flow demands.

Methods: A two-chamber test lung (TTL) was modified to simulate spontaneous breathing with the addition of a lift bar. A Hamilton Galileo was used to "drive" the lung model at a VT of 500 ml and three inspiratory flow demands (25, 37, and 50 L/min) using a constant inspiratory flow pattern and 5 cmH2O PEEP. Test lung compliance was 50 ml/cmH2O and a 6 or 8 mm ID standard endotracheal tube provided resistance. Two ventilators (840 and Evita4) with TC were used to ventilate the patient lung. At each condition, vents were set in spont mode with: 1) no PSV or TC, 2) 100% TC, and 3) 10 cmH2O PSV. Measurements included imposed WOB, peak negative pressure (PNP), peak inspiratory and expiratory flow rate, peak airway pressure, and VT.

Results: Imposed WOB increased and PNP became more negative as inspiratory flow demand increased. As a percentage of the total WOB imposed without TC or PSV, TC provides a more consistent reduction in WOB at each flow demand (65% @ 25, 71% @ 37, and 64% @ 50 L/min) versus PSV (89% @ 25, 66% @ 37, and 27% @ 50 L/min). Figure 1 reveals the imposed WOB (mJ/L) for each vent at all flow demands at all conditions tested with a 6.0 ET tube.
PIFR and PEFR increased as flow demand increased in all conditions, and both were highest with the E4. With PSV, VT was highest at low flow and was reduced as flow demand increased. With TC, VT was lowest at low flow and increased as flow demand increased. Imposed WOB was slightly lower on the 840 in all conditions.
Conclusion: TC offers advantages over PSV in reducing imposed WOB in response to varying inspiratory flow demand. (See Original for Figure)

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