The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts

Ventilator calculated values of Resistance: Are they accurate?

Jeffrey K. Goldman AS CRT, 1 Russell G. Peterson AS CRT, 2 Lonny J. Ashworth MEd RRT. 3 St. Luke’s Regional Medical Center; 1 Saint Alphonsus Regional Medical Center; 2 Boise State University, Boise, ID3

Rationale: The purpose of this study was to compare the accuracy of the displayed value calculated by the ventilator for airway resistance to the airway resistance calculated using the traditional method of calculating airway resistance in old and new generation mechanical ventilators.

Methods: Four ventilators were studied: Viasys Avea (VA), Drager Evita 2 (DE), Puritan Bennett 840 (840) and Puritan Bennett 7200 (7200). Each ventilator was attached to a mechanical lung model: compliance 0.015 L/cm H2O, #5 parabolic resistors in each lung and a #20 parabolic resistor as the trachea. Ventilator settings: Volume-targeted, Assist-Control; VT 500-1250 mL (increased in increments of 250 mL); flowrates 40, 60, 80 and 100 L/minute; square flow waveform; PEEP 0 cm H2O. Displayed values and calculated values for airway resistance were recorded after a breath-hold on five consecutive breaths at each flowrate and VT setting. Airway resistance was calculated as the (peak pressure – plateau pressure)/ Flow in LPS.

Results: At a flowrate of 40 L/minute and at all tidal volumes, the difference between displayed and calculated airway resistance with the VA, 840, and 7200 were all within 1 cm H2O/L/second of the calculated airway resistance; the DE was 4-11.8 cm H20/L/second greater than the calculated value. As the tidal volume increased, the difference between the displayed airway resistance and the calculated airway resistance increased. See figure below.

Conclusions: When evaluating airway resistance for ventilator patients, it is important to realize that the values displayed by some ventilators are not consistent with the traditional method of calculating airway resistance. The displayed resistance on the DE is actually an expiratory resistance, rather than the traditional inspiratory resistance. Future studies may determine whether clinicians should assess the inspiratory resistance or the expiratory resistance, and if they have different normal values.


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