The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts

Ventilator calculated values of Compliance: 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 value displayed by the ventilator for compliance to the calculated value for compliance using the traditional method of calculating static compliance 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 and #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); respiratory rate 10/minute (no auto-PEEP was present); flowrates 40, 60, 80 and 100 L/minute; inspiratory time 2.0 seconds; square flow waveform; PEEP 0 cm H2O. Compliance was calculated as the exhaled tidal volume/(plateau pressure – PEEP). Displayed values and calculated values for compliance were recorded after a breath-hold on five consecutive breaths at each flowrate and VT setting.

Results: The difference between the displayed compliance and the calculated compliance varied between the ventilators. The difference with the 840 and the VA were within 1 mL/cm H20 for VT less than 1250 mL. The DE averaged a 9-16 mL/cm H20 difference for all VT. The 7200 had a difference of 3.3-7.4 mL/cm H20 for most all VT and flowrate settings.

Conclusions: When evaluating the compliance for ventilator patients, it is important to realize that the values displayed by some ventilators are not consistent with the traditional method of calculating compliance.

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