The Science Journal of the American Association for Respiratory Care

1998 OPEN FORUM Abstracts

THE EFFECT OF AIRWAY LEAK ON RESPIRATORY MECHANICS CALCULATIONS

Harlan J. Brick, RRT, Robert L. Chatburn, RRT, University Hospitals of Cleveland, Ohio

BACKGROUND: Many ventilators and bedside monitors display values of resistance and compliance, regardless of whether underlying assumptions of the mathematical models they use are met. For example, an airway leak would theoretically cause displayed resistance to be falsely low and compliance to be falsely high. The purpose of this study was to determine if monitors react according to theory and how much of an error is introduced by a simulated airway leak. Methods: Airway leak during mechanical ventilation was simulated with an IngMar Medical Adult/Pediatric lung model. We ventilated one side of the lung model using an NPB 7200 ventilator using volume controlled, continuous mandatory ventilation with a rectangular flow waveform. Two load levels (high resistance/ low compliance and low resistance/high compliance) and two tidal volumes (400 mL and 800 mL) were set. The lung model allowed a simulated ET tube leak (approximately 40%) to be switched on or off. Measurements of dynamic resistance (R) and dynamic compliance (C), using a linear regression model, were made with either the NPB 7200 or the Novamatrix CO_{2}SMO+. Each of the 8 experimental conditions (ie, load high and low, tidal volume high and low, leak on and off) was examined 6 times using the NPB 7200 and 6 times using the CO_{2}SMO+. The effects of airway leak on mean values for R and C, under different loads and tidal volumes were compared using paired t-tests (significance set at p < = 0.01). Results: For every combination of tidal volume and load, airway leak had a significant effect on R and C. Thus, combined results are illustrated below (mean ┬▒ SD).

(See original for figure)

Conclusions: An airway leak causes significant error in displayed values of dynamic resistance and compliance. The error was as much as 19% for R and 118% for C, which may be clinically important. Furthermore, the CO_{2}SMO+ showed an increase in displayed resistance with a leak, contrary to the theoretically expected result. The NPB does notify the operator of questionable results for static mechanics if a leak causes unstable plateau pressure. But neither the NPB 7200 nor the CO_{2}SMO+ questioned results for dynamic mechanics. Clinicians should be aware that monitors behave differently due to differences in calculation algorithms as well as tubing compliance effects. Manufacturers should (a) strive for universal standards, (b) evaluate device performance under error causing conditions like airway leaks and (c) use rejection criteria that prevent display of R and C when airway leak is detected.

The 44th International Respiratory Congress Abstracts-On-Disk®, November 7 - 10, 1998, Atlanta, Georgia.

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