The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

Evaluation of a system to measure FUNCTIONAL RESIDUAL CAPACITY (frc)

Joseph Orr, Ph.D.1, Lara Brewer, M.S. 1, Michael Sherman, M.S. 1, and Dinesh Haryadi, Ph.D. 2   1. Anesthesiology, University of Utah, Salt Lake City, Utah, United States.

2. Respironics Inc, Wallingford, Connecticut, United States.

Background: Functional Residual Capacity (FRC) is the volume of gas that remains in the lungs at the end of expiration. Direct measurement of FRC is routinely done in the pulmonary function laboratory but is difficult, if not impossible, in the ICU. We compared the accuracy of a prototype system designed to measure FRC in the ICU to body plethysmography FRC measurements in healthy volunteers.

Materials and Methods: We assembled a prototype system to measure FRC, using a multiple breath nitrogen washout technique, based on signals acquired from typical critical care monitoring devices. Airway flow and CO2 data were acquired from a respiratory profile monitor (NICO2, Respironics, Wallingford, CT). The O2 signal was acquired from an anesthesia gas analyzer (Capnomac, Datex). Nitrogen concentration was calculated as the balance gas using the equation fN2 = 1 - fCO2 - fO2. FRC was calculated by analyzing the alveolar volume required to clear the nitrogen from the lungs in response to a step increase in FiO2. We applied the standard clinical protocol for FRC measurements using the body box in 20 healthy volunteers (11 male, 9 female). The FRC measurement protocol for the nitrogen washout system required volunteers to breathe spontaneously through a T-piece while the flow, CO2 and O2 signals were recorded and analyzed. After the end-tidal N2 concentration had stabilized, the concentration of O2 flowing through the T-piece was rapidly increased from 50% to 100%. The subjects continued to breathe through the T-piece for at least 5 minutes following the change in fiO2. Data from the first 20 breaths following the fiO2 change were used to calculate FRC. Two measurements were made for each subject using each method. The average FRC measurements from each of the two methods for each subject were compared using regression analysis and Bland-Altman analysis.

Results: Body box FRC measurements ranged from 2550 to 5410 mL. The difference between the two measurements was -17±322 mL (mean ±1 S.D.).  Regression analysis showed a correlation of r2 = 0.89 with a slope of 1.02.

Discussion: In earlier work, we demonstrated that the prototype N2 washout technique yields reasonable and repeatable FRC measurements in intubated animals with healthy and injured lungs. These data show that this same method also correlates well with the pulmonary function laboratory standard FRC measurements in healthy volunteers.


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