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.