2007 OPEN FORUM Abstracts
THE EFFECT OF ALTITUDE ON END-TIDAL CARBON DIOXIDE MEASUREMENT USING THE ALARIS ETCO2 CAPNOGRAPHY MODULE IN NORMAL SUBJECTS
J. B. Waugh1, C. Beckett2, S. A. Stanley3, T. R. Stalcup2, R. Nichols3, K. H. Niebel4, W. M. Granger1
Background: Capnography is used to detect breaths and provide a means to assess alveolar ventilation, the integrity of the airway, and ventilatory function by measuring the partial pressure of exhaled carbon dioxide (PETCO2) throughout the breathing cycle. Measurement errors at high altitudes with earlier capnometers of various brands have been reported in the literature. The purpose of this study was to compare the PETCO2 of healthy volunteer subjects in Flagstaff, Arizona (over 7,000 ft/2134 m) with a similar group in Boston, MA (9 ft/2 m) using the Alaris® Monitoring System (Alaris Medical Systems Inc, San Diego, CA) with Oridion technology to verify reliable measurement at both altitudes.
Methods: Two groups of spontaneously breathing, normal, healthy volunteers were measured in an indoor setting with IRB approval (barometric pressure and relative humidity recorded). Group One was measured at an altitude of approximately 2134 m (n=30) and Group 2 near sea-level (n=33). Resting ventilation data (PETCO2, respiratory frequency), oxygen hemoglobin saturation by pulse oximeter (SpO2), and heart rate were recorded every 30 seconds (ten minutes total). The participants were coached to breathe at a normal resting pattern.
Results: The subjects in each group had normal spirometry (based on NHANES III predicted normal FEV1, FVC, FEV1% values). The altitude group had a lower mean PETCO2 (33.2±2.7 mm Hg) than the sea level group (38±3.6 mm Hg) showing a significant difference by two-sample t-test (p<0.001). The distribution of each group is graphically displayed below (sea level group on the right) and the coefficients of variation for Grp One/Two were 0.082/0.096. The Shapiro-Wilk normality test indicated normal distributions (Grp One/Two p-values of 0.160/0.371).
Conclusions: The PETCO2 measurements at two different altitudes had similar standard deviations and coefficients of variation for these normal subjects. The observed altitude differences were similar to PaCO2 changes in other related studies (Ann Inter Med 1994;121(9):658-662; Chest 1998;113(3):571-575). When determining PETCO2 alarm setting on a patient monitor, it is advisable to take into consideration the altitude of the clinical setting since the ambient air pressure as a function of altitude does affect measured CO2 partial pressure.
[Supported by Oridion Capnography Inc.]