Journal of Applied Physiology, Vol 79, Issue 3 1032-1038, Copyright © 1995 by American Physiological Society
Effect of analyzer on determination of mixed venous PCO2 and cardiac output during exercise
L. Hornby, A. L. Coates and L. C. Lands
Division of Respiratory Medicine, Montreal Children's Hospital-McGill University, Quebec, Canada.
Cardiac output (CO) during exercise can be determined noninvasively by
using the indirect Fick CO2-rebreathing technique. CO2 measurements for
this technique are usually performed with an infrared analyzer (IA) or mass
spectrometer (MS). However, IA CO2 measurements are susceptible to
underreading in the face of high O2 concentrations because of collision
broadening. We compared an IA (Ametek model CD-3A) with a MS (Marquette
model MGA-1100) to see the effect this would have on mixed venous PCO2
(PVCO2) and CO measurements. After calibration with room air and a gas
mixture of 5% CO2-12% O2-83% N2, both devices were tested with three
different gas mixtures of CO2 in O2. For each gas mixture, IA gave lower
CO2 values than did the MS (4.1% CO2: IA, 3.85 +/- 0.01% and MS, 4.13 +/-
0.01%; 9.2% CO2: IA, 8.44 +/- 0.07% and MS, 9.19 +/- 0.01%; 13.8% CO2: IA,
12.57 +/- 0.15% and MS, 13.82 +/- 0.01%). Warming and humidifying the gases
did not alter the results. The IA gave lower values than did the MS for
eight other medical gases in lower concentrations of O2 (40-50%).
Equilibrium and exponential rebreathing procedures were performed. Values
determined by the IA were > 10% higher than those determined by the MS
for both rebreathing methods. We conclude that all IAs must be checked for
collision broadening if they are to be used in environments where the
concentration of O2 is > 21%. If collision broadening is present, then
either a special high O2-CO2 calibration curve must be constructed, or the
IA should not be used for both arterial PCO2 and PVCO2 estimates because it
may produce erroneously low PVCO2 values, with resultant overestimation of
CO.
