Effect of analyzer on the determination of mixed venous pco2 and
cardiac output during exercise.
Hornby, Laura, Allan L. Coates, Larry C. Lands.
Division of Respiratory Medicine, Montreal Children's Hospital
-McGill University, Montreal, Quebec, Canada
APStracts 2:0165A, 1995.
Cardiac output () during exercise can be determined non-invasively
using the Indirect Fick CO2-rebreathing technique. CO2 measurements
for this technique are usually performed using an infrared analyzer
(IA) or mass spectrometer (MS). However, IA CO2 measurements are
susceptible to under-reading in the face of high O2 concentrations,
because of collision broadening. We compared an IA (Ametek model CD
-3A) to a MS (Marquette model MGA-1100), to see the effect this would
have on mixed venous PCO2 (PCO2) and cardiac output () measurements.
After calibration with room air and a gas mixture of 5% CO2, 12% O2,
balance N2, both devices were tested with 3 different gas mixtures of
CO2 in O2. For each gas mixture (GM), IA gave lower values than MS
(GM: 4.1; IA: 3.85+/-0.01; MS: 4.13+/-0.01; GM: 9.2; IA: 8.44+/-0.07;
MS: 9.19+/-0.01; GM: 13.8; IA: 12.57+/-0.15; MS: 13.82+/-0.01).
Warming and humidifying the gases did not alter the results. The IA
gave lower values than the MS for eight other medical gases in lower
concentrations of O2 (40%-50%). Equilibrium and exponential
rebreathing procedures were performed. IA determined was more than
10% higher than that determined by 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 oxygen
is greater than 21%. If collision broadening is present, then either
a special high O2, CO2-calibration curve must be constructed, or the
IA should not used for both PaCO2 and PCO2 estimates, for it may
produce erroneously low PCO2 values, with resultant overestimation of
.
Received 20 April 1994; accepted in final form 14 April 1995.
APS Manuscript Number A362-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 25 April 1995.