Simultaneous determination of the accuracy and precision of closed-circuit cardiac output rebreathing techniques

Foreign and soluble gas rebreathing methods are attractive for determining cardiac output (Q̇c) because they incur less risk than traditional invasive methods such as direct Fick and thermodilution. We compared simultaneously obtained Q̇c measurements during rest and exercise to assess the accuracy and precision of several rebreathing methods. Q̇c measurements were obtained during rest (supine and standing) and stationary cycling (submaximal and maximal) in 13 men and 1 woman (age: 24 ± 7 yr; height: 178 ± 5 cm; weight: 78 ± 13 kg; V̇o2max: 45.1 ± 9.4 ml·kg−1·min−1; mean ± SD) using one-N2O, four-C2H2, one-CO2 (single-step) rebreathing technique, and two criterion methods (direct Fick and thermodilution). CO2 rebreathing overestimated Q̇c compared with the criterion methods (supine: 8.1 ± 2.0 vs. 6.4 ± 1.6 and 7.2 ± 1.2 l/min, respectively; maximal exercise: 27.0 ± 6.0 vs. 24.0 ± 3.9 and 23.3 ± 3.8 l/min). C2H2 and N2O rebreathing techniques tended to underestimate Q̇c (range: 6.6–7.3 l/min for supine rest; range: 16.0–19.1 l/min for maximal exercise). Bartlett's test indicated variance heterogeneity among the methods ( P < 0.05), where CO2 rebreathing consistently demonstrated larger variance. At rest, most means from the noninvasive techniques were ±10% of direct Fick and thermodilution. During exercise, all methods fell outside the ±10% range, except for CO2 rebreathing. Thus the CO2 rebreathing method was accurate over a wider range (rest through maximal exercise), but was less precise. We conclude that foreign gas rebreathing can provide reasonable Q̇c estimates with fewer repeat trials during resting conditions. During exercise, these methods remain precise but tend to underestimate Q̇c. Single-step CO2 rebreathing may be successfully employed over a wider range but with more measurements needed to overcome the larger variability.

Effect of analyzer on determination of mixed venous PCO2 and cardiac output during exercise

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.