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 (Q̇_c) because they incur less risk than traditional invasive methods such as direct Fick and thermodilution. We compared simultaneously obtained Q̇_c measurements during rest and exercise to assess the accuracy and precision of several rebreathing methods. Q̇_c 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; V̇O_2max: 45.1 ± 9.4 ml·kg ^-1·min^-1; mean ± SD) using one-N ₂O, four-C₂H₂, one-CO₂ (single-step) rebreathing technique, and two criterion methods (direct Fick and thermodilution). CO₂ rebreathing overestimated Q̇_c 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). C₂H ₂ and N₂O rebreathing techniques tended to underestimate Q̇_c (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 Q̇_c estimates with fewer repeat trials during resting conditions. During exercise, these methods remain precise but tend to underestimate Q̇_c. Single-step CO₂ rebreathing may be successfully employed over a wider range but with more measurements needed to overcome the larger variability.