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

¹Department of Kinesiology, Pennsylvania State University, University Park, Pennsylvania; ²Institute for Exercise and Environmental Medicine, University of Texas Southwestern Medical Center at Dallas and Presbyterian Hospital of Dallas, Dallas, Texas; ³Department of Medicine, University of California at San Diego, La Jolla, California; ⁴Department of Physiology, State University of New York at Buffalo, Buffalo, New York; ⁵Hartford Hospital, Hartford; ⁶Premise Development Corporation, Hartford, Connecticut; ⁷Cardiology Division, Department of Internal Medicine, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas

Submitted 2 October 2006 ; accepted in final form 30 May 2007

Foreign and soluble gas rebreathing methods are attractive for determining cardiac output (_c) because they incur less risk than traditional invasive methods such as direct Fick and thermodilution. We compared simultaneously obtained _c measurements during rest and exercise to assess the accuracy and precision of several rebreathing methods. _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; O_2max: 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 _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). C₂H₂ and N₂O rebreathing techniques tended to underestimate _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 _c estimates with fewer repeat trials during resting conditions. During exercise, these methods remain precise but tend to underestimate _c. Single-step CO₂ rebreathing may be successfully employed over a wider range but with more measurements needed to overcome the larger variability.

foreign gas rebreathing; physiological gas rebreathing; direct Fick; thermodilution