What is the clinical evidence from this study?

Study design: Observational. Intervention: Closed-circuit gas rebreathing techniques (CO2, C2H2, N2O) vs. Direct Fick and thermodilution. Primary outcome: Cardiac output (Qc) accuracy and precision (p=<0.05).

June 7, 2007

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

Q: What are the key findings of this study?

CO2 rebreathing overestimated cardiac output compared to direct Fick and thermodilution during maximal exercise (27.0 vs 24.0 and 23.3 l/min), while foreign gas rebreathing tended to underestimate it.

Key Result

CO2 rebreathing overestimated cardiac output compared to direct Fick and thermodilution during maximal exercise (27.0 vs 24.0 and 23.3 l/min), while foreign gas rebreathing tended to underestimate it.

Study Design

Type

Observational (n=14)

Structured PICO

Do closed-circuit gas rebreathing techniques accurately and precisely measure cardiac output compared to direct Fick and thermodilution in adults during rest and exercise?

Population

14 participants (13 men, 1 woman, mean age 24 years) underwent simultaneous cardiac output measurements using rebreathing and invasive criterion methods during rest and exercise.

Exposure

Closed-circuit cardiac output rebreathing techniques (one-N2O, four-C2H2, one-CO2 single-step) during rest and exercise.

Comparator

Criterion methods (direct Fick and thermodilution) measured simultaneously.

Outcome

Accuracy and precision of cardiac output (Q(c)) measurements during rest (supine and standing) and stationary cycling (submaximal and maximal).surrogate

Foreign gas rebreathing provides reasonable cardiac output estimates at rest but underestimates during exercise, whereas single-step CO2 rebreathing is accurate over a wider range but has larger variability.

Main Result

p-value: p=<0.05

Limitations

CO2 rebreathing has larger variability, requiring more measurements to overcome it.

Abstract

Foreign and soluble gas rebreathing methods are attractive for determining cardiac output (Q(c)) because they incur less risk than traditional invasive methods such as direct Fick and thermodilution. We compared simultaneously obtained Q(c) measurements during rest and exercise to assess the accuracy and precision of several rebreathing methods. Q(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; Vo(2max): 45.1 +/- 9.4 ml.kg(-1).min(-1); mean +/- SD) using one-N(2)O, four-C(2)H(2), one-CO(2) (single-step) rebreathing technique, and two criterion methods (direct Fick and thermodilution). CO(2) rebreathing overestimated Q(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). C(2)H(2) and N(2)O rebreathing techniques tended to underestimate Q(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 CO(2) 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 CO(2) rebreathing. Thus the CO(2) 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 Q(c) estimates with fewer repeat trials during resting conditions. During exercise, these methods remain precise but tend to underestimate Q(c). Single-step CO(2) rebreathing may be successfully employed over a wider range but with more measurements needed to overcome the larger variability.

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