J Thorac Cardiovasc Surg 2006;131:504
© 2006 The American Association for Thoracic Surgery
Assumed oxygen consumption in the determination of cardiac output: Assume carefully ...
R.M.F. Berger, MD, PhD,
A. Bergstra, BSc
Beatrix Children's Hospital, Department of Pediatrics, Division of Pediatric Cardiology, Thorax Center, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands
To the Editor:
With great interest we read the article of Fakler and coworkers
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in which they warn of large errors in determination of cardiac output when using the Fick principle with assumed oxygen consumption (VO2).
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We share the concerns of the authors, especially in specific patients with potentially deviating VO2 compared with a control group, such as children with congenital heart disease.
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However, methodological errors in the study of Fakler and associates may surpass the error introduced by the use of assumed VO2 in these patients.
Although we hope that our major concern is based on typographical errors, we fear a major flaw in the analysis in this study. According to the described equation in the Methods section, the assumed VO2 values derived from the LaFarge and Miettinen equations are being divided by body surface area (BSA) to reach indexed values. However, LaFarge and Miettinen
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derived their equation for indexed VO2! In other words, the presented equations are incorrect and will lead to highly erroneous values, resulting in false high values in smaller patients (BSA < 1 m2) and false low values in larger patients (BSA > 1 m2). In their original study, Lafarge and Miettinen
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included over 800 patients and found indexed VO2 values between 112 and 162 mL · min–1 · m–2. From Figure 4 in the current article, it can be estimated that in this study more than 70% of the VO2LaFarge/Miettinen values are outside this range, whereas the authors claim that the studied patients were in the same age range as the original LaFarge and Miettinen population. This strongly suggests that the authors indeed applied a wrong equation to assume indexed VO2. In this population with a mean age of 11.3 years, this could be one explanation for the large overestimation of assumed VO2.
By analyzing indexed values of VO2 instead of the measured quantity, the authors make it impossible to analyze whether the occurring errors correlate with the absolute magnitude of VO2. In general, this should be done when comparing measurement methods for accuracy.
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Second, it can be questioned whether absolute values of VO2 determined with the Deltatrac II system (Datex, Engström, Helsinki, Finland) are acceptable as a gold standard. Although this system has been validated in vivo, this was mainly for its use in metabolic studies, requiring accurate and stable respiratory quotients. It has not been well validated for absolute values of VO2 in mechanically ventilated children, in whom the prevention of air leakage is extremely cumbersome but imperative for accurate measurement of absolute values.
Finally, in contrast to the statement of the authors, in patients with congenital heart defects, adequate values for VO2 are not necessary for the determination of shunt ratio.
In conclusion, we absolutely agree with the authors that routine use of assumed VO2 may result in errors in the determination of Fick cardiac output. Unfortunately, we question whether the presented data can be used to support this common opinion.
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References
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- Fakler U, Pauli C, Hennig M, Sebening W, Hess J. Assumed oxygen consumption frequently results in large errors in the determination of cardiac output. J Thorac Cardiovasc Surg 2005;130:272-276.[Abstract/Free Full Text]
- Bergstra A, van Dijk RB, Hillege HL, Lie KI, Mook GA. Assumed oxygen consumption based on calculation from dye dilution cardiac output. an improved formula. Eur Heart J 1995;16:698-703.[Abstract/Free Full Text]
- Berger RMF, van Poppelen R, Kruit M, van Vliet A, Witsenburg M, Hess J. Impact of discrepancy between assumed and measured oxygen consumption for the calculation of cardiac ouput in children during cardiac catheterization. Neth J Cardiol 1992;4:156-160.
- LaFarge CG, Miettinen OS. The estimation of oxygenconsumption. Cardiovasc Res 1970;4:23-30.[Abstract/Free Full Text]
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