Gold standard must be solid gold
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W. P. de Boode S. L. Vrancken J. Lemson A. Nusmeier S. M. Tibby
Gold standard must be solid gold Accepted: 3 April 2013 Published online: 19 April 2013 Ó Springer-Verlag Berlin Heidelberg and ESICM 2013
Dear Editor, In search of an optimal haemodynamic treatment strategy, much interest has risen in advanced haemodynamic monitoring in paediatric patients. Therefore, it was with great interest that we read the article by Floh and colleagues [1]. However, we do have concerns about their validation study of the transpulmonary ultrasound dilution (TPUD) technology in paediatric patients after biventricular repair.
Fig. 1 Example calculation to demonstrate the effect of the use of erroneous venous oxygen saturation values to calculate cardiac output using the Fick equation
CO RRESPONDENCE
In this study, TUPD is validated against the oxygen-Fick (O2F) method, that can be regarded as the gold standard of cardiac output (CO) measurement in a clinical setting. Prerequisites for the O2F method are: accurate measurements of oxygen consumption, arterial oxygen content and mixed venous content (CmvO2). Any error in measurement in these vital parameters will directly result in an inaccurate calculation of CO. CmvO2 calculation demands sampling of blood via a pulmonary artery catheter. In this study, central venous blood was sampled from the internal jugular vein (86 %), subclavian vein (6 %), femoral vein (6 %) and right atrium (3 %). It is known that central venous oxygen saturation (ScvO2) does not reflect
mixed venous oxygen saturation (SmvO2) and that the difference between these parameters is influenced by the venous sampling site, presence of left-to-right shunt, redistribution of blood flow, level of consciousness (anaesthesia) and myocardial oxygen consumption. Limits of agreement (LOA) between SmvO2 and ScvO2 range from ± 15 to ± 25 % (see Table. 1). Use of erroneous venous oxygen saturation in this order to calculate venous oxygen content, which is used in the Fick equation, may overestimate cardiac output up to sixfold (see Fig. 1) Hence, the O2F method is not applied as should be and therefore, cannot be regarded as gold standard under these conditions.
Table 1 Limits of agreement between mixed oxygen saturation (PA) and central venous oxygen saturation (SVC) Patients
LOA (= 1.96xSD) (%)
Adults in shock [2] Children post cardiac surgery [3] Day 0 Day 1 Day 2 Day 3 Day 4
±21.3 ±25.3 ±15.9 ±16.5 ±16.5 ±14.3
1331
2. Edwards JD, Mayall RM (1998) Importance of the sampling site for measurement of mixed venous oxygen saturation in shock. Crit Care Med 26:1356–1360 3. Ra¨sa¨nen J, Peltola K, Leijala M (1992) Superior vena caval and mixed venous oxyhemoglobin saturations in children recovering from open heart surgery. J Clin Monit 8:44–49 4. de Boode WP, van Heijst AFJ, Hopman JCW, Tanke RB, van der Hoeven HG, Liem KD (2010) Cardiac output measurement using an ultrasound dilution method: a validation study in ventilated piglets. Pediatr Crit Care Med 11:103–108 5. Vrancken SL, de Boode WP, Hopman JC, Singh SK, Liem
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