Validation of pulse contour derived stroke volume variation
Kiel, Germany
* E-mail: bein{at}anaesthesie.uni-kiel.de
Editor—We read the article by Dr Kubitz and colleagues reporting their results comparing stroke volume variation (SVV) derived by different methods of SV determination with great interest.1 The authors should be commended for this study, that for the first time compared SVV derived from the widely used PiCCO monitor (Pulsion Medical Systems, Munich, Germany) to a true reference standard, that is, aortic transit time ultrasound. Although their approach is ambitious, however, some methodological remarks are necessary. First, the authors state that there was good agreement between SVV derived from pulse contour analysis and that derived from the aortic flow signal. This conclusion, however, is not supported by the data presented. After the introduction of Bland–Altman2 plots for method comparison in 1986, for more than a decade the judgement of bias and limits of agreement was left to the clinician, and identical values were interpreted differently. The pivotal work by Critchley3 for the first time suggested a comprehensive mathematically derived criterion for assessment of observed variability. Given an inherent variability of ±20% for each method under comparison, the combined variability (i.e. limits of agreement) should not exceed ±30% of the mean SVV. Applying these strict criteria to the data, agreement of methods studied by the authors was unacceptable. Secondly, the authors compared SV derived from different methods at specified experimental conditions. Unfortunately, no information is provided as to the ability of these methods to reflect changes of SVV consistently. It has been suggested recently to analyse the change in a variable after a specific intervention also with a Bland–Altman plot, comparing the mean per cent change of both methods against the difference.4 This analysis would have revealed quickly, if these methods do track ensuing changes of SVV in a comparable fashion. This is a very important issue, since changes of SVV are thought to reflect a change in the fluid responsiveness of an individual patient. Thirdly, the authors compared methods during a single tidal volume (12 ml kg–1). This is quite surprising, as mechanical ventilation has an important impact on SVV, and consequently agreement between methods may be influenced by depth of tidal volume applied.5
1 Munich, Germany
2 Hamburg, Germany
* E-mail: dreuter{at}uke.uni-hamburg.de
Editor—We thank Dr Bein and colleagues for their comments on our recent article and for the opportunity to reply. First, they are correct that Critchley and Critchley3 suggested a very useful approach to ‘quantify acceptable limits of agreement between two measurement techniques’. From a meta-analysis comparing absolute values of clinical cardiac output measurement techniques, they concluded that combined limits of agreement should not be above 30% according to an error-gram. This is a possible point for discussion; however, in contrast to our investigation, this was focused on absolute values of cardiac output and a comparison of two clinical methods vs each other. Secondly, there is no analysis of changes in SVV provided in our article, as no change in SVV occurred after alteration of mean arterial pressure. Thirdly, the influence of tidal volume on functional preload indices is well known,6 7 and thus tidal volume was not altered in our study.
References
1 Kubitz JC, Annecke T, Forkl S, et al. Validation of pulse contour derived stroke volume variation during modifications of cardiac afterload. Br J Anaesth (2007) 98:591–7.
2 Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet (1986) 1:307–10.[CrossRef][Web of Science][Medline]
3 Critchley LA, Critchley JA. A meta-analysis of studies using bias and precision statistics to compare cardiac output measurement techniques. J Clin Monit (1999) 15:85–91.[Medline]
4 Linton NW, Linton RA. Is comparison of changes in cardiac output, assessed by different methods, better than only comparing cardiac output to the reference method? Br J Anaesth (2002) 89:336–7.
5 Renner J, Cavus E, Meybohm P, et al. Stroke volume variation during hemorrhage and after fluid loading: impact of different tidal volumes. Acta Anaesthesiol Scand (2007) 51:538–44.[CrossRef][Web of Science][Medline]
6 Szold A, Pizov R, Segal E, Perel A. The effect of tidal volume and intravascular volume state on systolic pressure variation in ventilated dogs. Intensive Care Med (1989) 15:368–71.[Web of Science][Medline]
7 Reuter DA, Bayerlein J, Goepfert MS, et al. Influence of tidal volume on left ventricular stroke volume variation measured by pulse contour analysis in mechanically ventilated patients. Intensive Care Med (2003) 29:476–80.[Web of Science][Medline]
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