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Re: Microvascular Accuracy
- Hans Knotzer, Stephan Maier, and Walter Hasibeder (1 April 2009)
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Microvascular Accuracy
- Paul WG Elbers, Bektas Atasever, Dpt of Cardiothoracic Surgery, VUMC, Amsterdam, NL (12 March 2009)
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Hans Knotzer , Stephan Maier, and Walter Hasibeder
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Response to: Electronic Letter Microvascular Accuracy from Paul Elbers and Bektas Atasever. Editor - We agree with Drs. Elbers and Atasever that a uniform investigation process would facilitate the comparison between different studies using the same measuring technique. We are also aware of the round table conference report by excellent investigators in this field published in September 2007, at a time-point, where our present study already had begun, which is of course no excuse.1 But let us look again on the present investigation:2 The conclusion of the presented study demonstrates that alteration of the systemic vascular conductivity (= systemic blood flow/arterial perfusion pressure) has direct effects on the microcirculation. In detail, arterial blood pressure increase induced with phenylephrine at constant cardiopulmonary bypass blood flow leads to a microvascular blood flow shunting in the sublingual mucosal microcirculation.2 We arrived at this conclusion mainly by interpretation of the quantitatively assessed microcirculatory measurement techniques, namely laser Doppler flowmetry and tissue reflectance spectrophotometry.2 The sidestream dark-field (SDF) imaging was used to underline our microcirculatory results, as it provides us with a semi-quantitatively analysis of microcirculatory blood flow, which is correctly stated in the letter by Drs. Elbers and Atasever. In general, to the best of our knowledge, this is the first study presenting qualitatively and quantitatively assessed microcirculatory parameters using different measurement techniques at the same region in humans. With this combination of measurement devices the viewpoint of the microcirculation can be enlarged giving us a better insight in the physiology and pathophysiology in the microcirculation and tissue oxygenation. So, the accuracy of microvascular measurement, as stated in the letter by Drs. Elbers and Atasever, would not only improve by a uniform investigation process, but also by the use of different measurement techniques focusing not only on flow quality but also on oxygen supply to tissues and organs.3 Hans Knotzer Stephan Maier Walter Hasibeder Innsbruck and Ried, Austria e-mail: johann.knotzer@uki.at 1 De Backer D, Hollenberg S, Boerma C, et al. How to evaluate the microcirculation: report of a round table conference. Critical Care 2007; 11: R101 2 Maier S, Hasibeder W, Hengl C, et al. Effects of phenylephrine on the sublingual microcirculation during cardiopulmonary bypass. Br J Anaesth 2009; 109: 485-91 3 Knotzer H, Hasibeder W. Microcirculatory function monitoring at the bedside – a view from the intensive care. Physiol Meas 2007; 28: R65-86 Conflict of Interest:None declared |
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Paul WG Elbers Department of Anesthesia, Intensive Care and Pain Management, St. Antonius Hospital, Nieuwegein, NL, Bektas Atasever, Dpt of Cardiothoracic Surgery, VUMC, Amsterdam, NL
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Dear Sirs, It is with great interest that we read the recent article by Maier et al, in which they assess the microcirculatory effects of phenylephrine during cardiopulmonary bypass (1). The authors are to be commended for focussing their research on the microcirculation, as that is where actual delivery of oxygen and nutrients to tissue takes place. Their study adds to the large and expanding body of evidence that systemic hemodynamic parameters do not necesarily reflect microvascular perfusion. However, having extensive experience with the technique used by the authors to visualise the microcirculation, i.e. SDF imaging, we feel the following comments may be of great importance. Many different scoring systems and video acquisition procedures have been developed in microcirculation research. This has the potential for hampered comparison between similar studies, or worse, false conclusions. This is why a round table conference was held in 2006. Using the Delphi methodology, a vast number of recommendations were made in order to improve quality and consistency in microvascular research (2). Unfortunately, Maier et al did not fully adhere to these recommendations. Although there are multiple differences from the consensus statement, two may be critical and are discussed below. First, it was recommended to measure at least three, but preferably five sites, per patient per time point, because of the intrinsic variability of the microcirculation. This also allows for better reporting of an index of heterogeneity. The latter is not stated by Maier et al. and they only measured two sequences per time point. It is therefore possible that the measurements they have made do not truly represent the state of the microcirculation. This may be reflected by the unexpectedly low baseline microvascular flow index of 2.5 in anesthetized patients before cardiopulmonary bypass. Second, it was recommended to report on both microvascular flow index (MFI), as the authors did, and an index of functional capillary density, which was omitted. The problem with this is that MFI is a semi- quantitative way to describe microvascular flow but does not take into account the number of vessels per area. This is important because with SDF imaging, different imaging sites are used at different timepoints which may hence have different capillary densities. Obviously from a perspective of oxygen and nutrient delivery to tissue, a large capillary density with some sluggish and some continuous flow, would be preferable over very low capillary density albeit with only continuous flow. Because an index of perfused vessel density was not reported, the given MFI values may not truly represent the state of the microcirculation. Of course, trade-offs have to be made in microvascular research between detail of analysis and the time it takes to perform said analysis. However, it was recently suggested that using a fast-track analysis approach, it is quite reasonable to report on both indices of microvascular perfusion (3). In summary, failure to adhere to the recommendations mentioned above, may have led to a biased representation of the microcirculation. This is not to say that the results are false by definition. However, we feel that they should be approached with caution. The conclusion by Maier et al would be greatly strengthened by reporting on more microvascular sites per time point and by extending the analysis to include an index of functional capillary density. While the former is not feasible, it is relatively easy to perform the latter using the existing videos of the microcirculation. Therefore, we would strongly recommend the authors to do so. Yours Sincerely, Paul Elbers, MD Bektas Atasever, MD (1) Maier S et al. Effects of phenylephrine on the sublingual microcirculation during cardiopulmonary bypass. Br J Anaesth. 2009 Feb 25. [Epub ahead of print] (2) De Backer D et al. How to evaluate the microcirculation: report of a round table conference. Crit Care. 2007; 11(5): R101. (3) Elbers PWG. Fast track microcirculation analysis. Crit Care. 2007; 11(6): 426. Conflict of Interest:None declared |
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