Journal
JOURNAL OF APPLIED PHYSIOLOGY
Volume 102, Issue 3, Pages 1235-1242Publisher
AMER PHYSIOLOGICAL SOC
DOI: 10.1152/japplphysiol.00740.2006
Keywords
distribution space; plasma protein; endothelial glycocalyx; endothelial surface layer; albumin
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The indocyanine green ( ICG) dilution technique ( DT) is frequently used for plasma volume ( PV) measurement. However, because of inadequate knowledge about the properties of this dye, lack of accuracy has been attributed to the method. The aim of this report is to provide physiological background information about the ICG- DT to avoid some profound misunderstandings. When performing tracer dilution, one has to consider the tracer's distribution space before interpreting the result. For ICG, the distribution space is the total PV, i. e., circulating + noncirculating PV, fixed within the endothelial glycocalyx. The distribution space of red blood cells and large molecules, in contrast, is only the circulating part of PV. Therefore, it is erroneous to compare directly PV derived from different tracer dilution methods. The transcapillary escape rate of ICG should not relevantly influence measured PV if the method is performed properly, i. e., if a short time window of measurement is subjected to monoexponential extrapolation. A major problem of PV measurement in general is that the target itself is very inconstant. Thus, checking for constancy of ICG- DT with two consecutive measurements is unreliable. Nevertheless, the ICG- DT is a useful tool for determining PV, provided it is well understood by the investigator to enable correct interpretation of the results.
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