Measurement of microvascular permeability to small solutes in man: limitations of the technique

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Abstract
Study objective – The aim was to evaluate the limitation on the measurement of the clearance rate of a small solute, 99mTc DTPA, from intravascular to extravascular spaces imposed by diffusion of the solute back into blood during the period of measurement. Design – The technique for measuring clearance also generates the regional plasma volume from which the solute is cleared. Back diffusion would result in an overestimation of this plasma volume. By applying the same technique to 99mTc labelled human serum albumin, which is not cleared over the period of measurement, a separate estimate of plasma volume can be made. Subjects – The subjects were eight patients undergoing routine 99mTc DTPA renography for suspected outflow obstruction. Measurements and results – The ratio of plasma volumes based respectively on 99mTc DTPA and human serum albumin in a region of interest below the kidney was 1.04 (SD 0.09). The technique requires a region of interest over the cardiac blood pool from which the blood level of 99mTc DTPA is continuously monitored. When a correction was made for extravascular 99mTc DTPA in this cardiac region of interest, the ratio was unchanged: 0.97 (0.12). Conclusion – The technique is capable of measuring the clearance of 99mTc DTPA although, because of its small molecular size, the transfer rate is blood flow dependent, ie, its PS product is greater than its clearance.