Noninvasive measurement of microvascular permeability to small solutes

Quantification of microvascular permeability to diffusible hydrophilic solutes has received little attention in a clinical setting, probably because of a lack of noninvasive techniques. We describe a gamma camera technique which measures the clearance of 99Tcm-DTPA from the intravascular to extravascular space in extra-renal regions of interest over the lumbar region, the tissues within which being predominantly skin and muscle. This regional clearance (PSr), normalized to 100 ml of tissue, represents the permeability-capillary surface area (PS) product, a classical concept for describing solute transfer across the capillary. We found a mean value in 14 patients with normal renal function of 1.0 (S.D. 0.2) ml/min/100 ml of tissue, which is broadly similar to values anticipated from published values of 51Cr-EDTA extraction fraction (in a single pass) in human skeletal muscle and resting muscle plasma flow. The technique also generates the plasma volume (tPV) within this subrenal tissue, which we found to be 5.0 (2.0) ml/100 ml. In an attempt to reproduce an index of extraction fraction we also calculated the ratio PSr:tPV and this was 0.23 (0.08) min-1. This approach could prove useful in a variety of clinical settings.