Albumin macroaggregates and measurements of regional blood flow: validity and application of particle sizing by Coulter counter.

Macroaggregates of albumin (MAA), most commonly labeled with 131I, have been used to evaluate the distribution of blood flow in a variety of organs, both in experimental animals and in man. Under clinical conditions, most workers have been concerned with a qualitative determination of regional pulmonary blood flow by external scintiscanning of the lungs after intravenous injection.1–6 MAA introduced into the inflow of an organ are assumed to mix uniformly in the bloodstream and to be distributed according to the regional flow. Ideally, all the injected particles should be trapped in the organ under study, they should be of such a size that they deposit in the capillaries or precapillary arterioles, and there should be no particles so small that they immediately traverse the capillary bed or so large that they block larger vessels to regions of physiologically significant size. Although deposition of MAA in the brain after intra-arterial injection has apparently produced no clinical or pathologic changes,5–7 avoidance of this last problem is particularly important in clinical studies of organs with an impaired blood supply. In practice, aggregate sizes vary widely, and this reduces their usefulness for the measurement of regional blood flow. Tow and co-workers1 reported that 25 per cent of injected radioactivity passed immediately through the lungs after intravenous injection of labeled MAA and through the brain after internal carotid artery injection.7 However, Taplin and associates8 reported that initial extraction of radioactivity by the lungs is between 80 and 95 per cent when particles with diameters in the range of 10 to 100 µ are injected. The disparity between these reports may be due to variation within and between batches of MAA and suggests that the distribution of particle sizes should be determined. In the present study, MAA were sized and counted with the Coulter counter, an instrument widely used for studying blood cells. This application of the Coulter counter has been briefly reported before.1 A further aim of the study was to determine which sizes of macroaggregates traverse the capillary bed immediately. During the period in which this investigation was undertaken, the conditions under which the MAA were synthesized underwent changes designed to improve homogeneity between batches, and an attempt was made to achieve a narrow and more satisfactory distribution of particle sizes.