I131-labeled serum albumin: its use in the study of cardiac output and peripheral vascular flow.

Abstract

During the past several years a method for measuring plasma volumes has been under investigation in our laboratory (1, 2). This method involved the use of a tagged serum protein (I131-tagged albumin) and gave promising results which appeared useful. It became obvious that the slow rate of disappearance of tagged albumin from the blood would make this a satisfactory substance for the study of vascular flow measurements. Before this could be achieved, however, it was necessary to develop a number of instruments which would make it possible to detect small amounts of radioactivity by their gamma emission and to record the events rapidly enough so that critical alterations in dilution and mixing of the radioactive albumin could be recorded. The essence of the method which will be described concerns itself with measuring and recording the manner and rate in which iodinated plasma is mixed and diffused through the blood. The character of the curve which is obtained on isolated arteries has proved suitable for quantitation. The curves recorded for peripheral flow over the extremities do not as yet permit quantitation but, as will be demonstrated later, there are definite qualitative changes which may signify alterations in peripheral flow. Description of Recording Equipment As intimated above, the inherent problems in measurement of any vascular flow are twofold. The first requisite is a detector sufficiently sensitive to measure and detect low levels of radioactivity by gamma emission. Since the standard method of detection, a Geiger counter, has poor sensitivity to the comparatively low-energy gamma rays of I131, a scintillation counter was deemed necessary (3) and an anthracene crystal counter was adapted for this purpose. This gave an increase of sensitivity of approximately fiftyfold and made it possible for the iodinated protein to be used in tracer amounts. At the present time, thallium-activated sodium iodide crystals are being utilized and show an even greater improvement. The second device required for adequate representation of vascular flow data was a recording apparatus that would report the concentration of the tagged atoms in the vascular system as a function of time. Since in many proposed studies of various flow channels the change in concentration during very short intervals (i.e., seconds) would be of interest, it was considered that the method of withdrawing blood every one to three seconds would be both unwieldy and restrictive in respect to the total volume of blood required. Because of this, an in vivo system of measuring seemed desirable. Recording by an ordinary counting rate meter is possible except that the counting rate at any time is influenced by the previous count on this instrument.