Radionuclide assessment of peripheral intravascular capacity: a technique to measure intravascular volume changes in the capacitance circulation in man

Changes in the capacitance vasculature influence venous return and cardiac performance, so an understanding of the effects of pathophysiologic states on the human capacitance vasculature is necessary to understand integrated cardiovascular function in man. Techniques available to assess the capacitance vasculature in man, however, have limitations. We performed radionuclide imaging of the calf or forearm in 51 patients whose erythrocytes had been labeled in vivo with technetium-99m, basing our approach on the principle that counts from the radiolabeled intravascular space are proportional to blood volume. Two-minute or 15-second count acquisitions were obtained from the calf in 42 patients. Counts obtained at rest demonstrated little variation. With veno-occlusion at 15 and 30 mm Hg, counts increased 8 +- 1% (+- SEM) (p < 0.001) and 28 +- 2% (p < 0.001), respectively. After 0.4 mg of sublingual nitroglycerin, counts increased 9 +- 1% (p < 0.001). With leg elevation, counts decreased 34 +- 4% (p < 0.001). Response patterns were similar with 2-minute and 15-second acquisitions. In nine patients who underwent forearm imaging (2-minute acquisitions), counts increased 14 + 2% (p < 0.001) and 26 +- 4% (p < 0.001) at 15- and 30-mm Hg veno-occlusion and 15 +- 3% (p < 0.001) after nitroglycerin. Volume displacements, recorded simultaneously with a fluid-filled plethysmograph about the contralateral forearm, correlated linearly in all nine patients. Thus, gamma camera imaging of the radiolabeled peripheral intravascular space provides a quantitative and reliable assessment of peripheral vascular capacity in man. The technique could be used in conjunction with gated cardiac imaging in order to assess the interactions of peripheral vascular capacity and ventricular performance