The principle and application of a new plethysmography using a radioisotope (RI) is described. This technique (RI plethysmography) is applied to the limb blood flow measurement in patients to whom RI has previously been administered for the purpose of routine angiography. In case of intravenous administration, RI is assumed to diffuse homogeneously into the blood. When venous occlusion of the forearm or the calf is performed in such subject, the RI count rate in a limb compartment increases according to the accumulation of the blood. The relationship between the changes of the count rate (ΔC) and the blood volume (ΔV) after venous occlusion can be expressed as
ΔV/V=β·ΔC/(C-C0),
where V is the volume of the tissue compartment, β is the volume rate of the blood to the tissue (=Vb/V=2.5ml/100ml), and C and C0 are the count rates of the compartment before venous occlusion and of the avascularization state, respectively. In this study, C0 was tentatively determined by compressing the compartment using a pneumatic cuff with the pressure of more than 180mmHg. The blood flow per unit tissue volume can be calculated as the change in the blood volume per unit time (ΔV/V/Δt). Using this technique, calf blood flows of the patients with cardiovascular diseases have been measured and valuable results of the diagnosis of the disorders in peripheral circulation have been obtained.
