SHEAR RATE AND HEMATOCRIT DEPENDENCE OF FLUORESCENCE FROM RETINAL-VESSELS IN FLUORESCEIN ANGIOGRAPHY

The purpose of this work was to obtain more quantitative knowledge abo ut the yield of fluorescence from retinal vessels during fluorescein a ngiography. The influence of shear rate, concentration of sodium fluor escein, hematocrit, and layer thickness on the yield of fluorescence f rom blood were investigated. Measurements were performed in vitro on s amples of human blood in a cone-plate shear chamber using frontal illu mination. Application of physiologically relevant levels of shear (> 8 8/sec) decreased the yield of fluorescence from the blood sample consi derably as compared with stasis. The yield of fluorescence was proport ionally related to the logarithm of the sodium fluorescein concentrati on in blood up to a sodium fluorescein concentration of 1.2 mg/ml. Abo ve that concentration quenching occurred. An increase in layer thickne ss at a hematocrit of 45% resulted only in an increase of the yield of fluorescence up to a layer thickness of 25 mu m. In conclusion, the s odium fluorescein concentration in blood is the only important factor that determines the yield of fluorescence from the larger retinal vess els in the successive phases of the fluorescein angiogram in a subject with a given hematocrit and hemoglobin concentration. The yield of fl uorescence from retinal vessels (> 25 mu m) is proportionally related to the logarithm of the sodium fluorescein concentration over a broad range of concentrations.