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The effect of panretinal photocoagulation (PRP) on retinal volumetric blood flow rate and its regulatory response to hyperoxia was investigated in a main temporal vein of 15 eyes with proliferative diabetic retinopathy, using bidirectional laser Doppler velocimetry and monochromatic fundus photography. At an average of 4 ± 2 months after the beginning of PRP, retinal venous diameter decreased from an average of 174 ± 20 μm to 162 ± 19 μm (P < 0.01), red blood cell velocity decreased from 1.3 ± 0.4 cm/sec to 1.1 ± 0.4 cm/sec (P < 0.01), and blood flow decreased from 11.5 ± 3.8 μl/min to 8.4 ± 3.3 μl/min (P < 0.001). The regulatory response to hyperoxia, characterized by the percentage decrease in retinal blood flow during 100% oxygen breathing, improved from 20 ± 15% prior to treatment to 45 ± 12% following treatment (P < 0.001). The decrease in retinal blood flow is most likely due to a decrease in viable retinal tissue and an improvement in retinal oxygenation following PRP. The increase in the regulatory response to 100% O2 breathing following PRP may also result from such an improved retinal oxygenation.