Regional retinal blood flow and vascular autoregulation

The temporal retina is larger than the nasal retina and contains the metabolically active fovea. The variation in the distribution of blood between the temporal and nasal retinal circulations was investigated in 15 healthy volunteers, and the autoregulatory capacity of the temporal and nasal circulations was quantitated using 60% oxygen in the inspired air. Retinal blood flow was determined from red cell velocity using laser Doppler velocimetry and retinal vessel diameter from retinal photographs using a digital image analysis system. Volume flow was lower in the nasal than the temporal circulation by 52.49% (p < 0.001). This is a consequence of both significantly smaller vessels (20.4%, p < 0.001) and slower blood velocity in the nasal circulation (24.64%, p = 0.003) compared with the temporal vessels. After breathing 60% oxygen for 10 minutes, there was significant vasoconstriction (temporal, 10.42 +/- 1.24%, p < 0.001; nasal, 7.66 +/- 1.48%, p < 0.001), slower red cell velocity (temporal, 27.10 +/- 3.92%, p < 0.001; nasal, 27.36 +/- 5.51%, p < 0.001) and a significant reduction in the volumetric flow rate (temporal, 41.16 +/- 3.64%, p < 0.001; nasal, 37.99 +/- 5.07%, p < 0.001). The reduction in the haemodynamic parameters was comparable in the temporal and nasal circulations, indicating similar autoregulatory capacity. Retinal vascular conductance was calculated from volume flow and retinal perfusion pressure. It was 53% larger in the temporal than the nasal circulations. This provides an index of the metabolic needs of the different regions of the retina.