Physiological perturbation of ocular and cerebral blood flow as measured by scanning laser ophthalmoscopy and color doppler imaging

Retinal blood flow regulation in health remains poorly described. We hypothesized that retinal perfusion is controlled to provide constant O₂ delivery to that tissue, and that changes in retinal blood flow in response to chemical stimuli parallel changes in carotid and retrobulbar perfusion. Accordingly, in 11 young adults with normal eye examinations, we measured retinal blood flow indices (via scanning laser ophthalmoscopy [SLO] during fluorescein angiography) and carotid, ophthalmic, and central retinal arterial blood flow indices (via Doppler imaging [CDI]) under control, hypoxic (alveolar PO₂ = 55 ± 3 mmHg) and hyperoxic (alveolar PO₂ = 655 ± 18 mmHg) conditions. The three conditions were counterbalanced in order and isocapnia was maintained in each. Retinal arterial mean dye velocity and arteriovenous passage time, as measured by SLO, were slowed by hyperoxia and accelerated by hypoxia, in rough proportion to the changes in arterial O₂ content (± 10%; p < 0.05). In the seven subjects in which relative measurements of retinal arterial diameters were obtained, neither hypoxia nor hyperoxia significantly altered vessel diameter. At the same time, mean retinal capillary transit velocity was independent of PO₂, suggesting that, in health, retinal capillaries may be recruited as PO₂ falls. O₂-induced changes in carotid, ophthalmic, or central retinal arterial blood flow velocities (via CDI) were not found, though a wide coefficient of variation (30% for CDI vs. 14% for SLO) may have contributed to this failure. We conclude that, under isocapnic conditions, retinal perfusion may be regulated to provide constant O₂ delivery. The lack of precision of CDI in this study leaves open the possiblity that chemical regulation of the retinal flow may parallel that of other brain regions.