Acute effects of oxygen and carbon dioxide on retinal vascular network geometry in hypertensive and normotensive subjects

The optimal design of vascular networks maximizes circulatory efficiency wh ile minimizing power costs. We investigated the effects of acute changes in vascular tone on retinal arteriolar network geometry. Six hypertensive and six normotensive subjects each breathed air, 5% CO2 (with 12% O-2) and 100 % O-2 for 5 min periods in random order. Retinal photographs were taken at the end of each test period. Bifurcation angles and arteriolar diameters we re measured using operator-directed image analysis, and junction exponents were calculated. Arteriolar diameters narrowed on breathing O-2. The magnit ude of this change was significantly greater in normotensive than in hypert ensive subjects. Angles narrowed in normotensive subjects, but not signific antly in hypertensive subjects. Arteriolar diameters increased significantl y on breathing CO2 in normotensive but not in hypertensive subjects, but th ere were no changes in angles. Despite changes in diameter, junction expone nts did not change under any conditions. Vascular reactivity in the retinal arteriolar bed appears to be diminished in hypertensive subjects. The fail ure of junction exponents to change, despite alterations in diameter, sugge sts that arteriolar diameters at retinal bifurcations adhere to optimality principles when exposed to acute vasoactive stress. As vasoconstriction is associated with the narrowing of bifurcation angles, previous observations showing narrowed angles in hypertensive subjects could be explained by incr eased tone in the retinal arteriolar bed.