Retinal and choroidal blood flows in hypoxic and hypercarbic newborn lambs.

We studied the effect of changes in the arterial tensions of oxygen and carbon dioxide on blood flow to the retinal (RBF) and choroidal ( ChBF ) capillary beds in 20 lambs. One to three days after placement of catheters in the left ventricle, abdominal aorta, and brachiocephalic artery, different gas mixtures were delivered to a bag enclosing the lamb's head. One group of lambs was studied at normal and low oxygen tensions while normocarbic , and another group was studied at normal and high carbon dioxide tensions while normoxic. RBF and ChBF were measured using the radioactive microsphere technique. RBF increased as PaO2 and, thus, arterial oxygen content [( O2]a) fell; in contrast, ChBF was not related to [O2]a. Oxygen delivery to the capillary bed of the retinal artery (i.e., RBF X [O2]a) was independent of arterial oxygen content because the change in [O2]a was balanced by a reciprocal change in RBF. Oxygen delivery to the choroidal capillary bed, however, rose with [O2]a because there was no reciprocal decrease in ChBF . Both RBF and ChBF increase as arterial carbon dioxide tension rose. Although an increase in arterial carbon dioxide tension produced an increase in RBF with no change in [O2]a, oxygen delivery to the retinal capillaries rose. Similarly, oxygen delivery to the capillaries of the choroid rose with carbon dioxide tension. Oxygenation of retinal tissue, which depends on oxygen diffusion from both the retinal and choroidal capillary beds, may change with variations in arterial oxygen content (increase in oxygen delivered to the choroidal capillary bed) or arterial carbon dioxide tension (increase in oxygen delivered to both the retinal and choroidal capillary beds).