The influence of intraocular pressure and systemic oxygen tension on the intravascular pO2 of the pig retina as measured with phosphorescence imaging.
The intravascular oxygen tension (pO2) of the pig retina was determined by measuring the phosphorescence lifetime of an intravenously injected dye. Pseudocolor images of the intravascular retinal pO2 were obtained. The method is noninvasive except for the application of the dye. The measurement system was adapted to a fundus camera. The systemic arterial oxygen pressure (PaO2) as well as the intraocular pressure (IOP) were altered. In the measurements the pO2 of the choriocapillaris was close to the systemic PaO2. Under normoxia, the retinal veins showed a lower pO2 of about 40 mm Hg. We found evidence of an autoregulation of the retinochoroidal pO2. The retina and the optic nerve head showed an autoregulation of the intravascular pO2 at low IOP, but were more sensitive to a moderate elevation of the IOP of about 40 mm Hg, as compared to the choroidal pO2. The pO2 of the choriocapillaris seemed to change little until the considerably high IOP value of about 50 mm Hg was attained. This behavior is due to either the high perfusion reserve capacity of the choroid or to autoregulation. Although our experiments refer to the perfusion of the pig eye, the results provide indirect evidence that even during a glaucoma attack the human choroid might be able to maintain a reasonable oxygen supply to the retina, whereas the intravascular pO2 of the retinal vessels and of the optic nerve head decrease strongly. The adaptation to a fundus camera facilitates a future clinical application if a nontoxic dye can be developed.