A Method for Chorioretinal Oxygen Tension Measurement

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Abstract
Purpose: To report an optical imaging system that was developed to measure oxygen tension (pO2) in the chorioretinal vasculatures. The feasibility of the system for the measurement of changes in pO2 separately in the retinal and choroidal vasculatures was established in rat eyes by varying the fraction of inspired oxygen and inhibiting nitric oxide activity. Methods: Our optical section phosphorescence imaging system was modified to provide quantitative measurements of pO2 separately in the retinal and choroidal vasculatures. A narrow laser line was projected at an angle on the retina after intravenous injection of an oxygen-sensitive probe (Pd-porphyrin), and phosphorescence emission was imaged. A frequency-domain approach allowed measurements of the phosphorescence lifetime by varying the phase relationship between the modulated excitation laser light and sensitivity of the imaging camera. Chorioretinal pO2 was measured while varying the fraction of inspired oxygen and during intravenous infusion of Nω-nitro-L-arginine (Nω-NLA), a nonselective nitric oxide synthase inhibitor. Results: The systemic arterial pO2 varied according to the fraction of inspired oxygen. The pO2 in the retinal and choroidal vasculatures increased as the fraction of inspired oxygen was increased. Compared with baseline, choroidal pO2 decreased during infusion of Nω-NLA, whereas the pO2 in the retinal vasculatures remained relatively unchanged. The choroidal pO2 decreased markedly with each incremental increase in Nω-NLA infusion rate, in the range 1–6 mg/min, and there was no additional change in the choroidal pO2 at Nω-NLA infusion rates above 6 mg/min. Conclusions: An optical method combining pO2 phosphorescence imaging with chorioretinal optical sectioning was established that can potentially be applied for better understanding of retinal and choroidal oxygen dynamics in physiologic and pathologic states.