A Method for Chorioretinal Oxygen Tension Measurement

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 N^ω-nitro-L-arginine (N^ω-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 N^ω-NLA, whereas the pO₂ in the retinal vasculatures remained relatively unchanged. The choroidal pO₂ decreased markedly with each incremental increase in N^ω-NLA infusion rate, in the range 1–6 mg/min, and there was no additional change in the choroidal pO₂ at N^ω-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.

KEYWORDS:

• Choroid
• imaging
• oxygen
• retina