Alterations in Retinal Oxygen Delivery, Metabolism, and Extraction Fraction During Bilateral Common Carotid Artery Occlusion in Rats

PURPOSE: The purpose of the current study was to investigate alterations in retinal oxygen delivery, metabolism, and extraction fraction and elucidate their relationships in an experimental model of retinal ischemia. METHODS: We subjected 14 rats to permanent bilateral common carotid artery occlusion using clamp or suture ligation, or they underwent sham procedure. Within 30 minutes of the procedure, phosphorescence lifetime imaging was performed to measure retinal vascular oxygen tension and derive arterial and venous oxygen contents, and arteriovenous oxygen content difference. Fluorescent microsphere and red-free retinal imaging were performed to measure total retinal blood flow. Retinal oxygen delivery rate (DO(2)), oxygen metabolism rate (MO(2)), and oxygen extraction fraction (OEF) were calculated. RESULTS: DO(2) and MO(2) were lower in ligation and clamp groups compared to the sham group, and also lower in the ligation group compared to the clamp group (P ≤ 0.05). OEF was higher in the ligation group compared to clamp and sham groups (P ≤ 0.03). The relationships of MO(2) and OEF with DO(2) were mathematically modeled by exponential functions. With moderate DO(2) reductions, OEF increased while MO(2) minimally decreased. Under severe DO(2) reductions, OEF reached a maximum value and subsequently MO(2) decreased with DO(2). CONCLUSIONS: The findings improve knowledge of mechanisms that can maintain MO(2) and may clarify the pathophysiology of retinal ischemic injury.