Control of retinal blood flow levels by selected combinations of cervical arterial ligations in rat

The accumulated oxygen deficit as an indicator of the ischemic retinal insult

We here attempt to improve quantification of the ischemic retinal insult, that is, what is imposed on the retinal tissue by ischemia, especially in experimental models of ischemia. The ischemic retinal insult initiates the ischemic retinal injury (or outcome). Accordingly, it is reasonable to assume that the better the quantification of the insult, the better the correlation with, and thereby estimation of, the injury. The insult seldom has been quantified in terms of the relevant physiological factors, especially in connection with the rate of oxygen delivery (DO2). We here propose the accumulated oxygen deficit (AO2D) as an indicator of the ischemic retinal insult. We hypothesized that AO2D is correlated with the rate of oxygen metabolism measured 1 h after reperfusion following an episode of ischemia (MO2_1_Hr). Previously, we showed that MO2_1_Hr is related to the electroretinogram amplitude and the retinal thickness when they are measured seven days after reperfusion. We studied 27 rats, as well as 26 rats from our published data on retinal ischemia in which we had measurements of DO2 and duration of ischemia (T) of various levels and durations. We also measured DO2 in 29 rats treated with sham surgery. Ischemia was induced by either ipsilateral or bilateral common carotid artery occlusion or by ophthalmic artery occlusion, which gave a wide range of DO2. DO2 and MO2_1_Hr were evaluated based on three types of images: 1) red-free images to measure vessel diameters, 2) fluorescence images to estimate blood velocities by the displacement of intravascular fluorescent microspheres over time, and 3) phosphorescence images to quantify vascular oxygen tension from the phosphorescence lifetime of an intravascular oxygen sensitive phosphor. Loss of oxygen delivery (DO2L) was calculated as the difference between DO2 under normal/sham condition and DO2 during ischemia. AO2D, a volume of oxygen, was calculated as the product DO2L and T. Including all data, the linear relationship between AO2D and MO2_1_Hr was significant (R2 = 0.261, P = 0.0003). Limiting data to that in which T or DO2L was maximal also yielded significant relationships, and revealed that DO2L at a long duration of ischemia contributed disproportionately more than T to MO2_1_Hr. We discuss the potential of AO2D for quantifying the ischemic retinal insult, predicting the ischemic retinal injury and evaluating the likelihood of infarction.

Retinal Oxygen Delivery and Metabolism Response to Hyperoxia During Bilateral Common Carotid Artery Occlusion in Rats

Purpose

The purpose of the current study was to test the hypothesis that responses of total retinal blood flow (TRBF), inner retinal oxygen delivery (DO₂), metabolism (MO₂), and extraction fraction (OEF) to hyperoxia are higher after minutes of bilateral common carotid artery occlusion (BCCAO) as compared to days of BCCAO.

Methods

Twenty-eight rats were subjected to BCCAO for 30 minutes (n = 12), 1 day (n = 8), or 3 days (n = 8). Eight of the 12 rats were also evaluated at baseline, prior to BCCAO. During room air breathing (RA) and 100% O₂ inspiration (hyperoxia), blood flow and phosphorescence lifetime imaging were performed to measure TRBF and vascular O₂ contents, respectively. DO₂, MO₂, and OEF were calculated from these measurements.

Results

After 30 minutes or 3 days of BCCAO, TRBF did not differ between RA and hyperoxia conditions (P ≥ 0.14) but decreased under hyperoxia after 1 day (P = 0.01). Compared to RA, DO₂ and MO₂ were increased under hyperoxia after 30 minutes of BCCAO (P ≤ 0.02). Additionally, MO₂ was decreased under hyperoxia after 1 day of BCCAO (P = 0.04). OEF was decreased under hyperoxia compared to RA (P < 0.001). Under hyperoxia, TRBF and DO₂ were reduced after all BCCAO durations compared to baseline (P ≤ 0.04), whereas MO₂ did not differ from baseline after 30 minutes of BCCAO (P = 1.00).

Conclusions

The findings indicate that hyperoxia introduced minutes after ischemia can reduce DO₂ impairments and potentially return MO₂ to approximately normal values. This information contributes to the knowledge of the effect of supplemental oxygen intervention on TRBF, DO₂, MO₂, and OEF outcomes after variable durations of ischemia.