We presented a rat model that provides graded, temporally controllable acute, retinal ischemic insults. This could allow correlation of these two key prognostic factors, namely grade and duration, with the resulting ischemic retinal injury. While the many models of retinal ischemia
1–11 have various advantages and have permitted discovery of many features of retinal ischemic injury, only elevated IOP can produce graded ischemic insults. He et al.
13,14 produced multiple grades of retinal ischemia by progressively increasing IOP in rats. They calculated OEF (called OER) from ocular laser Doppler flow and preretinal PO
2 measurements taken between major retinal vessels. However, their F, PO
2, and OEF measurements included contributions from the choroidal circulation, whereas the values in our study were derived only from the retinal circulation. Furthermore, they showed that a substantial component of the injurious effects of elevated IOP was not related to ischemia. In contrast, our method does not add injurious effects to those of ischemia, manipulate or distort the eye, or cause corneal clouding, which precludes clear imaging of the ocular posterior segment and measurement of O
2-related factors.
Our model permitted preliminary noninvasive measurements of F, DO
2, OEF, and MO
2 during graded ischemia. To our knowledge, there have been no previous reports in which a comparable combination of these inner retinal O
2-related factors were measured in experimental graded ischemia. Of note, we showed for the first time to our knowledge in retinal ischemia that OEF reaches 1 (indicating that all vascular oxygen is being extracted), even while there still is substantial F. The relationship of F to the percent CA occlusion appeared to be nonlinear in which changes in F occurred mainly with compressions greater than 60%. This was the result of retinal vascular compensatory vasodilation, which eventually became maximized, and, probably, a nonlinear relationship between CA compression and the resulting cross-sectional area of the vessel. We interpreted the findings of DO
2, OEF, and MO
2 in the following manner. At mildly reduced levels of F and DO
2, MO
2 is maintained by extracting a higher percentage of the O
2 in the blood, that is, by increasing OEF. With further reductions in the F and DO
2 levels, OEF eventually reaches its maximum value of 1 when all O
2 in the blood is extracted. At more severely reduced levels of F and DO
2, MO
2 decreases proportionately and, when maintained for long enough durations, energy failure and the complex metabolic sequences of ischemic retinal injury must supervene, which terminate in cell death and loss of visual function.
26
At 100% occlusion of the CA there may have been a small amount of F, at least in some rats. This is consistent with findings of other studies with long-term total bilateral CA occlusion that showed various electroretinographic, biochemical, and histologic changes.
27–32 In fact, with total CA occlusion, the circle of Willis may provide some blood flow to the ophthalmic artery either directly into the orbit
33 or by way of retrograde flow through the internal carotid artery and then antegrade flow into the pterygopalatine artery (this artery is the normal blood supply to the eye in rats). Nonetheless, in the present short-term study, MO
2 was essentially reduced to zero with maximal compression of one CA and ligature of the other CA.
Our study had limitations. First, this method reduced flow in the retinal and choroidal circulations, which differs from clinical retinal vascular occlusions, which do not involve the choroid. Thus, findings here of DO2, OEF, and MO2, with F may differ from those that occur during retinal vascular occlusions only. However, it does mimic carotid occlusive disease and ophthalmic artery occlusion. Second, we made measurements during progressive grades of occlusion. Accordingly, some effects of the previous levels of occlusion probably were continuing at each grade of compression, as depicted in the presented data variability. However, we were particularly interested in the general pattern of how DO2, OEF, and MO2 related to F over a wide range. Future studies designed to collect data in groups of animals at each F reduction level will introduce less variability and better delineation of the relationships. Third, the CA blood flow was not measured. Thus, there may have been some variability among animals in the actual amount of compression. This also would contribute to measurement variability. Fourth, we did not measure the arterial pH, which can be reduced in ischemia. Lowering the pH shifts the oxygen hemoglobin dissociation curve to the right and causes SO2 to become lower for a given PO2 value. While our SO2 and O2 content values may be somewhat high, it is unlikely that this would have a major impact on the overall relationships among the O2 biomarkers. Fifth, the sample size was small. Nonetheless, it was adequate to demonstrate the feasibility of generating the model and obtaining data.
In conclusion, we described a new model for acute retinal ischemia in rats that can be delivered with controlled grade and duration without otherwise injuring the eye. We also showed that the methods we described previously to measure O2-related factors can be applied, and we presented preliminary combined measurements in this model of retinal ischemia.