Purpose:
In the brain, transient hypoxic insult evokes hemodynamic compensatory responses such as vasodilation. Recovery is accompanied by hyperperfusion. In the retina, these compensatory responses are not well characterized. Moreover, given the differential blood-flow (BF) regulation between the retinal and choroidal vasculature, they may respond differently to hypoxia. We employed, herein, laser speckle imaging (LSI) to investigate BF changes in the retina associated with transient and mild "hypoxia" at high temporal and spatial resolution.
Methods:
Three male SD rats were anesthetized with 1% isoflurane, paralyzed and mechanically ventilated. Mild retinal "hypoxia" was induced by transiently stopping the ventilator for 60s. LSI was acquired at 25Hz at 4µm using near infrared light (785nm). BF %-change maps and time courses were calculated [Cheng & Duong, Optics Let 2007]. BF changes were analyzed for different vessel types. Importantly, albino rats allowed analysis of a few large choroidal vessels in regions without large retinal vessels.
Results:
Figure 1 shows the response time courses from different retinal regions and the BF index maps before, during and after retinal "hypoxia." Within 10s of stopping the ventilator, BF increased 28±8% above baseline in the arteries, 25±3% in the capillaries, 22±3% in the veins, 9% in the optic nerve head, but no change in the choroid, suggesting differential responses to transient "hypoxia" between the retinal and choroidal vasculature. As "hypoxia" progressed, BF in all vessels decreased below baseline. Resuming ventilation markedly increased BF 58±18%, 43±15%, 49±8% and 28% above baseline in arteries, capillaries, veins and optic nerve head, respectively, with similar trends as described.
Conclusions:
We demonstrate that LSI can detect robust BF compensatory responses in the rat retina following transient and mild "hypoxia" at high spatial and temporal resolution. This approach offers a means to image BF changes and to investigate neurovascular couplings in the retina. Future studies will combine LSI with intrinsic optical imaging to visualize oximetry and blood volume changes, and apply this integrated approach to study focal retinal ischemia.
Keywords: hypoxia • retina • blood supply