Purpose
To determine if retinal capillary filling is preserved in the face of acutely elevated intraocular pressure (IOP) in anesthetized rats, despite a reduction in total retinal blood flow (RBF), using optical microangiography/optical coherence tomography (OMAG/OCT).
Methods
OMAG provided the capability of depth-resolved imaging of the retinal microvasculature down to the capillary level. Doppler OCT was applied to measure the total RBF using an enface integration approach. The microvascular pattern, capillary density, and total RBF were monitored in vivo as the IOP was increased from 10 to 100 mm Hg in 10 mm Hg intervals and returned back to 10 mm Hg.
Results
In animals with mean arterial pressure (MAP) of 102 ± 4 mm Hg (n = 10), when IOP was increased from 0 to 100 mm Hg, the capillary density remained at or above 80% of baseline for the IOP up to 60 mm Hg [or ocular perfusion pressure (OPP) at 40 mm Hg]. This was then decreased, achieving 60% of baseline at IOP 70 mm Hg and OPP of 30 mm Hg. Total RBF was unaffected by moderate increases in IOP up to 30 mm Hg, beyond which total RBF decreased linearly, reaching 50% of baseline at IOP 60 mm Hg and OPP 40 mm Hg. Both capillary density and total RBF were totally extinguished at 100 mm Hg, but fully recovered when IOP returned to baseline. By comparison, a separate group of animals with lower MAP (mean = 75 ± 6 mm Hg, n = 7) demonstrated comparable decreases in both capillary filling and total RBF at IOPs that were 20 mm Hg lower than in the initial group. Both were totally extinguished at 80 mm Hg, but fully recovered when IOP returned to baseline. Relationships of both parameters to OPP were unchanged.
Conclusion
Retinal capillary filling and total RBF responses to IOP elevation can be monitored non-invasively by OMAG/OCT and both are influenced by OPP. Retinal capillary filling was relatively preserved down to a perfusion pressure of 40 mm Hg, despite a linear reduction in total RBF.