Evaluation of the Effect of Elevated IOP on Retinal Capillary Bed and Total Retinal Blood Flow in Rats Using Optical Microangiography

Abstract

 

Purpose

 

To perform a comprehensive evaluation of the effect of acutely elevated intraocular pressure (IOP) on the retinal capillary bed and total retinal blood flow (RBF) in anesthetized rats using Optical microangiography (OMAG).

 

Methods

 

Seven adult Brown Norway rats were anesthetized with inhalational isoflurane (2.0%) and the systemic blood pressure (BP) was monitored by cannulation of the tail artery. IOP was elevated by cannulation of the anterior chamber using a 31 gauge needle connected to a reservoir filled with balanced salt solution. The height of the reservoir was used to elevate IOP, in 10 mm Hg increments, between 10 mmHg and 100 mmHg, and then returned to 10 mmHg. At each IOP level, one 3D data volume was captured using ultrahigh sensitive OMAG to provide retinal microangiograms with resolution to the capillary level. The capillary microvasculature within the NFL/GCL, IPL and OPL, respectively, were segmented and the vessel density was analyzed at each level. Another 3D scan was performed with dense A-scan to obtain a 3D axial velocity around the central retinal artery, which was then integrated to calculate total RBF.

 

Results

 

The BP of each rat was stable during each measurement session and the average BP for all animals was 76.0±6.4 mmHg. UHS-OMAG provided microangiogram maps within the different retinal layers (NFL/GCL, Fig.1 A and OPL, Fig.1 B). From the microangiograms and the capillary density change curve (Fig.1 C), the capillary bed filling was not affected up to an IOP of 40 mmHg, while an abrupt decrease was noted by 60 mmHg. At 70 mmHg, capillary flow is negligible. Total RBF decreased linearly as the IOP increase (Fig.1 D). Both capillary density and total RBF were completely restored to baseline following pressure normalization.

 

Conclusions

 

OMAG allow us to monitor the effect of elevated IOP on both retinal capillary beds and total RBF. Our results indicate that these microvascular beds respond differently to lower levels of IOP elevation, which may be a manifestation of vascular autoregulation.

 

 

Fig. 1 Microangiograms show response of the microvasculature within the NFL/GCL (A) and OPL (B) to acutely elevated IOP. (C) Relative OPL Capillary density (Mean ± SD) demonstrates a significant reduction once IOP exceeds 50 mmHg. (D) Total RBF decreases linearly with increasing IOP. Both measures return to baseline when IOP returns 10 mmHg.

 

Keywords: 551 imaging/image analysis: non-clinical • 568 intraocular pressure • 688 retina