Predicting Pulsatile Ocular Blood Volume Non-invasively

Abstract

Purpose : To develop a method of estimating pulsatile ocular blood volume (POBV) from measurements taken during an ophthalmic exam with a tonometer capable of measuring intraocular pressure (IOP) and ocular pulse amplitude (OPA).

Methods : A data set compiled from a previous accuracy study (Boehm 2008) on the PASCAL dynamic contour tonometer (DCT) provided central corneal thickness (CCT) and axial length (AL), as well as IOP and OPA using DCT and intracameral (ICM) measurements for 60 cataract patients. Briefly, once supine intracameral pressure was set to 15, 20, and 35 mmHg (randomized sequence) using a fluid-filled manometer, the system was closed by a stopcock, and IOP and OPA measurements were acquired at each manometric set-point (DCT and ICM simultaneously). The statistically significant correlation of ocular rigidity (OR) to the natural log of AL from a previously published study invasively measuring ocular rigidity through fluid injection was applied to the data set to estimate OR from AL. Friedenwald’s original pressure volume relationship was used to derive the estimated blood volume delivered to the choroid with each heartbeat (POBV) as a function of OR, IOP, and OPA, according to the equation POBV = log((OPA+IOP)/IOP)) / OR. Linear regression analyses were performed comparing OPA to OR and calculated POBV at each of the three manometric set-points. POBV was also compared to the factor OPA/IOP with all data points combined. Significant threshold was p < 0.05.

Results : OR estimated from AL showed a significant positive correlation to OPA for both DCT (p < 0.011) and ICM (p < 0.006) at all three manometric pressure set-points, with a greater slope for lower IOP. Calculated POBV also showed a significant positive correlation to OPA (p < 0.001) with greater slope at lower IOP. In the combined analysis, POBV showed a significant positive correlation to OPA/IOP (p < 0.001) in both ICM and DCT measurements with R² = 0.9685, and R² = 0.9589, respectively (see Figure).

Conclusions : POBV provides a straight-forward method to estimate pulsatile ocular blood volume noninvasively. Higher IOP with lower OPA results in the lowest values of POBV. The OPA/IOP factor may provide a useful clinical tool for evaluating changes in ocular blood flow in diseases with a vascular component, such as diabetic retinopathy and normal tension glaucoma. Future studies are warranted.

This is a 2021 ARVO Annual Meeting abstract.

 

Significant correlation of OPA/IOP to the calculated POBV for DCT measurements