4.2 Discrepancy Between In-Vivo Measure and Ex-Vivo Calculation of Pulse Wave Velocity in Retinal Arteries

Abstract

Background

Pulse wave velocity (PWV) in large arteries is a pressure-dependent marker of arterial stiffness. The retinal vasculature provides unique access to the microcirculation. There is inconsistency between reported values of retinal PWV (rPWV). The pressure dependency of rPWV was measured in-vivo and calculated ex-vivo using retinal artery material properties to investigate the inconsistencies.

Methods

High-speed fundus videos (125 fps) from three Sprague Dawley rats were recorded simultaneously with electrocardiogram and blood pressure. rPWV was measured using the cardiac component of retinal artery diameter waveforms at two retinal sites across a physiological range (70–130 mmHg) of mean arterial pressure (MAP). Ex-vivo tensile testing was performed on bovine retinal arteries, rat retinal arteries being too small for myography. Diameter and wall thickness of the retinal artery adjacent to the optic disc were measured using optical coherence tomography. Tensile testing was performed using a wire myograph in 9 bovine retinal artery specimens.

Results

In-vivo results showed a significant positive correlation between rPWV (4.9 ±1.8 mm/s) and MAP (R2 = 0.58, p < 0.001) as expected. Ex-vivo, calculated rPWV using material stiffness and geometry measurement ranged between 4.6 and 7.0 m/s at effective distending pressures between 70 and 100 mmHg.

Conclusions

Ex-vivo and in-vivo results differed by three orders of magnitude but should be the same. Ex-vivo results are in the same order as measured in-vivo in large arteries. In-vivo rPWV was lower than expected yet was responsive to changes in MAP. Further studies are required to uncover what rPWV is a measurement of, if not arterial stiffness.