Calculation of vessel pulse wave velocities in retinal vein segments within the optic disc centre.

The carotid-femoral pulse wave velocity (PWV) method is used clinically to determine degrees of stiffness and other indices of disease. It is believed PWV measurement in retinal vessels may allow early detection of diseases. In this paper we present a new non-invasive method for estimating PWVs in retinal vein segments close to the optic disc centre, based on the measurement of blood column pulsation in retinal veins (reflective of vessel wall pulsation), using modified photoplethysmography (PPG). An optic disc (OD) PPG video is acquired spanning three cardiac cycles for a fixed ophthalmodynamometric force. The green colour channel frames are extracted, cropped and aligned. A harmonic regression model is fitted to each pixel intensity time series along the vein centreline from the centre to the periphery of the OD. The phase of the first harmonic is plotted against centreline distance. A least squares line is fitted between the first local maximum phase and first local minimum phase and its slope used to compute PWV. Five left eye inferior hemi-retinal veins from five healthy subjects were analysed. Velocities were calculated for several induced intraocular pressures ranging from a mean baseline of 14 mmHg (SD 5) to 56 mmHg in steps of approximately 5 mmHg. The median PWV over all pressure steps and subjects was 20.77 mm/s (IQR 29.27). The experimental results show that pulse wave propagation direction was opposite to flow in this initial venous segment.