Fig. 2

Schematic showing how the segment of the vein centreline used to compute the PWV is selected. (a) Shows the optic disc (OD) and selected vein. The vein centreline is shown as a dashed black line. The arrow at P indicates the direction of the measured venous pulse wave. The arrows originating from F indicate the direction of blood flow within the vein. \({L}_{\text{boundary}}\) (\({L}_{\text{b}}\)) is the length of the centreline path from the origin (point closest to the centre of the OD) to the OD boundary. The segment of the path, of length \({L}_{\text{segment}}\left({L}_{\text{s}}\right)\), used to compute the PWV is determined from (b), which shows a plot of the phase of the first harmonic of the fitted harmonic regression model and the HRWA against distance along the vessel centreline. The segment corresponds to the closed interval, of width \({L}_{\text{segment}}\), defined by the first local phase maximum and first local phase minimum from the origin (both indicated by dotted red circles). The slope of the least squares line fitted to the distance-phase points in this interval is then used to compute the PWV (see text for details).