P168 Feasibility Study of Local Pulse Wave Velocity Estimation in the Carotid Artery with Multi-Beam Laser Doppler Vibrometer

Background An innovative device using Laser Doppler Vibrometry (LDV) has been designed [1] to measure the transit time of the pulse wave between two locations along the course of the carotid artery (CA) from skin surface vibrations for assessment of local pulse wave velocity (PWV) [2].Aim: Tests were conducted on in-vitro models to assess the feasibility of the LDV to estimate the local PWV; preliminary in-vivo measurements were also performed. Methods Two CA geometries embedded within a soft-tissue-mimicking hydrogel were considered: i) a straight latex tube and ii) a patient-specific CA silicone-rubber model including the bifurcation. Models were pressurised in a water-filled loop and pulsatile flow was generated with a pump and/or high frequency impulses induced externally. For all measurements, two sets of six beams were used to measure surface displacement perpendicular to the external surface. PWV was calculated from the distance between selected beams and the delay between corresponding signals, using the time of the maximum of first and second derivatives of pressure (P-PWV) and displacement (LDV-PWV) as fiducial points [3]. A windowed cross correlation method [4] was also used for the in-vivo data analysis. Results PWV values for the in-vitro models are summarized in Table 1, while preliminary in-vivo LDV-PWV results are shown in Fig 1. Conclusions Good agreement between P-PWV and LDV-PWV in the tubular model was found under impulse loading, while complex waveforms measured under pulsatile flow and in-vivo conditions lead to more disparate effects when using different analysis methods. Further signal analysis is warranted. Table 1 P-PWV and LDV-PWV mean values under pulsatile flow and induced impulse conditions. Straight tube model Carotid model Pulsatile flow Induced impulse Pulsatile flow Induced impulse P-PWV (m/s) 14.1 5.7 13.6 4.1 LDV-PWV (m/s) 3.2 6.2 5.1 15.7 Fig 1 a) PWV mean values and standard deviation for a young (24 years) and old (72 years) subject, systolic foot and dicortic notch were used as reference points. Note the higher PWV in the older subject, as expected. Fig 1 b) PWV mean values and standard deviation from windowed cross correlation analysis around the dicrotic notch region.