Venous Pulse Wave Velocity evaluation using innovative tonometers

About 10% of patients entering the Emergency Department (ED) are subjected to an evaluation of their hydration conditions (volemic status) with the aim of evaluating and eventually improving their vital organ perfusion. For this purpose, different non-invasive techniques, like the measurement of inferior vena cava (IVC) pulsatility, are commonly adopted to understand the volemic status of patients. Unfortunately, these techniques proved to have some limitations, for example varying respiratory pattern in spontaneous breathing can affect IVC ultrasound reliability. Even though new methodologies to evaluate the vital organ perfusion have been developed, some studies about Venous Pulse Wave Velocity (vPWV) are still performed. vPWV, in fact, can be investigated as a new parameter to understand the patient’s conditions and the management of fluid therapies. The first research, dated back to 1967, have not been followed up, probably due to the ineffective adopted methodologies. vPWV has been investigated less than PWV of arteries for two different reasons: the lack of natural pulsatility in the veins and the strong dependence on the respiratory system. Recently, two researches have focused on vPWV overcoming the limits of the venous system; they generated an artificial compression from an extremity of upper or lower limbs in order to propagate the wave, synchronizing the impulse with both cardiac activity and the respiratory system. The evaluation of vPWV can be crucial: first, because veins are more compliant vessels than arteries, second they are a perfect fluid reservoir for our body. For these reasons, depending on the quantity of fluid in the venous district, venous vessels change their contraction and consequently blood’s velocity. The aim of this work is the measurement and evaluation of vPWV replacing the previous device with a system based on tonometry (ATHOS designed by Politecnico di Torino for the detection of arterial velocity): the pulse wave has been generated artificially on a limb extremity by a pneumatic system and it has been detected with the tonometers. After some preliminary tests aimed to understand the best positioning of tonometers in order to detect the signal of interest, vPWV was detected placing two sensors on the left arm, where the signal-noise ratio is better detected, using two bands. An experimental protocol was created for healthy subjects and then, an algorithm has been implemented in MATLAB to measure the latency between the two waves. Finally, a statistical analysis was made comparing the results given by tonometers and the results of Echo-Doppler (obtaining an intraclass correlation coefficients (ICC) >0,8 and an absolute mean error of 0,8 m/s).