# A Portable Device for the Measurement of Venous Pulse Wave Velocity
## 1 Introduction  
Nowadays, the estimation of volcanic status (or valo anemia) of a patient through a noninvasive method is not trivial but its development could be of great utility, given that interest in the patient's volcanic spans across several different hospital departments [1]. An innovative method for assessing the valoemic status of patients has recently been proposed: pulse wave velocity in veins, termed venous pulse wave velocity (vPWV) [2, 3]. The relation among vPWV, venous pressure, and changes in blood volume was first observed in the 1970s [4, 5], but the methodology was subsequently abandoned, possibly due to the difficulty of achieving reliable measurements. In fact, pulse wave velocity is commonly measured in the arteries and widely used as a marker of cardiovascular risk [6]. Both for arteries and veins, PWV is related with the elasticity of the vessel. This dependence is formalized by the Moens-Korteweg equation, according to which the value of PWV is proportional to the square root of the incremental elastic modulus of the vessel wall:  
\[\text{PWV}=\sqrt{\frac{E_{inc}\times h}{2r\rho}} \tag{1}\]  
where \(E_{inc}\) represents the incremental elastic modulus of the vessel wall, \(h\) is the wall thickness, \(r\) is the vessel radius, and \(\rho\) is the blood density. The incremental elastic modulusis proportional to the filling of the vessel. In case of veins, their filling should be a good proxy of the volcanic status of the subject, with veins being the capacitance vessels that are used as a blood volume reservoir [2]. PWV measurement is considerably easier in the arteries than in the veins due to higher blood pressure in the arteries and the presence of a natural cardiac pulsatility that is absent in veins. However, in recent studies, an improved methodology was devised in which a pneumatic compression applied to a limb extremity (e.g., the hand), generates the pressure pulse (i.e., the pulse wave) in the venous compartment, which centrally propagates and is proximally detected (e.g., at the basilic vein) by Doppler ultrasound. Knowing the distance between cuff and ultrasound probe, it is possible to calculate the vPWV. In addition, to limit the confounding effect of respiratory and cardiac modulation of venous blood pressure, compression is delivered synchronously with cardiac and respiratory activity. Indeed, venous flow has great intersubject variability. In some subjects, backward pulsatility related to right atrium contractions can be observed; in others flow, is almost constant. Sending an exogenous compression synchronised with cardiac and respiratory activity allows for the measurement to be applicable and repeatable over a large pool of subjects, regardless of whether they have inherent pulsatility [2; 3]. It was thus possible to measure vPWV with reasonable reliability and to confirm linear correlation between vPWV and venous pressure [2]. vPWV also exhibited good sensitivity to simulated changes in blood volume, associated with minor changes in venous pressure as observed in healthy subjects in response to passive leg raising (PLR) [3]. These results were also achieved thanks to the development of an automatic method for calculating vPWV by analysing the Doppler shift signal, as acquired by the echograph machine [2]. Thanks to this feature, the measurement can be carried out continuously and is thus adequate for long-term monitoring. Following these promising results in healthy subjects, clinical trials need to be carried out in order to assess the usefulness of vPWV as a diagnostic haemodynamic parameter. However, experimental series in hospital wards are not yet feasible due to cumbersome laboratory instrumentation and no dedicated device being available on the market. The aim of this study is to present a proof-of-concept prototype of a user-friendly, portable, and electrically isolated device for vPWV measurement.