Pulse wave velocity

# Overview
Pulse wave velocity (PWV) is a measure of arterial distensibility. It is defined as the velocity of the pressure wave produced by ventricular ejection that travels along an arterial segment. It is calculated from the interval between two pressure peaks located at two different positions in the arterial tree. Because of the principle that pressure waves travel faster in less distensible arteries, PWV measurement has been considered the best proxy for the assessment of arterial stiffness.

# Pulse Wave Velocity

## Physical Principle
- The pulse wave velocity as a measure stems from the biomechanics model that correlates the speed of propagation of a pulse to the elastic modulus of the arterial wall i.e. how elastic it is. In 1877, Moens & Korteweg were the first to describe an equation derived from Newton's second law of motion that estimates the pulse wave velocity and reads: PWV=(Einch/2R)
Where  Einc is the elastic modulus of the artery, h is the thickness of the arterial wall, R is the arterial radius, and  is the blood density. This equation assumes that no change in artery volume occurs and that the vessel wall elasticity is isotropic.
- Where  Einc is the elastic modulus of the artery, h is the thickness of the arterial wall, R is the arterial radius, and  is the blood density. This equation assumes that no change in artery volume occurs and that the vessel wall elasticity is isotropic.
- Many equations have since been proposed to calculate the pulse wave velocity; however, further investigation into the architecture and biology of the arterial wall showed that arteries are in fact not isotropic, that the elastic modulus for any artery varies nonlinearly with pressure, and with the frequency of an applied stress, and that pressure waves are reflected within arteries limiting their clinical application.

## Measurement Techniques
- In simpler terms, pulse wave velocity is the distance traveled by the pressure pulse per a given time period, warranting techniques that can accurately assess the trajectory traveled by a pressure pulse and the time difference delay of a pressure wave as it passes two checkpoints. Although a PWV measurement can be obtained in any artery, aortic PWV (aPWV) is considered the gold-standard in the clinical assessment of arterial stiffness  as it has the most researched outcome data and the most documented pathophysiological significance.
- Furthermore, the measurement of aPWV by means of assessing carotid-to-femoral pressure pulses was shown to be the best indicator of arterial stiffness among measurements at other sites (carotid-radial or femoral-posterior tibial).
- Different techniques have been studied and clinically applied including the Complior System, the SphygmoCor System, the Arteriograph System, the Vicorder System , the automated Doppler ultrasound recording , and the velocity-encoded MRI  among others.
- Most studies involving aPWV to date have involved two of those techniques which include probes placed on the carotid and femoral arteries;  Complior and Sphygmocor . The Complior System uses dedicated mechanotransducers that detect pressure signals for 2 arterial segments simultaneously and calculates the PWV from a 15s recording of pulse waves . The SphygmoCor System on the other hand uses a single high-fidelity applanation tonometer to obtain the carotid and femoral pulses, which are recorded sequentially rather than simultaneously. The transit time is then acquired from the delay between the R wave of the ECG and the recorded pulses.
- Using doppler ultrasonography, transit time may also be detected between recorded pressure pulses from the root of the left subclavian artery and abdominal aorta bifurcation . Velocity-encoded MRI detects changes in the aortic cross sectional area and in the maximal blood flow velocity during systole which are used to estimate aortic pulse pressure and to derive PWV.

## Value
- Pulse wave velocity is measured in m/s.
- The value is influenced by age, with higher velocities seen in older subjects even when accounting for risk factors for arterial stiffness (Collaboration, 2010).
- In 2007, The Task Force for the Management of Arterial Hypertension of the European Society of Hypertension (ESH) and of the European Society of Cardiology (ESC) included PWV as a factor influencing prognosis in hypertension with a cut-off value of  more than 12m/s indicative of subclinical organ damage .

# Clinical Significance
- Arterial stiffness is becoming all the more recognized as an important representation of cardiovascular disease. For that, PWV has become one of the techniques used to determine the presence of subclinical cardiovascular disease in patients with pre-existing risk factors.
- Existing studies have demonstrated that large artery stiffness, measured via carotid-femoral PWV, is an independent predictor of the risk of incidence of cardiovascular events in clinical and community-based cohorts . PWV has been shown to predict all-cause mortality in end stage renal disease , hypertensive , and diabetic patients .
- It has also been investigated as an indicator of cardiovascular morbidity and mortality in the general population .
- In the 2007 ESH Guidelines for the Management of Arterial Hypertension, PWV was added to the recommended investigational tests for hypertensive patients. PWV is not yet as widely recommended as other screening tests mainly due to its low availability .