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The interpretation of estimated PWV values as representative of arterial stiffness can also be challenging because
of potential physiological, behavioral, and environmental
confounding factors [1], [4]. The most important physiological confounders are mean arterial pressure (MAP) and
heart rate, since an increase in MAP will distend the artery
and stiffen it, while an increase in heart rate can augment
PWV. It is also important to take the measurements when the
heart rhythm is stable, as arrhythmias can lead to unreliable
estimates of PWV. Behavioral confounders that should be
considered when assessing arterial stiffness include smoking, eating, and drinking alcohol within a few hours of the
measurement, while environmental confounders include
making measurements in variable room temperatures or at
a different time of the day. Finally, validation of devices that
non-invasively estimate PWV also poses a challenge because
the obtained values should be compared with invasive PWV
measurements [1].

Related Measurement Methods and Indices
Some measures, such as the Augmentation Index (AIx), which
refers to the augmentation of the early systolic peak relative to
the late systolic peak in the central aortic pressure waveform,
have been reported as surrogate measures of arterial stiffness.
However, these measures depend on other factors beyond just
the stiffness of the arteries, and so they will not be discussed
in this review (although as discussed elsewhere in this review, currently-used indices, such as PWV, are not themselves
pure measures of arterial stiffness) [1]. Furthermore, methods
for assessing local endothelial function will not be discussed.
These include the Flow-Mediated Dilation (FMD) test which
is used to assess the brachial artery in the arm and has been
found to be predictive of cardiovascular events. Rather, the
emphasis of this review will be the methods that assess arterial
health more globally by probing the stiffness of the aorta and
other large arteries.

Current Measurement Methods

Current Gold Standard: Pulse Wave Velocity
(PWV)
Pulse wave velocity is defined as the velocity of a pressure wave
that propagates between two points in the arterial system,
and it has been shown to be a predictor of cardiovascular outcomes that is independent of blood pressure measurements
[2]. PWV is measured as the distance travelled by the pressure
pulse along the arterial tree divided by the elapsed time. To determine the elapsed time, the R-wave of the ECG is often used
either as the starting point of the time interval, if the PWV is
measured from the heart to a peripheral site, or as the reference
time point when PWV is estimated using simultaneous measurements of the pressure pulse at two peripheral sites such
as at the carotid and femoral arteries [2]. PWV is usually measured over several beats and then averaged.
Higher PWV normally means higher arterial stiffness. The
general consensus is that cfPWV is accepted as the most established technique for assessment of arterial stiffness, at least in
North America and Europe [1], [6]. This measure represents
the propagation speed at which the pressure wave propagates
to the carotid artery relative to the femoral artery. As the arteries harden, PWV increases, and a cfPWV greater than 12 m/s
for middle-aged hypertensive patients is regarded as an indicator of high cardiovascular risk and damage.
The estimation of PWV can now be done non-invasively using one of several commercial devices that are either based on
tonometry or photoplethysmography (PPG). Mathematically,
the PWV is determined by the Moens-Korteweg equation:
	

PWV 

Eh
	(1)
2r 

where E is Young's elastic modulus of the arterial wall, h is the
wall thickness, r is the radius of the artery, and ρ is the density
of the blood.
Disadvantages of the assessment of the arterial stiffness
based on PWV include: the most popular/validated method
in North America/Europe (carotid femoral PWV) requires
a trained operator; PWV requires knowing travel distance
which can be difficult to estimate accurately; and PWV is really

The general architecture of systems for estimating arterial stiffness is presented in Fig. 1.
The arterial pulse waveform is influenced by both
the heart, affecting its morphology and duration, and
the vasculature, affecting
its morphology through
arterial stiffness and wave
reflection. The measurement of the arterial pulse
wave is used in a majority
of arterial stiffness estimation methods. Based on Fig.
1, Table 1 describes sensors,
models and features that
are used by each measureFig. 1. Block diagram of a general system for estimating arterial stiffness. Blocks and arrows with dashed lines exist in
ment method.
some systems; however, there are systems for estimating the arterial stiffness that rely only on one sensor.
April 2021	

IEEE Instrumentation & Measurement Magazine	55



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