Instrumentation & Measurement Magazine 24-2 - 56

Table 1 - Comparison of methods for estimation of arterial stiffness
Measurement
technique

Sensor

Additional sensors

Feature

Physical model

PWV

Pressure sensor
(tonometry,
oscillometry) or
ultrasound

Sensor of same type or
electrocardiogram
(ECG) electrodes, or
phonocardiogram
microphone

Time difference between
characteristic points of
the pulses

Moens-Korteweg
equation

CAVI

Oscillometric cuff

Oscillometric cuff and
phonocardiogram
microphone

PWV,
Systolic and diastolic
blood pressure

Bramwell-Hill equation
and stiffness index
Beta

Beta

Pressure sensor
(tonometry)

Carotid ultrasound
imaging

Systolic and diastolic
pressure,
Diameter of the artery
and change in the
diameter during the
cardiac cycle

Experimentally obtained
relationship between
the pressure and
diameter of the arteries
[5]

a surrogate measure of arterial stiffness and is not identical to
arterial stiffness itself.

Other Indices of Arterial Stiffness
As discussed above, one disadvantage of PWV as a measure
of arterial stiffness is that it depends on the mean arterial pressure level (MAP), as it varies naturally over different time
scales (hourly, daily etc.), with higher MAP leading to an increase in stiffness due to distension and subsequently to a
corresponding increase in PWV. As a result, there has been an
effort to develop other measures of arterial stiffness that are
based on physical models and experimental observations and
that are independent of blood pressure. However, it should be
noted here that even though PWV depends on MAP as a measured value, it has been shown to be independent of blood
pressure as a predictor of cardiovascular events [1], [2].
One measure that aims to be independent of blood pressure is the so-called arterial stiffness index Beta [5], [7], which
derives from the approximately exponential relationship between the pressure and arterial diameter over the physiological
pressure range. This index has been found to predict hardening
of the arteries that supply the heart [8] and is defined as:
	

D
 P
Beta    DBP  ln  SBP

D
Δ

  PDBP


   	(2)


where DDBP is the diameter of an artery at diastole, ΔD is the
change in diameter during the cardiac cycle, PSBP is the pressure at systole, and PDBP is the pressure at diastole.
As formulated above, the stiffness index Beta is difficult to
determine because it is dependent on accurate measurements
of arterial diameter. Consequently, a new index which can be
measured automatically, the Cardio-Ankle Vascular Index
(CAVI), was derived [9], [10]. This index is defined as:
	

56	

a  haPWV 2  2
CAVI 


PSBP  PDBP

P
ln  SBP

 PDBP


  b	(3)


where haPWV is the heart-ankle PWV, a and b are scaling
factors used for matching CAVI to the heart-femoral PWV.
CAVI can be measured using devices fabricated by Fukuda
Denshi Co., Ltd. (Tokyo, Japan) which owns the patent for
this method. The measurement of PWV is automatic and is
performed using a brachial cuff, an ankle cuff, and a phonocardiogram microphone. This approach has been used primarily
in Japan, and the measurement of brachial-ankle PWV in general has been shown to be useful for predicting cardiovascular
outcomes in Asian populations, but fuller validation in other
populations is needed [1].
Furthermore, it should be noted that recent work has suggested that the traditional formulations of the arterial stiffness
index Beta and CAVI remain, in fact, somewhat dependent on
blood pressure, and corrections were proposed that can readily make them blood pressure independent [11].

New Sensors, Techniques, and Models
Related to Arterial Stiffness
Sensors
This section will only discuss wearable technologies that have
potential to be used in future devices for estimating arterial
stiffness. The advancements in the technologies are achieved
through developments in sensor technology, miniaturization
of sensors, integrating multiple sensors on a single chip as well
as integrating the electronics for amplification and filtering on
the chip. Micro-electro-mechanical systems (MEMS) that combine small mechanical and electronic components on a silicon
chip are one of key enablers for these advancements. We analyzed articles published during and after 2018.
Ultrasound devices measure aortic PWV based on the Doppler effect. Ultrasound wall tracking can be used to acquire the
arterial pulse. There is new research on miniature ultrasound
devices that can achieve the goal of continuous monitoring of
the arterial pulse from the different sites of the body including

IEEE Instrumentation & Measurement Magazine	

April 2021



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