Non-Invasive Blood Pressure Monitoring with Positionable

Three-chamber Pneumatic Sensor

V. E. Antsiperov

1,2 a

, G. K. Mansurov

, M. V. Danilychev

and D. V.

Churikov

1,2,3

Kotelnikov Institute of Radio Engineering and Electronics of RAS, Moscow, Russia

Moscow Institute of Physics and Technology, Moscow, Russia

Scientific and Technological Centre of Unique Instrumentation of the RAS, Moscow, Russia

Keywords: Blood Pressure Non-Invasive Measurement, Continues Pulse-Waveform Monitoring, Local Pressure

Compensation Principle, Pneumatic Pressure Sensor, Sensor Positioning Problem, Multichannel

Measurements and Control.

Abstract: The main goal of the paper is to present a new type of a sensor for non-invasive continuous blood pressure

measurements. The principle of such a sensor operation, based on local pressure compensation, is in the centre

of discussion. The sensor presented has very small sensing pads (1 mm

or less) which permits accurate sensor

positioning directly on the elastic surfaces such as the human skin. Thanks to that it is possible to provide a

high quality of the blood pressure measurement, when keeping the continuity of the measurement parameters

and minimizing the level of disturbances. For this reason, the paper focuses on a detailed discussion of the

positioning problem and the results of developing approaches to its solving. As a promising method, a

positioning based on the pulse wave controlling by a three-chamber pneumatic sensor is proposed.

1 INTRODUCTION

The invasive method for measuring arterial blood

pressure (ABP) is direct and the most accurate.

However, due to strict professional personnel

requirements, it is used only in acute cases in a

hospital, under the supervision of a qualified and

certified medical personnel. This method does not

allow to continuously monitor the patient's every day

activities, to track objective hemodynamics and the

state of the cardiovascular system outside the

hospital.

Most of modern methods for non-invasive

measurement of ABP are based on manipulation of

counter-pressure in the cuff or the applicator,

compressing the artery (usually together with the

limb). The goal of these manipulations is to neutralize

the possible additional pressure caused primarily by

strained elastic artery walls (Settels, 2014).

Oscillometric method and Korotkoff method are

quantitatively dominant in ABP measurement, but

they determine only two pressure levels - systolic and

diastolic, along with pulse frequency, just enough for

everyday use. These methods, however, do not

https://orcid.org/0000-0002-6770-1317

present the shape of ABP pulse wave beat-to-beat.

Moreover, continuous monitoring of ABP is

impossible when the methods inherently intermittent

are used. In further development of Marey’s

proceedings, dated 1860-80, new methods of arterial

tonometry were proposed in 1960-70-s. For example,

when monitoring BP using the Peňáz method, for

these purposes, the principle of volume compensation

is used, exploiting the idea of dynamic “unloading of

vessel walls” (Peňáz, 1973). Pressman and Newgard

proposed the idea of local compensation of an artery

wall by a flat applicator with a plunger (rider) located

in it (Pressman and Newgard, 1963). The plunger was

provided by a tracking drive for mechanical push on

the artery wall (through the skin) in counterphase

with pressure in it. It was assumed that the force

required to keep the plunger stationary (according to

the displacement sensor) is proportional to the

pressure in the arteries. Looking ahead, we note that

the distribution of pressure on the applicator pad in

the projection of the artery is very uneven, so at best

this method could measure the change in average

pressure at the plunger pad. High demands on the

accuracy of manufacturing miniature mechanical

462

Antsiperov, V., Mansurov, G., Danilychev, M. and Churikov, D.

Non-Invasive Blood Pressure Monitoring with Positionable Three-chamber Pneumatic Sensor.

DOI: 10.5220/0007574904620465

In Proceedings of the 12th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2019), pages 462-465

ISBN: 978-989-758-353-7

assemblies and the presence of friction and backlash

in moving parts are the additional disadvantages of

this method.

The disadvantage of Peňáz method is that in order

to avoid stagnation of blood in the limb, it is

necessary to periodically relax the cuff, which breaks

the continuous regime of ABP monitoring. To

overcome this problem some new approach to a

continuous, non-invasive and indirect measurement

of ABP based on changes in pulse wave velocity

(PWV) was proposed in (Gesche et al., 2012). PWV

is the speed of the pressure pulse propagating along

the arterial wall and it can easily be calculated from

pulse transit time (PTT). The implementation of this

approach, based on MEMS pressure sensors was

reported in (Kaisti et al., 2017).

To avoid disturbances related to the inflation of

the cuff and the alterations of the ABP caused by such

disturbances, we also developed a new approach to

continuous, non-invasive blood pressure

measurement. But in contrast to PWV our method is

based on the principle of local pressure compensation

(Figure 1).

Figure 1: The measurement principle of blood pressure due

to local pressure compensation in the artery Part by pressure

in the measuring element Psen of the pneumatic sensor (A)

and the appearance of the element applied to the patient's

wrist (B).

Practical implementation of this method became

possible by the previously developed unique technic

of pressure compensating measurements on a very

small working area (1 mm2 or less) (Antsiperov et al.,

2016).

2 THE PRINCIPLE OF LOCAL

PRESSURE COMPENSATION

When analyzing well–known methods of non–

invasive continuous measurement of blood pressure,

we can conclude that the best results of monitoring

non–stationary dynamics of ABP are achieved by the

so–called compensation methods or methods similar

to them.

The compensation methods are used for

measurement of various physical quantities and are

based on the compensation of an unknown measured

value by controlled counter value and nullification of

their difference. The simplest example of the

compensation method is the use of balance scales on

which unknown mass is measured using a set of

counter -weights. The predetermined position of the

balance beam or the associated arrow serves as a null

indicator of the balance scales.

The compensation methods as methods of high

precision are used for electrical measurements, in

embodiments having bridge and half-bridge circuits.

Note that the compensation methods are usually used

to measure the static variables, for example the

constant unknown resistance in bridge circuits.

We consider the compensation method as the

fundamental basis to measure the varying blood

pressure. The application of this method for

measuring the non–static dynamic quantity has

become possible, for two reasons.

Firstly, the fact that blood pressure changes are

not so fast, its rhythm is of the order of one beat per

second, and its spectrum fits into the range of a few

tens of Hz. Secondly, there are relatively cheap, high

performance microcontrollers for which such a

change in pressure is quasi-static.The idea of local

compensation principle for measuring pressure in

inaccessible volumes of gas or fluid is as follows: if

the external force fails to make the shape of the

surface bounding the volume of the elastic membrane

locally flat, then the external pressure due to the lack

of longitudinal elastic stresses in the shell will be

equal to the internal. This principle is realized in the

method of applanation tonometry to measure

intraocular pressure (Goldmann and Schmidt, 1975).

For non-invasive pressure measurement in the

inaccessible volume of the artery this principle is

illustrated in Figure 1. Namely, if at some initial

moment the pressure in the measuring chamber of the

sensor element P

sen

is less than the pressure in the

artery P

art

then the tissue and the skin directly above

the artery snug against the air channel of the sensor

and lock it. Once P

sen

reaches P

art

, the channel outlet

is opened, and the excess air goes under the flat

surface of the measuring element being pressed

against the skin. If the air flow into the chamber is

chosen correctly (choice of pressure P

res

in the

receiver and the throttle position of the screw), the

laminar flow of air from the chamber will keep the

skin surface in a flat, minimally open state,

Non-Invasive Blood Pressure Monitoring with Positionable Three-chamber Pneumatic Sensor

463

automatically maintaining balance P

sen

≈ P

art

(even

with variable blood pressure).

In other words, we developed the pneumatic

pressure sensor with local compensation principle,

based on the idea of an automatic pressure relief valve

in the working chamber with a constant flow of air

from the outside (from the receiver).

3 POSITIONING PROBLEM

It turned out, however, that the advantages of locally-

compensatory blood pressure measurement are not

obtained free of charge, and they have to be paid for

by the problems arising here with the positioning of

the measuring sensor. Since the contact pad (outlet

channel hole) of the measuring element has

substantially smaller sizes than artery size, the in-

sensor pressure coincides with Р

art

(see Figure 1) only

when the sensor pad is located directly above the

artery. Defacements of blood pressure measurement,

associated with positioning of the sensing pads, are

illustrated in Figure 2.

Figure 2: Distinguishing shape of pulse wave signal from

sensor (A), depending on sensing pads position: ● — pad is

directly over the artery, ■. ♦ — pad is shifted to the left, to

the right from the centre of the radial artery.

Figure 3: Sensor with three-chamber measuring element

based on locally compensatory blood pressure

measurement (A), implementing the three-channel

synchronous measurement of the pulse wave (B).

A detailed study of the positioning problem

revealed the following. In the position just above the

artery the ABP signal has the greatest magnitude

between the major maxima and minima and wherein

the extrema are themselves more acute (see Figure 2).

As seen from Figure 3, the same symmetrical

positions with respect to the artery occur when the

pulse wave graphics substantially coincide.

4 SENSOR POSITIONING

METHOD

These observations led us to design a pneumatic

sensor for monitoring blood pressure comprising a

measuring unit with three chambers for locally

compensating pressure measurements (each with its

own independent pressure meter), arranged in a row

transversely to the direction of the artery. Thus, in

certain positions of the measurement element the

chamber pads are simultaneously over the artery. A

schematic view of the sensor and the result of

simultaneously measuring by the three-channel pulse

wave at the proper position upon the artery are shown

in Figure 3. Details of the technical implementation

of the sensor are presented in the patent (Mansurov et

al., 2018).

In the claimed design of three-chambered

pneumatic blood pressure measuring sensor lateral

chambers are used basically to control the positioning

of the measuring element. Namely, the correct

arrangement of the sensor corresponds to the

maximum coincidence of signals from the lateral

channels (see Figure 3). Thus, it is not essential that

these channels cannot achieve complete "unloading"

of the side walls of the artery and, therefore, ABP

signals are significantly distorted. It is important that

the coincidence of these signals ensures the central

chamber to be located exactly above the artery normal

and in this position an undistorted signal from central

chamber replicates the pressure in the artery.

The methodology of measuring three-chambered

pneumatic ABP sensor is closely associated with the

described features of its construction. Namely,

immediately before the measurement the artery

location is estimated by means of palpation (feeling

for the pulse) to the patient's wrist. Then the

measuring element is applied so that the sensing pads

were arranged in a row transversely to the direction

of the artery (see Figures 2, 3). Further, moving the

sensor in the direction transverse to the artery, such a

position is sought in which the side channels signals

would maximally coincide. After that, the sensor is

pressed to the arm to make the contact of the central

pad with artery wall (through skin) as flat as possible,

without artery occlusion (applanation principle).

HEALTHINF 2019 - 12th International Conference on Health Informatics

464

5 CONCLUSIONS

A new method of positioning the three-chamber

pneumatic sensor for ABP measurement based on

local pressure compensation is proposed in the paper.

The main idea of local compensation is discussed in

detail. It is shown that this idea is quite simple and in

part resembles the principle of the relief valve. But to

achieve the stability in ABP measuring it is necessary

to provide very small sizes (tens of microns) for the

outlet channel hole of the measuring element. While

the sizes of a pads become smaller than the artery

size, the in-sensor pressure coincides with ABP only

when the sensor pad is located directly above the

artery. So, the advantages of locally-compensatory

blood pressure measurement are not obtained free of

charge – they have to be paid for by the problems

arising here with the positioning of the measuring

sensor.

The features of the measuring element

construction that we found made it possible to

produce up to four or more working chambers with a

linear step from one and a half mm to two mm in one

block of the element. According to the results

obtained the variant with three working chambers is

optimal. We believe that this solution allows the

implementation of a sensor option with the automatic

positioning of the applicator over the artery, since

manual positioning restricts the applicability of the

sensor proposed.

In short, the test results of the three-chambered

pneumatic sensor positioning and the developed

calibration technique showed the following:

1) a significant improvement in the accuracy of

measuring of systolic, diastolic blood pressure

and pulse characteristics due to calibration and

correct positioning of the measuring element;

2) the possibility of continuously measuring blood

pressure beat-to-beat for a long time (many

cycles);

3) the possibility for qualitative measurements of the

patient’s hemodynamics in everyday life.

The most important task in prospect is to replace the

manual positioning of the sensor by the automatic

control of its position and to develop on this basis a

mobile device for continuous monitoring of ABP

parameters.

ACKNOWLEDGEMENTS

The authors are grateful to the Russian Foundation for

Basic Research (RFBR), grant N 18-29-02108 mk for

the financial support of this work.

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