BEHAVIOURAL ANALYSIS OF AN IMPLANTABLE FLOW AND

PRESSURE SENSING DEVICE

J. A. Miguel, R. Mozuelos and M. Martinez

Technology Electronics, Automatic and System Engineering Department, University of Cantabria

Avda de los Castros s/n, Santander, Spain

Keywords: Biomedical transducers, Implantable biomedical devices, Cardiology.

Abstract: This paper presents a simplified MatLab model of an implantable device for pulmonary artery blood flow

velocity measurement. A comparative review of the most popular blood flow measurement techniques has

been carried out, showing the better suitability of pressure-sensing approaches to provide useful information

to cardiovascular diseases monitorization. Different possible grades of stenosis in the pulmonary artery have

been simulated in order to obtain an early estimation of the device behavior under real conditions.

1 INTRODUCTION

The development of micro-electro-mechanical

systems (MEMS), the continuous evolution in the

miniaturization and integration of sensor structures

and electronic circuits in the same chip, together

with recent advances in the field of biocompatible

materials have boosted the development of wearable

implantable medical devices .

Blood flow measurement represents one of the

most common procedures performed in hospitals for

the monitorization of cardiovascular diseases.

Recently, several implantable electronic devices

with both flow sensing and wireless communication

capabilities have been developed and tested, but

their power consumption, dimensions and long-term

reliability remain as unsolved constraints.

In this article, an initial model of a proposed

pulmonary artery flow sensing device is carried out

under several stenosed conditions. Section 1 presents

the definition of intelligent stent (e-stent) and its

impact on cardiovascular treatments. In Section 2,

different methods for blood flow measurement,

compatible with an implantable intelligent stent

design, are described. A simplified model of an

implantable device for pulmonary artery blood flow

measurement, based on pressure sensing, is

described in section 3. Different possible grades of

stenosis in the artery are simulated and presented in

order to obtain a first approximation of the device

behaviour under real conditions.

2 INTELLIGENT STENT

A stent is a bio-compatible flexible tube, made of

plastic or metal mesh, and designed to be implanted

in the human body during an angioplasty procedure.

A collapsed stent is mounted at the tip of the

catheter and then expanded in the site of an arterial

or venous blockage to push the vessel wall.

The impact of stents in modern cardiovascular

medicine has been enormous, reaching about 70% to

80% of all percutaneous coronary interventions

(PCI) (Lau, Johan, Sigwart and Hung, 2004),

significantly decreasing the total number of acute

complications in patients.

Unfortunately, the pressure applied by the

inflated balloon during an angioplasty procedure can

damage the vessel walls. Besides that, the patients’

body can respond using physiological repair

mechanisms, such as spasms, plaque deposition and

smooth muscle cells proliferation. In-stent restenosis

(ISR), defined as blood vessels narrowing due to

neointimal tissue growth inside an implanted stent,

keeps on being the major drawback in stent

implantation, seriously compromising its long-term

results. ISR has ratios from 10% to 70%, regarding

the nature of the disease (Hoffmann and Mintz,

2000). The introduction a drug-eluting stents, coated

with anti-proliferative drugs, has lowered the ISR

ratio to nearly 10% at the expense of higher cost and

long term reliability issues.

An intelligent stent (e-stent) that incorporates a

sensor capable of monitoring and transmitting real-

269

A. Miguel J., Mozuelos R. and Martinez M..

BEHAVIOURAL ANALYSIS OF AN IMPLANTABLE FLOW AND PRESSURE SENSING DEVICE.

DOI: 10.5220/0003873902690273

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2012), pages 269-273

ISBN: 978-989-8425-91-1

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

time measurements of biological related parameters

for its clinical consultation can help to detect ISR.

The monitoring function is carried out through an

implantable sensor, which must fulfil certain

characteristics, like reduced dimensions, output

stability and reliability over an extended period of

time without recalibration, minimal invasiveness and

low power consumption and cost.

3 MEASUREMENT METHODS

Blood flow measurement is one of the most usual

techniques for monitoring various types of

cardiovascular diseases. There are three typical flow

measurement approaches compatible with intelligent

stent design, such as electromagnetic, ultrasonic and

pressure-based measurements.

3.1 Electromagnetic Flow

The stents with electromagnetic flow measurement

capability are based on a direct application of Hall-

effect. This effect is produced when an electrically

conductive fluid passes through an externally

applied magnetic field with a certain angle. Then,

the magnetic field exerts a transverse force on the

charge carriers in the flow, creating a voltage

difference perpendicular to the flow and to the

magnetic field itself. The magnitude of the voltage

measured by two diametrically opposed electrodes

attached to the vessel walls, is given by (Webster,

1999),





·cos·cos··

vBDV

MAXEM



(1)

Where D is the diameter of the blood vessel, B is the

magnetic flux density and v is the blood flow cross-

sectional mean velocity. The angles θ and φ,

represents the magnetic field alignment with the

flow and the generated electric field.

Electromagnetic flow-meters must overcome

some major drawbacks. First, a shift between the

actual positioning of the electrodes and the desired

diametrical line, due to a non-uniform expansion of

the angioplasty balloon, produces significant

deviations in blood flow measurements. Second,

these architectures present a strong dependence upon

the magnetic field orientation, so an efficient

correction method is needed (Takahata and

Gianchandani, 2006).

3.2 Ultrasound

Ultrasound flow measurement techniques are based

on a direct application of Doppler Effect. This

principle postulates that the frequency change

between an emitted sound wave and the received

one is proportional to the relative velocity between

the sound source and the observer. Doppler equation

can be applied to hemodynamic variables, like blood

flow, using the following expression (Webster,

1999),



cos





(2)

Where v is the blood flow velocity, c is the

propagation velocity of the sound waves through

human body tissues, Δf is the Doppler frequency

shift, f

is the transmitted sound wave frequency and

θ is the angle between the axis of the emitted sound

wave and the direction of the blood flow.

This approach is the base for thoracic and

esophageal Ecocardiogram, commonly used

nowadays in hospital procedures.

Low frequency ultrasound waves present high

tissue penetration and low measurement resolution,

while high frequency ultrasonic waves have a better

resolution but are only able to scan the surface of the

tissue. This is why external ultrasound Doppler

blood-flow meters are incapable to reach deep inside

patient’s body without sacrificing the degree of

resolution in the measurements.

Devices bringing the ultrasound transmitter and

receiver closer to the blood vessel can avoid the

previous limitations, using an ultrasound frequency

high enough to allow good resolution (Wang and

Chen, 2011). However, it is necessary to overcome

important physical constraints to make this device

implantable. Among them we can point out the

determination of the angle between the probe and

the vessel and the amount of energy needed to

generate the ultrasound waveform and to process the

received signal.

3.3 Pressure

Blood flow velocity in an obstructed vessel can be

expressed as a function of the pressure gradient

between both sides of the stenosis. The general

expression can be written as (Young, 1983),

RvRvRP ···



(3)

Where ΔP is the pressure gradient between two

separate locations in a stented vessel, v is the mean

BIODEVICES 2012 - International Conference on Biomedical Electronics and Devices

270

cross-sectional flow velocity in the unobstructed

vessel and R

, R

and R

are coefficients that depend

on fluid properties and the geometry of the

obstruction. R

represents losses due to fluid

viscosity and it is directly related to the length of the

stenosis. R

represents nonlinear losses due to the

flow difference between downstream and upstream

locations and it is determined by the relationship

between the transversal un-stented area of the

obstructed vessel and its total area. R

is related to

fluid inertial effects, and can be neglected under

circumstances of severe stenosis (Young, 1983).

The simplest implantable version of the pressure

sensor is made of a capacitive MEMS to measure

blood pressure, and an inductance to form the LC

tank that transmits the information by proximity

coupling (Takahata, Gianchandani and Wise, 2006).

More elaborated systems incorporate electronic

circuits to process the information within the chip to

enhance the system performance (Chow,

Chlebowsky, Chakraborty, Chappell and Irazoki,

2010).

A pressure-based measurement allows the

integration of the sensor and the electronic circuits

in the same silicon substrate, decreasing the overall

cost of the system. The low energy requirements of

its components help to reduce the system size since

it can be powered by a wireless link. Moreover, this

approach provides the absolute pressure in the

vessel, providing additional information to carry out

the ISR monitorization.

4 ELECTRONIC SYSTEM

A simplified model of an implantable device for

pulmonary artery blood flow measurement based on

pressure sensing is described throughout this section.

Figure 1 shows the aforementioned model

description, to be implemented in the mathematical

program MatLab. Different possible grades of

stenosis in the artery will be simulated, by varying

and R

parameters, in order to obtain a first

approximation of the device behavior under real

conditions.

Figure 1: Simplified model of the electronic system.

The F2dP block performs the conversion from

blood flow to differential blood pressure in the

pulmonary artery using R

and R

parameters of eq.

3 to reflect the geometry of the stenosis. The

parameter’s values, as seen in Table 1, have been

taken from medical research publications regarding

the relationship between blood flow velocity and

pressure gradient in stenosed coronary arteries

(Marques, 2001). These values have been estimated

in an indirect way, by applying the arteries pressure

and blood flow measurements in the formula

described in eq. 3. However, stenosis shape can also

be correctly estimated with additional direct

procedures, such as angiographies and intravascular

ultrasonographies.

Table 1: Characteristics of the instantaneous flow velocity

and pressure gradient relationship (Marques, 2001).

R1 R2

Normal artery

0.032±0.018 0.00030±0.0049

Intermediate stenosis

0.15±0.11 0.0021±0.0014

Severe stenosis

2.67±1.58 0.0014±0.010

The F2dP block also takes into account the

absolute pressure in the heart side of the stent, in

order to provide two pressure waveforms whose

values represent the magnitudes to be measured by

the sensors placed at both sides of the stent. Figure 2

shows a simplified model of the device, focusing on

sensor placement inside the stented artery; where P

is the pressure in to the heart side of the stent and ΔP

is the difference of pressure measured between both

sensors.

Figure 2: Sensor placement and measures.

The P2C module emulates the behavior of a

capacitive MEM sensor, where the applied pressure

produces a deformation in a diaphragm that reduces

the chamber size, increasing the capacitance

between the two-plate structure. Figure 3 shows a

simplified cross-section of a MEMS capacitive

sensor, based on a deflecting diaphragm and a fixed

backplate; where P is the uniformly distributed

pressure applied, w

is the deflection of the

diaphragm center, t

is the undeflected gap between

BEHAVIOURAL ANALYSIS OF AN IMPLANTABLE FLOW AND PRESSURE SENSING DEVICE

271

the diaphragm and the backplate and t

is the

thickness of the diaphragm.

Figure 3: MEMS pressure to capacitance transfer function.

The general expression regarding the relationship

between pressure and capacitance of a circular

diaphragm-based MEMS capacitive pressure sensor

can be written as (Chang, Lee and Allen, 2002),





















2282

242

1280

)1(9

)1(

gmgm

ttE

tEt





(4)

Where ε

is the dielectric permittivity of free space

and a, µ and E are the radius, the Poisson ratio and

the elasticity modulus of the diaphragm,

respectively.

The relationship between pressure and

capacitance of the actual pressure sensor, with an

average sensitivity of 9.1 fF/mmHg, is shown in

Figure 4. It can be seen that its pressure range has

been selected to fit the regular pulmonary artery

pressures, which ranges between 15 to 30 mmHg

during systole and 8 to 15 mmHg during diastole.

Moreover, the sensor must be able to measure even

higher pressure values, in order to be capable of

detecting pulmonary artery hypertension caused by

in-stent restenosis.

Figure 4: MEMS pressure to capacitance transfer function.

The last block C2V performs the conversion

from MEMS’ capacitance to an electric voltage,

reflecting the difference between the two measured

pressures and their absolute values. By this way, it is

possible to monitorize both the artery pressure and

the blood flow velocity, providing enough

information to make an early restenosis diagnosis.

The converter parameters have been selected to be

similar to the ones common to this class of

electronic circuits (Arfah, Alam and Khan, 2011),

and its response can be seen in Figure 5.

Figure 5: MEMS pressure to capacitance transfer function.

In this way, we can obtain a first approach to the

behavior of the electronic system. Figure 6 shows

the output voltage of the system (lower graph),

reflecting the differential pressure at both sides of

the stent when a blood flow signal (upper graph) and

a blood pressure signal (center graph) are applied to

a healthy artery (R

=0.032 and R

=0.0003).

Figure 6: Output voltage reflecting the measured

differential pressure under normal artery conditions.

This simplified model also allows to display the

mean value of the output voltage of the system for

different conditions of stenosis (R

and R

sweep)

BIODEVICES 2012 - International Conference on Biomedical Electronics and Devices

272

when reference waveforms of blood flow and

pressure are considered (Figure 7). As can be seen,

higher R

and R

magnitudes produce higher

pressure gradient along the stenosis, reflected in an

increased output voltage. In a similar way, as

expressed in eq. 3 formula, worse obstruction

conditions produce higher pulmonary artery

pressures, as expected according to medical reports

about stenosed arteries (Rothman, Perry, Keane and

Lock, 1990).

Figure 7: Output mean value of the system for R

and R

parameter sweep.

5 CONCLUSIONS

For the last forty years, several implantable

electronic devices with flow sensing and wireless

communication capabilities have been developed

and tested, but their power consumption, dimensions

and long-term reliability in such a hostile

environment as human body remain as unsolved

drawbacks. For this reason, a simplified model of an

implantable device for pulmonary artery blood flow

measurement based on pressure sensing has been

developed. This kind of devices present some

important advantages, such as its dual pressure and

blood flow velocity sensing capabilities, together

with an improved robustness and reliability

compared with the rest of the analyzed measurement

methodologies. Finally, different possible grades of

stenosis in the artery have been simulated, by

varying the obstruction geometry parameters

between ranges reported by medical studies, in order

to obtain a first approximation of the device

behaviour under real conditions.

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