Mechanism of Cuff-Less Blood Pressure Measurement Using MMSB

doi:10.4236/eng.2013.510B025

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Engineering, 2013, 5, 123-125

http://dx.doi.org/10.4236/eng.2013.510B025 Published Online October 2013 (http://www.scirp.org/journal/eng)

Mechanism of Cuff-Less Blood Pressure Measurement

Using MMSB

Yibin Li, Yangyu Gao, Ning Deng

Institute of Microelectronics, Tsinghua University, Beijing, 100084, China

Email: yb[email protected], [email protected]

Received December 2012

ABSTRACT

Continuous measurement of blood pressure based on pulse transit time (PTT) using GMR sensors is the state-of-art

non-invasive cuff-less method in which modulated magnetic signature of blood (MMSB) is used. In this paper, the me -

chanism of MMSB is inv e stigated. According to the experimental results, it is found that both blood pulse flowing

through the applied magnetic field and the displacement of the GMR sensor caused by blood pulse contribute to the

disturbance of magnetic fi e l d de t e c ted by GMR se nsors. The feasibility of MMSB method is discussed as well.

Keywords: Blood Pressure; Magnetic Sensor; Pulse Transit Time

1. Introduction

Blood pressure (BP) is one of the most important vital

signs for people’s healthcare. Recently, the MMSB com-

bined with electrocardiographic (ECG) was proposed to

continuously measure the blood pressure [1,4,10,12].

However , the origin of MMSB is not yet clear enou gh for

practical applications. Some researchers believ ed that the

signal detected by GMR sensors reflected the disturbance

caused by blood flow in an applied magnetic field [2,3],

while others argued that this disturbance could be neg-

lected. This means there should be other reasons respon-

sible for the detected signal [5]. Clarifying the origin of

MMSB signal is very important for developing a real

blood pressure measurement system.

In this paper, a prototype system is established to

measure the blood pressure by MMSB. The mechanism

of MMSB is investigated and verified by our experimen-

tal results. The feasibility of MMSB method is discussed

as well.

2. Experimental Setup

The prototype system consists of three main parts: MMSB

acquisition circuit (including the magnetic sensor), ECG

acquisition circuit and back-en d signal process ing system.

To obtain MMSB signal, a GMR sensor (AH002-02,

NVE Corp.) is used. A permanent magnet with diameter

of 2 cm is placed 2.5 cm away from the sensor. This

permanent magnet is used to provide a bias magnet field

to make the GMR sensor work in the linear region.

In order to show the MMSB signal clearly, an oscil-

loscope is used before the MMSB signal is sampled by

A/D converter. Back-end processing system includes two

parts: amplifiers and lo w-pass filters . Considering the

signal of MMSB is of the same order as the background

noise, the back-end processing system is carefully de-

signed so that the noise is suppressed as much as possi-

ble.

3. Origins of MMSB

In previous studies, the disturbance of magnetic field

cause by blood flow in the applied magnetic field is con-

sidered the only reason responsible to the MMSB [7,9].

Some research work showed that the disturbance of mag-

netic field caused by the blood flow is only about 10−5

Gauss [5]. Unfortunately, the GMR sensors experience

small movement caused by blood pulse during measure-

ment. Th e angle between the magnetic sensitive direction

of the GMR sensor and the geomagnetic field changes

with the position of the sensor. Accordingly, this will

result in an extra disturbance on the detected signal. We

believed that this displacement of sensors contributes to

the MMSB besides the change of magnetic field caused

by blood fl ow.

Hereafter we evaluate the disturbance caused by the

displacement.

Parameters used are defined as following.

Magnetic field intensity of the environment: B

Geomagnetic field intensity : BG

Inclined angle of the sensor: θ

Sensitivity of the sensor: S

Power supply voltage: V

Magnitu de of Se nsor outp ut: Voutput

Y. B. LI ET AL.

124

The movement of the sensor is illustrated in Figure 1.

Assume the sensor initially rested horizontally as

shown in Figure 1. Due to an arriving blood pulse be-

neath the sensor, the direction of the sensor may change a

small angle θ. Therefore the change of magnetic field

intensity of the environment along the sensitive direction

is Bsinθ. Because the movement is very small, we have:

sinBB

θθ

≈

Since the sensitivity of the magnetic sensor is defined

as the output of the sensor per unit power supply voltage

per unit magnetic field intensity changed, it can be de-

rived that the sensor output should be

= sin

output

VS VBSVB

θθ

×× ≈

For the GMR sensors we used in our experiments, the

range of S is 11.0 ~ 18.0

( )

mV/V Oe⋅

. The voltage

supply is 5 V. Assume the environment magnetic field is

only the geomagnetic field intensity BG with typical val-

ue of 0.5 Oe. And assume the angle θ is only 0.001,

which is small enough and can be hardly observed by

human eyes. Then we have

( )

11 /5 0.50.001=27.5

output

VmVV OeVOeV

≥⋅ ×××

The output of the sensor is amplified by 104 times be-

fore sent to the oscilloscope. Then the magnitude should

be more than 275 mV. We will use these values in next

part to investigate the origins of the MMSB.

4. Experimental Results and Discussions

Figure 2 shows the typical MMSB and ECG signals ob-

tained with our prototype system. Based on the method

in Ref.1, the calculated mean blood pressure (MBP) is 73

mmHg. The MBP measured with a standard sphygmo-

manometer is 72 mmHg. This result means the MMSB

method is valid to some extent. The peak-to-peak value

of the MMSB signal is 680 mV which is of the same

order the disturbance caused by sensor displacement.

This means both components of MMSB should not be

neglected for blood press u r e c a l c ulations.

As shown in Figure 3, we obtained MMSB signals

with different shapes by changing the direction of the

applied magnetic field. The first signal in Figure 3 is

similar to the photoplethysmography (PPG) signal [14]

Figure 1. Illustration of movement of the sensor caused by

blood pulse.

Figure 2. MMSB and ECG si gnals obt ai ned w it h our sys te m.

(yellow: MMSB, green: ECG).

Figure 3. MMSB signals with different shapes.

Y. B. LI ET AL.

125

which are also usually used in continuous BP measure-

ment systems [11,12,14]. The PPG signal is caused only

by the changes of blood flow. The MMSB signal should

have a similar shape as Figure 2’s signal if MMSB is

also caused only by blood flow under the magnetic sen-

sor like the case of first signal. Now the question is: why

did we obtain MMSB signals with deferent shapes? The

only reasonable explanation is that the both origins we

mentioned above contribute to the MMSB at the same

time. Which kind of shape we would get is determined

by the competition of the two components. The meanings

of the different shapes will be studied in other papers.

PTT is usually defined as the time delay between the

peak of the R wave of the ECG waveform and the up-

stroke of a peripheral pulse wave signal [1]. The output

of magnetic sensor due to the position variation makes

the measurement much more complicated. It seems that

this will cast a shadow on the feasibility of applying

MMSB on bloo d pre ssure measurement. Fortunately, this

is not the case. Although both origins will contribute to

the MMSB signals, the MMSB will be still highly related

to blood pressure. The only negative effect is the MMSB

signal could be too small to be detected if these two me-

chanisms lead to opposite change of the detected mag-

netic field. As a result, to improve the feasibility and

reliability of the measurement, shielding the geomagnetic

field is an effective solution.

5. Conclusion

In this paper, origin of the MMSB is investigated. We

confirm that the MMSB signal consists of two compo-

nents based on experimental results. One is from change

of the blood flow in the applied magnetic field. The other

is from the sensor displacement caused by the blood

pulse. According to our study, the MMSB method is still

feasible for blood pressure measurement. A method to

improve the feasibility and reliability of the measurement

is proposed as well.

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