Design and Development of Pulse Transit Time Based Cuffless Blood Pressure Monitoring System Harinderjit Singh
Mandeep Singh
Department of Academic Consultancy Service Division C-DAC Mohali, Punjab, India Email-
[email protected]
Department of Digital Electronics and Communication Division, C-DAC Mohali, Punjab, India Email-
[email protected]
Abstract- Blood Pressure (BP) is a major and most reliable measurement
for
continuous
patient
monitoring
for
their
personal health care and patient's cardiac system. It is observed that stress is the main reason for many serious diseases and cardiac disease is also one of it. To overcome this problem continuous method is required for BP measurement. For this, various techniques are available, i.e. invasive method and non invasive method, both of these methods are not continuous, So a non-invasive technique based on Pulse transit time (PTT) has been developed that can measure BP continuously for a long time. In PTT, the difference in time between the R-wave peak of electrocardiogram (ECG) signal and rising edge peak of the photoplethysmogram (PPG) signal will be taken out. This time difference is PTT which is used to calculate pulse wave velocity (pwv) that can be directly applied to the BP-PWV relationship to calculate Systolic and Diastolic BP (SBPIDBP).
Keywords- pulse wave velocity; pulse transit time; systolic blood pressure; diastolic blood pressure
I.
INTRODUCTION
Stressful lifestyle has a negative impact on a person's health [6], which lead to some serious diseases like heart disease, Asthma, Obesity, Diabetes, headache and premature deaths [17]. According to this research, it is generally due to non-continuous monitoring of blood pressure. Thus, a continuous monitoring of blood pressure is required [15] to provide proper medication on time for decreasing the prospects of severe cardiac diseases. The blood pressure is categorized into three groups low, normal, high with systolic (low) BP and diastolic (high) BP. Even the measurement of BP can be done with invasive and non-invasive techniques. II.
PREVIOUS WORK
In the past history, various techniques has been developed to measure BP. The First BP monitoring device was invented by Stephen Hales in 1733 by inserting tubes directly into the artery of animal [11]. He correlated this with millimeters of mercury (i.e. mm hg) and he also calculated 176 mm hg of blood pressure for humans around 7.5 feet. After his experiment a French surgeon inserted a pipe directly to patient's artery for measuring blood pressure and calculated BP value around 120 mm Hg. Later on these experiments various BP monitoring devices were developed, the Von Basch Sphygmomanometer device was developed in 1876,
978-1-4799-7678-2/15/$31.00 ©2015 IEEE
[11]. In this device a balloon is tied to the wrist and squeezed until no pulse is observed and this balloon is connected to pressure manometer. The methods developed till now were only experiments and yet these techniques cannot be well enforced. In 1896 an Italian physician, Riva Rocci developed a technique which was a cuff based mercury sphygmomanometer to measure blood pressure [II] and this method was very gentle to apply. This conventional cuff was attached with mercury manometer and it can measure systolic blood pressure (SBP). A few years after a new method was developed by Nicolai Korotkoff in 1904. In this method a stethoscope was used with cuff over the artery, this stethoscope was used to observe korotkoff sound at systolic blood pressure (when heart pumps) and also at low pressure i.e. diastolic blood pressure (when the heart relaxes) [3]. This method has given an approach to measure systolic as well as diastolic pressure and over the 100 years there had been very little development of this technique but the basic method was same. In 2014 SRM University's student Surendhra Goli, has given a new approach to measure blood pressure [1], his work was basically focused on pulse wave velocity which indirectly depends on pulse transit time technique. This pulse wave velocity was calculated by using the relation between velocity and time. For calculation of blood pressure a relationship was developed which consist of two unknown parameters, these parameters were estimated by using a least mean square algorithm. At the end, both systolic and diastolic blood pressure values were calculated and compared with the blood pressure calculated by using cuff and the error between these two is also calculated. III.
MEASUREMENT TECHNIQUES
A. Invasive Method For BP Measurement
This method is the primary calibration technique for Blood pressure measurement. This method yields high end of accuracy in BP at very low pressure. In this technique a pressure transducer is coupled with a catheter, which is inserted into brachial artery. This method was not adequate, because it requires a catheter needle to be inserted into an artery, it is painful and also there are chances of infection.
B. Non-Invasive Method For BP Measurement
There are many indirect methods used for blood pressure monitoring. BP is generally valued in millimeters of mercury (mm Hg) [3]. Several non-invasive blood pressure measurement techniques [2] are described as follows:
1) Palpation: This technique is easy to execute and often utilized in emergency situations. It is roughly estimated a minimum systolic (high) blood pressure, i.e. >70 mm Hg, hence diastolic (low) blood pressure and mean pressures are undetermined. This method is used before auscultatory method to get an estimate. 2) Doppler: This technique uses Doppler principle and is reserved for measuring Systolic (low) blood pressure. The primary role is performed by Doppler probe, as when blood runs away or towards it, a sound wave is reflected and is detected by it, by applying the concept of switching frequency. 3) Auscultation: It includes the Korotkoff sound (as described by Nicolai Korotkoff in 1905) caused by the bubble formation within the blood, turbulence within the vessel, sudden stretching of vessel wall and combination of all factors. These Korotkoff sounds are drawn for both systolic and diastolic blood pressures. 4) Oscillotonometry: This technique uses two cuffs and two bellows which are connected to measurement gauge. The two cuffs i.e. occluding cuff and sensing cuff are overlapped. Both bellows display the pressure received from two cuffs via a single gauge alternately using a lever. So that SBP and DBP can be measured accurately. 5) Liquid Manometers: The air present in the cuff can act on a liquid and force it up to manometer. There are two types of manometers, open manometer, which is used in sphygmomanometer to measure gauge pressure and closed manometer, which measures absolute pressure and is used in mercury barometers. Blood pressure can be measured using closed barometer. 6) Aneroid Gauge: The problem of mercury toxicity is avoided with this technique, by replacing the mercury column. Pressure is indicated by a pointer along a scale when a bellow is expanded by an increase in pressure. This proficiency involves a regular calibration as it can lose accuracy. 7) Electronic Systems: This technique utilizes the motion of a diaphragm which is due to changed air pressure and is detected and displayed on LCD. The minimum compressible component used in the system to increase its accuracy. This may measure systolic blood pressure/diastolic blood pressure via single gauge. 8) Pulse Transit Time technique: In this technique, time taken by the pulse of blood to travel from the heart to any extreme position [8] of the body is calculated. IV.
METHODOLOGY
A. Pulse Transit time technique
As technology is stacking up, new methods have been developed for Blood Pressure measurement. There are
basically three types of cuff-less devices that are developed in the last few years, i.e. Pulse Wave Velocity, Pulse Transit Time and Photoplethysmographic. These methods have been developed to continuously measure Blood Pressure for a long time without creating any irritation to the patient. It can be directly applied to patients in hospitals. In this technique Electrocardiogram (ECG) and Photoplethysmogram (PPG) signals are applied to measure blood pressure. Fig.1 shows that PTT is computed by taking change in time between R wave peak of ECG signal and rising edge peak of the PPG signal [1,12,13,14].
FIGURE I: PTT calculation using ECG and PPG waveforms
This is basically the time taken by the pulse of blood to travel from the heart to the finger [8] and this time is used to calculate PWV [12,13]. PWV is the velocity by which pulse of blood travel from the heart to fingertip. It is observed that with increase in the blood pressure, velocity of blood pumped by the heart increases and to decrease in blood pressure, velocity of blood pumped by heart decreases [16,7], from this we come to a conclusion that blood pressure is directly related to pulse wave velocity [17] and inversely related to PTT [16]. Fig.2 shows the basic block diagram of PTT based BP monitoring system.
FIGURE 2: Block diagram of PTT based BP monitoring system
B. Electrocardiogram Sensor
ECG sensor is used to extract electrocardiograph signal produced by our heart during pumping of the blood. This sensor consisting of three electrodes i.e. right arm, left arm and right leg. Right arm and left arm electrodes are used to take the difference in voltage and right leg electrode is taken
as a reference electrode. Figure 3 shows an ECG sensor that is interfaced with the hardware.
F.
PPG Peak Detection
For detecting PPG peak, the output of PPG sensor is given to interrupt pin of the microcontroller. As the rising edge of interrupt comes, it goes to high and PPG peak is detected. G. PTT calculation
FIGURE 3: EeG sensor
C.
Photoplethysmogram Sensor
This detector is employed along the finger of the patient. This PPG sensor is having IR emitter and a photo detector, as the heart pumps the pulse of blood, the volume of blood changes in finger, with respect to this light detected by observing changes. Figure 4 shown below is PPG sensor, which is interfaced with the hardware for extracting PPG signal from the finger of the subject.
For calculating PTT between both of these peaks, the timer of PIC microcontroller is used. Firstly, the reference value of ECG peak is selected by the user. Once an ECG peak value is selected, it will keep on monitoring this peak level. The timer will not start until this peak goes above selected value, once it goes above this level the timer starts and simultaneously it check for the interrupt, when interrupt signal is received from PPG sensor, the timer stops and value in the timer is converted into millisecond to display it on the LCD. H.
Pulse Wave Velocity calculation
PWV can be estimated by applying the dire�t relationship between velocity and time [5], Equation (I) rcpT��cnts thc relationship between PWV and PTT.
Pulse Wave Velocity,
(Distance between heart and finger tip, D)
Pulse Transit Time)
(1)
The distance between the heal1 and finger is calculated by using a relation as shown in eq. (2) D
FIGURE 4: PPG Sensor
BCF
x
height(in cm)
(2)
Where, BeF is a Body correlation factor. For adults, it is 0.6, based upon the survey on different subjects it is found that length of the artcry from hcart to fingcr is approximately 60 % of individual's height [8]. From (I) and (2), [t can be simplified to [5,9],
D. Noise and Artifact Reduction
In case of Photoplethysmogram sensor motion artifact occurs due to motion of hand at different situations, this generally comes about due to the deprivation of light signal during motion of the physical body. For this reason, the sensor is selected, in which very less loss of light occurs. For this, the sensor is taken with a silicon grip cover where the very least amount of loss of light occurs during motion of hand and in case of ECG sensor, initially we are using two electrodes at two different sites i.e. right arm and left arm to create a difference of potential. In this case much amount of dissonance is present, to bump off this character of noise a third electrode is needed to be inaugurated in the sensing element as a reference electrode.
=
PWV I.
=
height(cm) PTT(inms)
BCF
x
(3)
Blood Pressure Calculatiun
After calculation of pulse wave velocity of different subjects a relationship b e tween PWV and BP is developed [1,[0], eq. (4) and (5) showing the SBP and DBP relationship with PWV i.e. SBP
DBP
=
asys x
pwv
+
bsys
(4)
=
aDia x
pwv
+
bDia
(5)
Where a Sys, bsys> a Dia and bDia are constant parameters, these are determined by using a least mean square algorithm.
E. R-Peak Detection
1.
The output of ECG sensor is analog and this analog input is applied to analog channel of PIC microcontroller for analog to digital conversion and this digital output is used for R-peak detection. A constant level of ECG is set up by using external input switches. This R-wave peak varies from person to person due to this an option is provided to increase or decrease this level manually by using switches.
In this hardware PPG and ECG sensors is interfaced with PIC18F452 microcontroller, it comes with 10 bit ADC therefore it is useful for converting analog signal of ECG sensor output to a digital signal so that calculation of PTT becomes easy. Designed hardware of the prototype is depicted in figure 5.
Hardware Description
FIGURE 5: Controller part of the hardware
Four input switches are also interfaced with the hardware. Where, switch I is for increment the value of ECG step size and height of the subject, switch 2 is used to decrement the value of ECG step size and height of the subject, switch 3 for entering selected ECG and PPG values and switch 4 is for reset. K. Software Description
Figure 6 shows the flow chart of pulse transit time, pulse wave velocity and SBP/DBP calculation. The Following steps are followed in software program for calculation of blood pressure [4]:
S�l",,' ECG P�ak (ADC Valu�) Inlrlally, ECG Peak
�
me)
37() (Step
3) Set the height of the subject by using an increment and decrement switches (swl and sw2) and switch 3 for entering the [mal height in centimeters. 4) After entering both ECG peak step size and height of the subject, it keeps monitoring the ECG peak (ADC step size) value, if it is below the selected ADC value, it will keep on monitoring, once the selected point or greater peak comes the timer of microcontroller turns on. 5) Once the timer is turned on then it will wait for the interrupt signal from the PPG sensor, on reception of interrupt signal, the timer turns off and the value in timer is converted into decimal to display it on the LCD and this value is the PTT. 6) This PTT value is applied for calculating the PWV [13] by using mathematical relationship that is expressed in equation (3 ). 7) PWV calculated in step (vi) is used for calculating blood pressure, i.e. SBPIDBP and this value are also displayed on the LCD. V.
RESULT AND CALCULATIONS
An experiment was executed on different subjects in different health conditions, in which whole setup was used for different subjects at different time for calculation of pulse transit time and pulse wave velocity was too computed. As shown in Table 1, these are PWV value calculations by using PTT for male and female subjects of different age and sex groups. Table 1: Pulse wave velocity calculation using PTT [1]
Select Hei!ht (in Clrt) of Patient Initially, Heigb t
-
1 SO
i. Timer starts: nnd wain ((}r PPG Signal Interrupt ii. PPG Sienal Interrupt Occurs Timer Stops and value in time!'" i s c.onvened into millisfi.onds and this Vil1ue
is Pulse Transit Time
� i.
Pul§:@'Va,r@ V@lodty calculation u§:mg Pub.@ Transit
TIme. ii.
Sy§':toLic and Diastolic Blood Pr@ssur@ calculation
Subjects
Age
M/F
Height
PTT
(cm)
(ms)
PWV (cm/sec)
Subject 1
23
M
173
236
439
Subject 2
25
F
155
212
438
Subject 3
30
M
185
255
435
Subject 4
28
M
180
270
400
Subject 5
27
F
165
218
454
Subject 6
25
M
176
224
471
Subject 7
24
F
170
219
465
Subject 8
40
M
175
155
677
Subject 9
42
M
178
172
619
Subject 10
45
M
185
190
582
u,siug BP-p"ry n�.lat.iuusbip
I
1 SHl'flJ1U' value is Displayoo
00
LCD
FIGURE 6: Flow chart
I
1) Initializing the system, after connecting ECG electrodes on the body. 2) Set the ECG step size value by using an increment and decrement switches (swl and sw2) and switch 3 for entering the [mal ECG step size.
By using these readings, a graph is plotted between PWV and PTT. Figure 7 shows that "PWV" is inversely related to "PTT" i.e. with an increase in PTT, PWV decreases.
The calculated parameters are applied to the BP-PWV relationship to determine SBP and DBP. These values are calculated for different subjects and these are compared with reference values calculated with cuff based OMRON BP machine as shown in figure 9 and error is also calculated between them that is shown in table 2.
PTTV sPWV
675 650 625
�PWV/PTT
600 575 �
"
� 550
E
�
525
�
� Q.
500 475 450 425 400 150
170
190
210
230
250
270
FIGURE 9: SBP/OBP reading on both machines on same time.
PTT (in ms) FIGURE 7: Graphical representation between pulse wave velocity and pulse transit time
Later on the calculation of pulse wave velocity, least mean square algorithm (eq. (6)) is employed [1,10] for calculating constant parameters, i.e. asy" bsys , aDia and bDia. In this method the coefficients are initially determined by using BP value measured by cuff based machine and PWV measured by PTT method [1,10].
where, YK
=
[BP1] :
BPn
and
XK
=
[P�Vl] PWVn
Error percentage is calculated by using the formula between the mea ured value and true value (as shown in eq. 8) calculated from cuff based BP monitoring machine. %
Error
=
Measured value-T''Ue Value True Value
FIGURE 8: Complete hardware setup along with OMRON BP machine.
(8)
Table 2: Comparison between Cuff based BP machine and PTT based BP monitoring system
Subjects
SBP /OBP (in mm hg)
SBP
OBP% Error
UsingPPT method
Using cuff machine
% Error
I
112/77
116/71
3.4
8.4
Subject 2
112/76
117/73
4.2
4.1
Subject 3
111/75
112/72
0.89
4.1
Subject 4
106/70
108/73
1.8
4.1
Subject 5
114/78
119/80
4.2
2.5
Subject 6
116/81
120/83
3.3
2.4
Subject 7
115/80
118/76
2.5
5.2
Subject 8
145/90
147/93
2.7
3.2
Subject 9
137/85
143/87
4.1
2.2
Subject 10
132/84
135/82
2.2
2.4
(7)
The complete apparatus is setup up along with PPO sensor, ECG electrodes and OMRON BP monitoring machine are attached on body is shown in figure 8.
X 100
Subject
Figure lOis shows the graphical comparison between the reference value calculated by an OMRON BP machine and PTT based BP monitoring system.
of body during testing it can induce an error in measurements. In this adaptive filtering circuit can be inserted into this device for reducing motion artifacts.
Blood pressure comparison between cuff based and PTT based techniques 95
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The work presented in this research paper deal with the design and development of a cuff-less BP measuring system for non-invasive and continuous measurement of BP. There are various methods that have been studied for continuous measurement. It is observed that the PWV technique is the best method for calculating the blood pressure. In this method the error in SBP ranges from -5 mm hg to +2 mm hg and in DBP error range is from -6 mm hg to +5 mm hg.
[9]
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[15]
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