(PPG) Signal

International Conference on Information Technology, Electronics and Communications (ICITEC – 2013), Bangalore, India, March 30 – 31, 2013

Non Invasive Estimation of blood pressure using a linear regression model from the photoplethysmogram (PPG) Signal Shine Augustine, Manimegalai .P Dept .of Electronics and Instrumentation Karunya University [email protected],[email protected] ABSTRACT:- Nowadays most of the blood pressure measuring devices rely on a common concept of inflatable cuff to the arm which is based on auscultatory or oscillometry principle. These existing blood pressure meters, based on cuff are considered to be inconvenient for daily monitoring and these are very sensitive to artefacts due to the presence of cuff. The other non invasive methods like PTT, PAT require more no. of parameters in order to estimate the blood pressure. This paper is aimed at designing a non invasive cuff-less cardiovascular parameters estimation system from Photoplethysmography(PPG) Signals technique Using LabVIEW and analysis using MATLAB. A ppg signal is voltage derived signal generated by the photo detector from light with the changes in the blood volume. It does reflect the movement of blood in the blood vessel, going in a wave like motion from the heart to the fingertips through the blood vessels. It has been considered as non-invasive, reliable, simple, and cost-effective method for measuring arterial pulse waves in relation to changes in wave amplitude. This study aimed to investigate the possibilities of monitoring the pressure system status by using morphological features of PPG components. The specific characteristic feature of the PPG wave such as Pulse height and Pulse width has been taken into count for the Blood Pressure estimation. The estimation is further done using a mathematical tool such as regression analysis via MATLAB. Keywords: PPG, Regression, blood pressure, PAT, PTT,PWD

1 INTRODUCTION Demands to improve living styles causes most people not to really concerned about their healthiness , however the biggest known cause of disability and premature death through stroke is since the awareness of high blood pressure is, heart attack and heart disease, medical doctor recommended a regular self monitoring of blood pressure and pulse rate to make sure of the necessassity to control blood pressure and prevent it from taking the shape of either hypertension or hypotension. Blood pressure is a vital parameter in the evaluation of cardiovascular function and status. It has generally been accepted that blood pressure control is significantly affected by the resistance in the peripheral vessels and cardiac output that correspond to the blood pressure control during systole and diastole, respectively. The cardiovascular system is a closed loop comprising the heart and blood vessels, its internal pressure is continuously changing due to changes in blood volume and vascular capacity the pressure system status by using morphological features of PPG components. The specific characteristic feature of the PPG wave such as Pulse height and Pulse width has been taken into count for the Blood Pressure estimation. The estimation is further done using a mathematical tool such as regression analysis via MATLAB. On the basis of this physiological foundation, change in blood pressure and vessel status can be evaluated by monitoring the change in blood flow. Photoplethysmography (PPG) is a noninvasive optical technology that detects changes in blood volume in the vascular system. The Photoplethysmography (PPG), introduced by Hertzman in 1938, reflects the blood volume changes in the finger arterioles. It has been considered as non-invasive, reliable, simple, and cost-effective method for measuring arterial pulse waves in relation to changes in wave amplitude.

2 EXISTING SOLUTIONS & PROBLEMS

From literature study it is found that, blood pressure measuring devices rely on a common concept of inflatable cuff to the arm which is based on auscultatory or oscillometry principle. These existing blood pressure meters, based on cuff are considered to be inconvenient for daily monitoring and these are very sensitive to artefacts due to the presence of cuff.. Many researchers have proposed solutions in this regard with other non invasive methods like PTT (Pulse Transit Time), PAT (Pulse Arrival Time),PWD(pulse wave delay) require more no. of parameters like ECG and PPG in order to estimate the blood pressure but still to find another more reliable easy accessible way has been the interest of research. In the later study Blood pressure is calculated from PPG signal taking ECG as reference parameter ,estimating the relation between transit time and blood pressure. The detection of the velocity of pulse wave is based on the registration of the mechanical movements of blood vessel’s walls and computing the time delay in different regions of the human body using an ECG as a reference signal.Thus the signal which is of most important is ECG as it is required to estimate the Pressure . In a scientific paper pulse wave transit time (PWTT) of the toe, finger and nose was measured and the difference between each site’s PWTT was calculated.To develop a non-invasive measurement method of circulatory parameter by analyzing waveforms of digital PPG, mathematical modeling of input (observed PPG waveforms) relations were performed [25]. A new method for cuff-less blood pressure monitoring was presented, where signals from plethysmographic sensors were integrated with a mathematical model to estimate beat-to-beat blood pressure waveform.

3 PULSEPLETHYSMOGRAPHY Plethysmograph (PG) is a combination of the ancient Greek words ‘plethysmos’, meaning increase, and ‘grapho’ is the word for write, and is an instrument mainly used to determine and register the variations in

International Academic and Industrial Research Solutions (IAIRS)

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International Conference on Information Technology, Electronics and Communications (ICITEC – 2013), Bangalore, India, March 30 – 31, 2013 blood volume or blood flow in the body which occur with each heartbeat. PPG was one of the oldest methods for measuring blood flow in the extremities.By 1938 Hertzman found a relationship between the intensity of backscattered light and blood volume in the skin. 3.1 NON INAVSIVE TECHNIQUE Photoplethysmograph (PPG) is an optical technique which typically operates using infrared light, allowing the transcutaneous registration of venous and/or arterial blood volume changes in the skin vessels. PPG signal is generated with the complex interaction between connective vasculature and the heart, The fundamental of this technology is the detection of the dynamic cardiovascular pulse wave generated by the heart as it travels throughout the body .generally the illuminating PPG wavelength is chosen to provide weak absorption in tissue, yet stronger absorption by blood, to provide a high degree of optical contrast. Infrared radiation is often employed and provides a convenient illumination source. It provides a signal proportional to changes in skin blood volume.The pulsatile component of the is often called the ‘AC’ component and of PPG waveform having its fundamental frequency, 1 Hz, depending on heart rate. This AC component is superimposed onto a large quasiDC component that relates to the tissues and to the average blood volume. This DC component varies slowly due to respiration, vasomotor activity and vasoconstrictor waves. .

The pulse width correlates with the systemic vascular resistance better than the Systolic amplitude. Awad et al. [10] used the pulse width as the pulse width at the half height of the systolic peak. The pulse width in the PPG wave is shown in Fig. 2.The another approach to calculate the Pulse width is by finding the pulse inflection point (Pi) lying between the Pulse peak (Pp) and the pulse valley (Pv) further subtracting previous Pv from(Pi).The expression can be given as : PW (PULSE WIDTH) = Pi- Pv…………….(1) Table 1:Different factors of Systolic amplitude

4 PROPOSED SOLUTION The proposed work aims at estimating the blood pressure from the different features of PPG components. The PPG waveform has been segregated into respective sections and the processing is done for each of the sections. The PPG wave can be segregated as per the respective morphology pulse width, pulse height, area under the curve. The systolic and the diastolic peak of a PPG waveform can be co-related with the blood pressure values. A number of features based on the PPG have been described in literature: 4.1 Systolic Amplitude: Dorlas and Nijboer found that systolic amplitude is directly proportional to local vascular distensibility over a remarkably wide range of cardiac output [6].the different factors effecting the systolic peak has been evaluated and mentioned in table 1.The systolic amplitude is an indicator of the pulsatile changes in blood volume which is caused by arterial blood flow around the measurement site [8, 9] as shown fig. 2.It is also has been suggested that a more suitable measure than pulse arrival time (PAT) is systolic amplitude is potentially used for estimating continuous blood pressure [7]. Pulse amplitude was calculated as the difference between the PPG values of the cardiac peak and the preceding valley 4.2 Pulse Width

Figure 2: The systolic component

4.3 Pulse Area: The pulse area is measured as the total area under (AUC) the PPG curve. The AUC was calculated by integral minus the DC area. Seitsonen et al. [5] found the PPG area response to skin incision to differ between movers and non-movers. AUC = Integral –DC area …………… (2) Wang et al. [11] have divided the pulse area into two areas at the dicrotic notch. They found that the ratio of the two areas, see Fig. (4), can be used as an indicator of total


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International Conference on Information Technology, Electronics and Communications (ICITEC – 2013), Bangalore, India, March 30 – 31, 2013 peripheral resistance. This ratio is called the inflection point area ratio ( IPA) and is defined as:

PPG signals are taken from individual persons using Photoplethysmography unit with a sampling rate of 4000 samples/second. The frequency response for PPG it is 0.05-15Hz. It is obtained by using transmissive type USB PPG sensor. 5.1 Block Diagram

IPA= A2/A1 …………………….. (3)

Figure 3. figure showing of pulse amplitude, pulse width, and area under the curve (AUC) extracted from the photoplethysmogram (PPG) waveform.

The respective amplitude, width, and area of the PPG signal were calculated and further were normalized with respect to their baseline values. Inorder to study the patterns while accounting for individual physiologic variability.

Figure 5:Block diagram of the proposed work

The signals are then acquired using LabVIEW software and further analysis is done by implementing the algorithm. LabVIEW aids in real time implementation. The block diagram as shown in Fig5, the acquisition of the required PPG signal from normal and abnormal subjects were obtained. 5.2 PROPOSED ALGORITHM The proposed algorithm has been implemented using LabVIEW software and further regression has been applied via MATLAB tool. Step 1: Acquiring the PPG signals Step 2: Filtering the signal using low pass filter (15 Hz) Step 3: Estimating the DC value Step 4: calculate the respective parameters i.e peaks and valleys Pp and Pv Step 5: calculating the Inflection points Pi. Step 6: Calculating the Pulse height, width, and are under the curve. Step 7: Applying regression tool to the obtained Parameters (Pp, Pv, Pi) Step 8: calculating the Pressure values from the equations drawn from regression Step 9: co-relating the pressure values with the measured blood pressure values.

Figure 4. Original fingertip PPG A1 and A2 are the areas under the whole PPG wave separated at the point of inflection Pi.

5. PROCEDURE Participants were provided with pre-test guidelines to reduce the impact of external influences on measurements. No tobacco consumption and caffeine for at least an hour prior to data acquisition. Room temperature was measured and maintained at a minimum of 20 to 22°C at both sessions to prevent vasoconstriction of digital arteries.

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6 ANALYTICAL EVALUATION The normal subjects where within the age of 40. The signals were obtained for about a min after the subject completely relaxes. Further the pressure readings were measured for later reference. The data analysis was performed in LabVIEW 2011 the respective morphological feature were extracted after the algorithm implementation. The morphological features such as Pulse height, Pulse width, area under the curve were extracted and further analysis was done using mathematical tool called


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International Conference on Information Technology, Electronics and Communications (ICITEC – 2013), Bangalore, India, March 30 – 31, 2013 regression analysis. The equations obtained were used to calculate the SBP values.

The calculated pulse height and width analysis were performed and the regression analysis was done the MATLAB results are as follows:

Figure 9: Pulse height analysis via Linear regression in MATLAB

Figure 7:Front Panel of The Proposed Work

Figure 10: Pulse width Analysis via linear regression using MATLAB

Figure 8: Area under the curve via LabVIEW

The respective area was calculated and the Inflection point ratio was calculated for each every signal. IPA =A2/A1

The equations used for calculating the Blood pressure values from Pulse height and pulse width are: Y=0.8x+ 99…………………..(4) Y = -37.6x+161………………(5) The respective calculation was done from the above extracted parameters and SBP values were estimated .The given table shows the data obtained : Table 2: Estimated SBP values from PPG S.NO.

The generalized equation for regression analysis is given below: Y=β0+β1xi + u i The IPA index was calculated and using regression in Matlab was performed and the relation showed that IPA is inversely proportional to the SBP.

PUL SE RAT E (BP M)

Pulse height

SBP (estima ted)

Pulse width

SBP SBP (estima (Measured ted) BP)

Patient1

75

33

128

122.1

126.91

123

Patient2

63

44

110

130.5

121.12

118

Patient3

80

47

140

79.41

156.29

150

Patient4

95

48

133

98.4

143.49

139

Patient5

93

49

137

93.77

146.41

142

Patient6

70

51

127.5

116.0

131.21

129

Patient7

80

49

129

122.4

126.71

123

Patient 8

104

53

129

105.2

138.53

136

Patient 9

72

52

137

110.8

134.65

131

Patient 10

101

49

130

100.7

141.60

138

Figure 9: Matlab results


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International Conference on Information Technology, Electronics and Communications (ICITEC – 2013), Bangalore, India, March 30 – 31, 2013

7 CONCLUSION In this paper, Estimating SBP using PPG is proposed using an algorithm, in which the respective morphological features were extracted using LabVIEW And the data analysis has been performed using MATLAB/ Our simulation results show tthe Pulse height (systolic amplitude) is directly proportion to the systolic blood pressure and it increases for higher cardiac output but with decreased time span. The Pulse width and area is inversely proportional to the Systolic blood pressure .

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