Implementation of a Photoplethysmographic Heart Rate Monitor with SMS Alert

Transcription

1 9 Implementation of a Photoplethysmographic Heart Rate Monitor with SMS Alert James T. Tingir 1 and Jonathan A. Enokela 1, Department of Electrical and Electronics Engineering, Federal University of Agriculture, P.M.B. 33, Makurdi, Benue State, Nigeria 1 jaytinter@gmail.com, jonajapeno@gmail.com ABSTRACT Heart rate is an essential parameter for checking Cardio-Vascular Diseases (CVD), which are responsible for the highest number of deaths globally. The Heart Rate Monitor (HRM) in this work uses a photoplethysmographic (PPG) sensor, which is composed of a green light emitting diode (LED) and a phototransistor as detector. The user places his finger on the sensor and pulses taken from the finger are filtered and amplified using the LM38 dual operational amplifier.the pulses are then sent to a microcontroller that processesthe pulses over a period and sends the information to a Liquid Crystal Display (LCD) which displays the heart ratein beats per minute (BPM). A buzzer alarm is activated if the heart rate falls above the stipulated range of 100 BPM to indicate Tachycardia conditions. A Global System for Mobile Communication (GSM) module is incorporated in the circuit to send the information to an enabled handset via Short Messaging Service (SMS). The measurement period is adjustable. The results obtained show that at the measurement period of 0 seconds the heart beats obtained from this heart rate monitor compared favorably with those obtained from a standard Littmann's stethoscope, giving errors that range from 0.38% to.%. However, as measurement periods decreased, the errors increased. Due to the low power consumption of this heart rate monitor, it is suitable for use in the field. Keywords: Photoplethysmography, Cardio-Vascular, Heart rate Monitor, ATMEGA 3, GSM. 1. INTRODUCTION Among the major causes of death worldwide, cardio-vascular diseases (CVDs) are responsible for the largest number, with an estimate of 1. million deaths in 01representing 31% of all deaths globally (WHO, 01). In Nigeria, cardiovascular diseases rank fourth among the most prevalent diseases in the country (Health Profile Nigeria, 01). Oguoma et al. (014) pointed to lack of facilities for screening at early stages as one of the major issues affecting developing countries as it relates to fatality from these cardio-vascular diseases. The heart of an adult human being generally beats at around sixty to one hundred beats per minute when at rest (Paradkar and Chowdhury, 01). Three major conditions associated with heart rates are Bradycardia, Tachycardia and Arrhythmia. Bradycardia is defined as a condition where theresting heart beat rate is less than 0 beats per minute. It is also called Athletic Heart Syndrome as it is common among very fit athletes and is not necessarily an adverse condition of the heart. There are, however, medical conditions such as hypothyroidism that are associated with Bradycardia (Wikipedia, 01). Tachycardia on the other hand is the condition where the heart beat rate of the individual goes above the normal range and is often associated with blood pressure, stroke, anemia and hyperthyroidism pathologically. Short term causes include fever, pregnancy, anxiety or stress. Arrhythmia is an abnormal heart rhythm. Heart Rate Monitors (HRM) are devices which check the heart rate of an individual and give the appropriate results. The earliest Heart Rate measuring device is the stethoscope. Heart rate monitoring devices now use sensors and electrodes to measure the heart rates in real time. The earliest models of the wireless heart rate monitor consist of a monitoring box with electrodes attached to the chest (Wikipedia, 01). Ibrahim and Buruncuk(00) designed a simple, low cost Heart Rate measuring device with LCD output using infrared optical sensors and a PIC1F84 Microcontroller. The results were tested against manual measurements and were seen to be satisfactory. However, Fallow(01) showed in a report that the Green Light Emitting Diode (LED) with wavelength of 30nm offers a better resolution during heart rate measurements when compared to the Red LED (30nm) and the infrared LED (880nm).

2 9 Hrushikesh et al.(010) also designed a PPG based, low cost microcontroller based heart rate monitoring device with temperature sensor which had an LCD output. The sensor however used a Light Dependent Resistor (LDR) as detector. The drawback in the design was the photo resistor sensitivity which is less compared to the photo transistor and photodiode. Babiker et al. (011) designed and developed a PPG microcontroller based heart rate monitor which made use of Discrete Fourier Transforms algorithm. The results showed a 1.4% error against the 4.1% error of the zero crossing technique. It also showed a high degree of accuracy when compared with ECG signals even under intense activity. The drawback was the absence of any indicator for Arrhythmia or Tachycardia conditions and a transmission medium for a medical personnel to interpret the measured results. Kara et al. (00) designed a low cost, small size, portable LCD screen ECG device with a phonocardiograph. The system had the advantage of small size, ease of use, and the ability to see heart beat waveforms and hear the heartbeat sounds simultaneously. The major drawback is in the ECG probes which are invasive and cost a lot more than PPG sensors. The system designed in this project offers the following advantages over most existing models: i. Use of Green LED which has a better resolution than other models using Red and Infrared LED according to the study by Fallow (01). ii. Use of GSM helps the system to communicate with a physician even if they are not present. iii. The system uses a buzzer as an indicator for Tachycardia condition. iv. It has an adjustable measurement period (from to 0 seconds with an interval of seconds). MATERIALS AND METHODS.1. Operation of the System The system designed in this project uses a Green LED (0 nm) and a VTT111 phototransistor (VTT Phototransistor Datasheet, 01) in its sensor unit, an LM38 (LMx8-N Datasheet, 014) dual operational amplifier to filter and amplify the signal from the sensor and an ATMEGA 3 (ATMEGA 3 Datasheet, 010) microcontroller as the main processing unit and the LCD, GSM Module and Buzzer as the output units. The sensor picks the pulses from the finger as a result of the systolic and diastolic phases of blood flow in the finger artery and sends them to the LM38 dual operational amplifier through a passive high pass filter. This is to remove the steady part of the signal which is due to bone mass, and nonpulsating blood as well as interaction with other tissues. The pulse is then filtered using an active low pass filter to remove noise and other interferences before being amplified. The ATMEGA 3 microcontroller receives the signal, processes it and gives the output in beats per minute (BPM) on the LCD. It also sends the result to the GSM module through a MAX 3 Integrated Circuit (MAX 3xDatasheet, 01) to the preprogrammed phone number. The buzzer is activated when the heart rate goes above the stipulated range off 100 BPM. The system has an adjustable measurement period because according to Kobayashi (013) pulse rate accuracy relates to the duration of measurement... System Architecture The block diagram of the Heart Rate Monitor is depicted in figure 1 and is subdivided into four stages: The Input, Signal Conditioning, Processingand Output. Mobile handset Photoplethysmographic sensor Filter/Amplifier Microcontroller GSM Modem Buzzer LCD Keypad INPUT STAGE SIGNAL CONDITIONING STAGE PROCESSING STAGE OUTPUT STAGE Figure 1: Block Diagram of the Heart Rate Monitor

3 Input Stage The input stage consists of the PPG sensory unit (which is made up of the Green LED and the VTT111 phototransistor and a customized - button Keypad which has the and keys for inputs and three others keys for " Menu", "Reset" and "Enter" instructions. The green LED is connected via a current limiting resistor R d through a supply side by side with the phototransistor, which is biased through the resistor R t. The heart rate signal is received at the collector of the phototransistor as shown in Figure. Equations for calculating the LED current limiting resistor R L and biasing resistor R are R = V L R = V a Rd Rt to filter circuit LED NPN PHOTOTRANSISTOR Figure : The Sensor Unit... The Signal Conditioning Stage The Signal Conditioning Stage involves the use of LM38 in two stages to filter and amplify the signals from the sensor. A passive high pass filter is designed using a capacitor in parallel with a resistor to remove the non-pulsating components of the signal and an active low pass filter is used to remove the noise and interference signals. The LM38 is used to amplify the signal with a gain of 101 for each operational amplifier stage. The equations for the cutoff frequencies for the filters shown in Figure 3 are given by = = L = L = π R a (3) π R a (4) πc R a () π R a () R R R3 C R C4 From the Sensor C LM38 1 C3 8 4 LM38 to the Microcontroller R1 R4 Figure 3: The Signal Conditioning Stage

4 E International Journal of Advanced Engineering Research and Technology (IJAERT) Processing Stage The ATMEGA 3 microcontroller is used to handlethe processing of the signal. It receives the signals, counts the pulses using its internal Timer and gives the output in beats per minute (BPM), using the relation R i B M = T l _c () Where T p l is the desired measurement period and _c is the number of pulses counted from the Conditioning stage. The measurement period is set by the user. The output is sent to the LCD and SIM 900 via the MAX 3 Integrated circuit for transmission. The microcontroller also sends AT commands to the Module along with the programmed cellular phone number for transmission. The ATMEGA 3 sends a signal to the Buzzer if the Heart Rate exceeds 100BPM...4. Output Stage The 1 LCD displays the value of the Heart Rate in BPM;it also displays the instruction and initialization processes of the system. The LCD is connected in a 4-wire mode. The SIM 900 GSM module receives the heart rate values from the microcontroller via a MAX 3 IC which is used to convert logic levels from the TTL to the RS3 protocol that is handled by the SIM 900 module. AT commands are transmitted to the SIM 900 module to send the received heart rate value to the programmed cellular phone number. The Buzzer is programmed to activate when the Heart rate goes above 100BPM. The Buzzer is driven via a BC4 NPN transistor. The complete schematic diagram of the heart rate monitor is shown in figure 4. LCD1 LM01L BUZ1 BUZZER Q VSS VDD VEE RS RW D0 D1 D D3 D4 D D D BC4 R 10k R 10k RESET 10K LEFT ENTER MENU RIGHT pf C C pf FREQ=8MHz U1 9 RESET 13 XTAL1 1 XTAL 40 PA0/ADC0 39 PA1/ADC1 38 PA/ADC 3 PA3/ADC3 3 PA4/ADC4 3 PA/ADC 34 PA/ADC 33 PA/ADC 1 PB0/T0/XCK PB1/T1 3 PB/AIN0/INT 4 PB3/AIN1/OC0 PB4/SS PB/MOSI PB/MISO 8 PB/SCK ATMEGA3 Vcc GND PC0/SCL PC1/SDA PC/TCK PC3/TMS PC4/TDO PC/TDI PC/TOSC1 PC/TOSC PD0/RXD PD1/TXD PD/INT0 PD3/INT1 PD4/OC1B PD/OC1A PD/ICP1 PD/OC AREF AVCC C1+ C1-11 T1IN T1OUT 1 R1OUT R1IN 10 TIN TOUT 9 ROUT RIN VCC MAX3 VS+ VS- GND GSM MODEM K 0K K 0K C+ C- 4 D4 0 39K.uF LM38 1.uF 8 4 LM38 NPN R1 BC4 100K 4K 10k PHOTOTRANSISTOR Figure 4: Complete Schematic Diagram of the Heart Rate Monitor.3. Software Design and Flowcharts The program for the heart rate monitor was written using the C programming language in the ATMEL STUDIO.0 (ATMEL, 01) environment. The hardware design was simulated using Proteus 8.4 (Labcenter Electronics, 01).The flowchart for the program of the heart rate monitor is shown in Figure.

5 00 Start Configure I/O Ports, ADC, Timer, Initialize LCD and GSM Modem Is desired period 0 sec? YES NO Set Measurement Period LCD Displays "PRESS ENTER TO PROCEED" LCD Displays "INSERT FINGER AND PRESS ENTER" "ENTER"key turns sensor on Countpulses Received Calculate heart rate, HR, in BPM HR = 0 P i l NO Is Heart Rate <100? YES Display HR(BPM) on LCD NO Activate Buzzer Give AT+COMMAND To GSM to Send HR through SMS to GSM Figure : Flowchart of the program for the heart rate monitor 3. RESULTS The PPG based heart rate monitor that has been designed was constructed in the laboratory. Figure shows the internal circuit of the construction. The setup, process, measurement, and sending of the results to a cellular phone are shown in figure. The heart rate monitor was tested on 1 random test subjects (age range between 0 to 4 years) for four different measurement intervals (0 seconds, 30 seconds, 0 seconds and 10 seconds). Three measurements were done for each interval. The results were averaged and compared with manual measurements made using a Littmann's stethoscope. The percentage error was calculated using the relation: % E = R h R i R h % (8) The results of measurements and calculation of errors are shown in Table1

6 01 Table 1: Calibration of PPG Heart Rate Monitor against Littmann s Stethoscope Heart Rate Average Heart Rate in (BPM) BPM for given intervals (in %Error % % % S/No Test Stethosco seconds) for PPG Heart at 0 s Error at Error Error at Subject pe Rate Monitor period 30 s at 0 s 10 s period period period 1. User User User User User User User User User User User User User User User Figure : Complete Internal View of the Circuit of the Heart Rate Monitor

7 0 Figure : Heart Rate Measurement and Screenshot of Result sent to Mobile Phone 4. DISCUSSION The resultsin table 1 show an error range of 0.38% -.% for 0 - second measurement period, 1.03% -.1% for 30 - second measurement period, 0% -.81% for 0 - second measurement period and 1.3% -.04% for 10 - second measurement period. The results show that the designed PPG Heart Rate Monitor is most accurate using the default measurement period of 0 - seconds even though the other measurement periods gave acceptable results. The results also show that the accuracy of the measurement tends to decrease with reduced measurement time.. CONCLUSION The PPG based heart rate monitor was designed and implemented successfully. Heart rate measurements were taken and results were sent to a programmed phone number. The heart rate monitor that has been designed is suitable for use by a medical personnel to monitor his patients who are located far from him through the mobile network. REFERENCES [1] WHO, Cardiovascular diseases (CVD): (accessed 0th July, 01) [] Health Profile: Nigeria: (accessed 1 st July, 01)

8 03 [3] Oguoma, V.M.,Nwose, E.U., and Bwititi, P.T.,(014). Cardiovascular Disease Risk Prevention: Preliminary Survey of Baseline Knowledge, Attitude and Practices of a Nigerian Rural Community. North American Journal of Medical Sciences, Vol. (9): [4] Paradkar, S.N., and Chowdhury, S.R.,(01). Fuzzy Entropy based Detection of Tachycardia and Estimation of Pulse Rate through Fingertip Photoplethysmograph. Journal of Medical and Bioengineering, Vol. 4(1): pp [] Heart Rate: (accessed th May, 01) [] Heart Rate Monitor: (Accessed, August 3 rd, 01). [] Ibrahim D., and Buruncuk K.,(00). Heart Rate Measurement from the Finger Using a Low- Cost Microcontroller. Article of Faculty of Engineering, Near East University, Turkish Republic of Northern Cyprus. [8] Fallow B. A.,(01). Influence of Skin Type and Wavelength on Light Wave Reflectance. Thesis presented to Faculty of the Graduate School, The University of Texas at Austin. [9] Babiker, S. F., Abdel-Khair, L. E., and Elbasheer, S. M.,(011). Microcontroller Based Heart Rate Monitor using Fingertip Sensors. University of Khartoum Engineering Journal (UofKEJ), Vol. 1 (): pp 4-1. [10] Hrushikesh, P.V.R.R., Raman, H.R., and Sri Harsha, U.,(010). Heart Rate Monitor & Temperature Measuring Device.Project Report,Department of Bio-Medical Engineering, GokarajuRangaraju Institute of Engineering and Technology, Hyderabad. [11] Kara, S., Kemaloglu, S., and Kirbas, S.,(00). Low-cost compact ECG with Graphic LCD and Phonocardiogram System Design. Journal of Medical Systems, Vol. 30 (3): pp [1] LMx8-N Low-Power, Dual-Operational Amplifiers datasheet. (014).Texas Instruments Incorporation, 014. [13] VTT111H Phototransistor, PerkinElmer Optoelectronics p.103, (Accessed, 1 th August, 01) [14] ATmega3 Data Sheet. (010). Atmel Corporation, 010. [1] MAX3x Dual EIA-3 Drivers/Receivers datasheet.texas Instruments Incorporation, 01. [1] Kobayashi, H.,(013). Effect of measurement Duration on Accuracy of Pulse-counting. Ergonomics, Vol. (1): pp [1] Atmel Studio (Version: ). Atmel Corporation, 01. [18] Proteus Design Suite 8.4, Release 8.4 SP0 (build 109) with Advanced Simulation. Labcenter Electronics, 01.