TelehealthSolution: Portable Pulse Oximeter

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1 TelehealthSolution: Portable Pulse Oximeter The advancements in technology and connectivity open up previously unexplored avenues in the field of personal health monitoring. It is indeed true that prevention is better than cure. But In today s fast paced and busy lifestyle, it often becomes difficult to take out time for frequent health checkups. An early detection of any medical condition can result in timely preventive action and remediation. This is where portable healthcare solutions can make a meaningful impact by providing us the ability to monitor our health related parameters easily. These portable solutionscan be connected to Smartphones forsharingone s health parameters with Health Consultants and Hospitals on a secure network. The Health Consultant and Hospitals can recommend additional tests or schedule an appointment with the relevant specialist if any discrepancy is detected. The complete solution opens the concept of Telehealth as shown in Figure 1. Figure 1: Telehealth eco-system In this article, we talk about Pulse Oximeter designusingdynamicnfc/rfid (Near Field Communication /Radio Frequency Identification)tag.NFC tag used in healthcare designs provides additional features using Smartphones such as data acquisition and management. Pulse Oximeter:It allows the user to measure SpO2 (Saturation of Peripheral Oxygen) and Heart rate non-invasively. SpO2 is basically a measure of the blood s capability to carry oxygen to the remotest part of the body.this measurement is important for high altitude mountain climbers, athletes and people suffering from anemia. NFC:NFC (Near Field Communication)isan RFID (Radio Frequency Identification) basedwireless communication technology commonly available in many Smartphones. The term near field signifies a working distance of less than 4-5 cms. There are different RFID standards available for different applications such as animal tagging (ISO 11784/ISO 11785), item management (ISO/IEC18000), freight containers (ISO 23389), etc. STMicroelectronics Dynamic NFC/RFID tag (M24LRxxE) featuresi2c as well as NFC interface is compatible with ISO RFID standard.the ISO15693 is one of the prominent standards for NFC interface and has the added advantage of being readable from a greater distance as

2 compared to other proximity cards. It operates at a carrier frequency of MHz with data rate ranging from 106 Kbit/s to 424 Kbit/s. ST Android App:STMicroelectronics had developed a free android application called ST Health Care for NFC enabled Smartphone.This application allows the user to share their data via Internet, Bluetooth and SMS. The data can also be uploaded on Microsoft HealthVault account. This allows the health data to be available on internet and can be accessed by any authorized person at any time. This patient-centric concept of bringing the healthcare from the hospital to the patient at home is helping people to overcome the health challenges and improve quality of life. Pulse Oximeter HardwareDesign:Pulse Oximeter hardware block diagram as shown in Figure 2is a portable battery operated design based on STM32L. The on-chip USB interface, 12 bit ADC/DAC (Analog to Digital Converter/Digital to Analog Converter)and integrated RTC (Real Time Clock) with its low power consumption makes the STM32L ideally suited for this application. Analog Front End comprises TSV612A rail to rail input/output CMOS op-amp with low input bias current (1pA typically). The low input bias current op-amp is critical for accuracy when used as a trans-impedance amplifier to convert photodiode current into voltage. The internal ADC and DAC are used to measure the conditioned sensor output and to remove DC offset respectively. MCU converts the sensor data into SpO2 and Heart rate information and displays it on a 128x64 OLED (Organic Light Emitting Diode) display. Figure 2: Hardware Block Diagram

3 The power management section comprises STC4054 for Li-ion battery charging using USB and two STLQ015XX30 ultra low quiescent current LDO (Low Dropout) regulators for powering the analog and digital sections independently. The standby consumption of the circuit with OLED is approx.250ua out of which OLED consumption was measured approx. 243uA. The STM32L low power MCU and rest of the circuit has standby consumption of approx. of 7uA. Working Principle: Pulse Oximeter uses photo plethysmography probe comprising red andir (Infrared)LEDs (Light Emitting diodes)on one side and a photodiode on the opposite side. The probe is placed on the periphery of body, most commonly on a finger or a toe to measure SpO2 and Heart rate. Red and IR LEDs are used to pass light through the fingertipwhile the photodiode measures the transmitted light. The Red and Infrared LEDs are pulsed alternatelyand the same photodiode is used to measure the light intensity of both LEDs. The arterial and venous blood has different light absorption characteristics at different wavelengths. As shown in Figure 3, Oxy-hemoglobin (HbO2) absorbs more infrared light (940 nm), whereas Deoxy-hemoglobin (Hb) absorbs more red light (660 nm). The transmitted light received by the photo-detector changes with respect to time because of the pulsatile nature of blood flow. Figure 3: Hemoglobin light absorption The light absorbed by the fingertip can be attributed to the bloodless tissues, the venous blood and the arterial blood. The volume of arterial blood changes with pulse which in turn changes the absorption of light.the Photodiodereceivesa large DC signal (99%) representing the absorptionof light by bloodless tissuesand venous blood whilea small portion of AC signal (approximately 1%) represents pulsating arterial blood(although some constant level of arterial blood is also a part of DC offset).the signal obtained from the ADC is used to generate a proportional DAC signal to remove the DC offset. The AC component of the signal is further amplified to ensure good SNR (Signal to Noise Ratio) when measured using the ADC. This AC signal needs further filtering of 50Hz/60Hz line and other high frequency noise. This filtering can be realized in either analog or digital domain.since the IR and red LEDs are multiplexed at approximately 1 khz, and share the same photodiode to measure both signal, we cannot use low pass filters tuned to lower frequencies to filter out the 50/60Hz AC noise. Therefore we would require two

4 individual signal conditioning circuits after the photodiode signal is demultiplexed to ensure proper filtering. Whereas the use of FIR (Finite Impulse Response) digital filters reduces hardware complexity without adversely affecting measurement accuracy. The ratio of measured red and IR signals (both AC and DC) are used to calculate the value of SpO2 as per the formulae below: = ACrms(660nm)/DC(660nm) ACrms(940nm)/DC(940nm) IR light due to its higher absorption to arterial blood gives higher variation and better SNR for heart rate calculation. Heart rate is calculated by measuring the time duration of a fixed number of peaks in the AC signal and interpolating it to BPM (Beats per Minute). The measured SpO2 and Heart rate values arestored into the Dynamic NFC/RFID tag memory using I2C interface. Users can then retrieve thishealth data stored in memory using the ST Android App on their NFC based Smartphones.The Pulse Oximeter demo board is shown infigure 4. The OLED display, joystick and SpO2 probe connector can be seen in the top view while the Dynamic NFC/RFID tag daughterboard can be seen in the bottom view. Figure 4: Pulse Oximeter PCB top and bottom view Android Application: To complete the Telehealth communication chain, STMicroelectronics had developed a free android application called ST Health Care which can be downloaded from the Google Play Store ( application enables Android Smartphones/Tablets featuring NFC interface tocommunicate with Dynamic RFID/NFC Tag (M24LRxxE).Figure 5describes the architecture of this application..

5 Figure 5: Android Application Architecture The App allows users to createe their personalized account with unique username and password. The Login Credentials once created are used to authenticate the user and ensure security of personal health information. RTC Date & Time can be configured using the application along with the Manufacturer details. Date Time of the actual event is synchronized with the board before recording SpO2 readings through NFC interface. Reading & Sharing Health Data from device using Application Configuration Data It includes the configuration of data like device name, manufacturer, model, etc. This needs to be done only once for any health device.

6 Figure 6: Pulse Oximeter android app default screen Health Data: Application can be used to read the health data recorded in device by bringing the NFC enabled mobile phone in close proximity. Application displays the reading of SpO2 and Heart rate with date and timefrom Dynamic NFC/RFID tag as shown infigure 7.It can be shared with doctors at the same time via short message service (SMS), Bluetooth, electronic mails or secure healthcare database websites such as Microsoft HealthVault. The application also maintains a secured local database of patient health data measured with the device. Microsoft HealthVault is a web-based platform from Microsoft to store and maintain health and fitness information. This website is accessible at and addresses both individuals and healthcare professionals. Microsoft HealthVault service is based on license agreement. The OEM has to sign the license agreement with Microsoft to allow their application to interact with the database. Figure 7: Pulse Oximeter Real-Time Data

7 Multiple readings saved over time can be viewed in the plot or char from. The application currently supports two plots: SpO2 vs.date-time Heartrate vs.date-time Figure 8: Snapshot of plot and history for SpO2 and Heart rate Conclusion: Portable Personal Healthcare devices enable convenient monitoring of user health parameters on a regular basis. Although they do not replace the specialized clinical equipment, they can be effective tools for screening and early diagnosis. For patients suffering from certain chronic diseases, they can reduce the possibility of key health parameters going critically low or high and remain unnoticed.this can significantly improve their quality of life. Taking advantage of STMicroelectronics wide product portfolio, several portable personal healthcare designs with NFC connectivity have been developed. Compatible free Android App ST Health Care provides secure health data management and Microsoft HealthVault integration to demonstrate the complete Telehealth ecosystem. This App supports Pulse Oximeter, IR Thermometer and Iontophoresis reference designs. These designs can be further used as a starting pointfor developing other portable personal healthcare devices.