Analysing the effects of cold, normal, and warm digits on transmittance pulse oximetry

Non-invasive estimation of arterial oxygen saturation (SpO2) and heart rate using pulse oximeters is widely used in hospitals. Pulse oximeters rely on photoplethysmographic (PPG) signals from a peripherally placed optical sensor. However, pulse oximeters can be less accurate if the sensor site is relatively cold. This research investigates the effects on PPG signal quality of local site temperatures for 20 healthy adult volunteers (24.5 ±4.1 years of age). Raw PPG data, composed of Infrared (IR) and Red (RD) signals, was obtained from a transmittance finger probe using a custom pulse oximeter (PO) system. Three tests were performed with the subject’s hand surface temperature maintained at baseline (29 ±2°C), cold (19 ±2°C), and warm (33 ±2°C) conditions. Median root mean square (RMS) of PPG signal during the Cold test dropped by 54.0% for IR and 30.6% for RD from the baseline values. In contrast, the PPG RMS increased by 64.4% and 60.2% for RD and IR, respectively, during the Warm test. Mean PPG pulse amplitudes decreased by 59.5% for IR and 46.1% for RD in the cold test when compared to baseline, but improved by 70.1% for IR and 59.0% for RD in the warm test. This improvement of up to 4x in signal quality during the warm condition was associated with a closer match (median difference of 1.5%) between the SpO2 values estimated by the PO system and a commercial pulse oximeter. The differences measured in RMS and mean amplitudes for the three tests were statistically significant (p < 0.001). Overall, warm temperatures significantly improve PPG signal quality and SpO2 estimation accuracy. Sensor site temperature is recommended to be maintained near 33°C for reliable transmittance pulse oximetry

Sensor de fotopletismografia por reflexão sem fios: projeto e desenvolvimento de hardware

Nos anos oitenta do último século começaram a surguir oxímetros wearable que se estabeleceram como um standart para a monitorização da saturação de oxigénio no sangue e actividade cardíaca, de forma não intrusiva. Os referidos oxímetros medem a percentagem de hemoglobina totalmente saturada com oxigénio (SPO2), transmitindo luz com comprimentos de onda diferentes, vermelha e infra-vermelha, através dos tecidos. Os dispositivos wearable atuais são frequentemente desenhados de forma modular, em que o módulo de medição e de display são integrados num único dispositivo. O armazenamento e tratamento de dados é difícil uma vez que são dispositivos de tamanho reduzido; baixo consumo de energia; baixo custo e baixa capacidade de processamento de dados. Tendo em conta que a quantidade de dados recolhidos é relativamente baixa, a sua transmissão de forma wireless é conveniente. Nesta dissertação é desenvolvido e testado um oxímetro de pulso em modo refletivo capaz de cálcular a saturação de oxigénio no sangue, o batimento cardíaco e enviar os dados de forma wireless para outros dispositivos. O hardware desenvolvido engloba quatro módulos funcionais: fonte de alimentação constituída por um conversor DC-DC e um regulador de tensão linear, circuito de carga e monitorização da bateria que controla os ciclos de carga e descarga da bateria, um módulo de rádio frequência que permite que o oxímetro comunique com outros dispositivos de forma wireless e um microcontrolador responsável por gerir todas as comunicações e pelo processamento de sinal. O sofware desenvolvido divide-se em duas partes: uma interface gráfica escrita em Matlab que permite a comunicação entre o computador e o oxímetro e o firmware do microcontrolador que engloba todos os algoritmos de cálculo do SPO2, do batimento cardíaco, drivers de periféricos, gestão das comunicações e aquisição e processamento dos dados.Ever since the early 80s from the last century, wearable oximeters appear as the established standard for non-invasive monitoring of arterial oxygen saturation (SpO2) and heart activity wearable oximeters can monitor arterial SpO2, which is the percentage of arterial hemoglobin that is fully saturated with oxygen, by transmitting red and infrared light through the finger, where it is sensed. The current wearable oximeters are frequently designed as single modular devices, namely, the measurement and display modules are integrated on a single device, which are responsible for several problems. Such devices lack effective data management functions and by being limited by size, power consumption and cost, advanced operating systems cannot be embedded to such wearable oximeters, making difficult to store and manage data. Bearing in mind that the amount of data pulse wave signal collected is small, transmit it wirelessly is convenient and effective. In this thesis a reflective pulse oximeter is developed and tested capable of assessing the oxygen blood saturation (SpO2), the heart rate and send the acquired data through wireless communication to other devices. The developed hardware comprises four functional modules: the power supply made of a DCDC converter and a linear voltage regulator, the charging circuit and battery monitoring system which controls the charging and discharging cycles of the battery, a radio-frequency module that allows the device to connect through wireless communication to other devices and a microcontroller responsible for the management of the communications and for the signal processing. The software developed in this thesis is made of two parts. One being the Matlab graphical interface that allows the communication between the oximeter and the PC while the other one being the microcontroller which comprises all the algorithms of SpO2, heart rate, management of the communication, drivers, and data acquisition and processing