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A High-Performance Wireless Reflectance Pulse Oximeter for Photo-Plethysmogram Acquisition and Analysis in the Classroom

Abstract

Pulse oximetry is an essential health-monitoring technique in both clinical and home care environments. From an engineering education perspective, pulse oximeter technology offers excellent study material in areas such as light-based sensor construction, embedded system design, control theory, and digital signal processing. However, off-the-shelf pulse oximeters do not provide suitable educational platforms for several reasons: (a) their design layouts and internal data flows are inaccessible to the user, (b) units that display photo-plethysmographic/ pulse waveforms or make those signals available to the user provide data that have already been filtered in an unspecified manner, and (c) sensor sites are usually constrained to fingertips, ear lobes, or other locations for which commercial transmittance probes are already constructed.

This paper presents the novel design and initial application of a high-performance, wireless reflectance pulse oximeter that exhibits a filter-free design and offers full access to relevant waveform data. A unique embedded control system extracts the DC (baseline) and AC (time- varying) signal components from the original reflectance signal without the use of any analog filter circuitry, ensuring high-fidelity pulsatile waveforms that can offer several thousand peak- to-valley digitization levels. All red and near-infrared signal data can be stored on the unit or transmitted in real-time to a personal computer via a wireless ZigBee or wired USB link mapped to a serial port. Sampling frequencies of 240 Hz or higher can be easily realized for all waveform channels simultaneously, so all signal information is retained, and ambient noise (e.g., 120 Hz flicker from room lights) is unaliased. These data are logged and displayed graphically by a MATLAB interface, which also incorporates a communication control panel and a set of pre- defined signal processing modules for signal compensation, real-time filtering, fast Fourier transformation, and oxygen saturation level calculation.

The pulse oximeter and MATLAB interface were initially employed in a Fall 2009 biomedical instrumentation laboratory. For this laboratory, students viewed photo-plethysmograms from various body locations, applied linear-phase filters for artifact removal, and determined cardiovascular parameters such as respiration rate, reflection index, and stiffness index. Overall, the students’ experience was highly positive, and all of the formal learning objectives were successfully addressed.

I. Introduction

A pulse oximeter is a medical device that can non-invasively yield cardiopulmonary data that are useful in both clinical and ambulatory care environments.1 Photo-plethysmograms obtained by the optical sensing circuitry in a pulse oximeter provide information similar to that offered by an electrocardiogram2 (e.g., pulse rate and respiration rate3) but can also be used to determine arterial blood oxygen saturation, blood pressure,4 and other vascular parameters.5, 6 For example, a reflection index correlates to endothelial function,5 and a stiffness index can provide an arterial stiffness measure that relates to pulse wave velocity.6

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Li, K., & Warren, S. (2010, June), A High Performance Wireless Reflectance Pulse Oximeter For Photo Plethysmogram Acquisition And Analysis In The Classroom Paper presented at 2010 Annual Conference & Exposition, Louisville, Kentucky. 10.18260/1-2--16621