Pulse oximetry non-invasively measures blood oxygen saturation (SpO2) and heart rate using a photo sensor to track the amount of absorbed light emitted by a red and infra-red LED. It allows for the rapid diagnosis of hypoxemia (low blood oxygen levels), which is difficult to identify clinically until the blood turns blue (a late sign). Since becoming the standard of care during anesthesia in the early 1980s, pulse oximetry has extended throughout much of the hospital, greatly improving patient safety due to early detection of clinical deterioration. This can rescue a patient from the permanent effects of lack of oxygen, such as brain damage or death.

Fig. 1 – The phone oximeter is designed to be used by non-specialist healthcare workers to monitor pulse oximetry across most common mobile phone platforms. (Credit: 2011 Electrical and Computer Engineering in Medicine, The University of British Columbia, Vancouver, Canada)

This life-saving technology has many other potential applications for both monitoring and diagnosis of medical conditions. The pulse oximeter produces a graphical display of blood volume changes (the photoplethysmograph [PPG]). The PPG is currently used by clinicians to visually identify signal quality (artifact identification) and perfusion levels, but advanced signal processing methods have the potential to extract many other clinically important parameters from PPG, such as respiratory rate (RR), heart rate variability (HRV), pulse pressure variation (PPV) and capillary refill time (CRT), all of which are key to the diagnosis and management of respiratory disease.

Lack of training and resources, however, has limited the uptake of pulse oximetry across much of the developing world, where death rates from anesthesia remain 100–1000 times higher than in the developed world, mostly due to preventable causes related to airway and respiratory problems leading to hypoxemia.

Key to wider adoption of this life-saving technology is the availability of affordable, robust pulse oximeters that can be used by non-specialist healthcare workers. Pairing pulse oximeters with ubiquitous mobile phones will catapult the assessment and management of respiratory disease from the hospital into non-hospital settings. The inherent computing power of the mobile phone, its peripheral resources (LCD display; audio, serial and USB connectivity, wireless networking, battery) and everyday availability offer the opportunity to create a low-cost standalone device that can even be used by patients at home. The real-time wireless communication of results (no delay in interpretation, or response to critical events) offers a distinct advantage over traditional pulse oximeters. Engineers at the University of British Columbia are developing just such an intelligent mobile device, called the Phone Oximeter.

Currently, the Phone Oximeter combines an FDA-approved pulse oximeter probe and electronics module (Nonin Medical, Plymouth, MA) with an iPod Touch (Apple Inc., Cupertino, CA). The software implementation is designed to allow the majority of development to be performed outside of the proprietary software development kit (SDK) frameworks. This maximizes portability across the most common mobile phone platforms currently available.

Fig. 2 – The prototype combines an FDA-approved pulse oximeter probe and electronics module with an iPod Touch. The next generation will eliminate third-party hardware and incorporate all signal processing on-board the mobile phone. (Credit: 2011 Electrical and Computer Engineering in Medicine, The University of British Columbia, Vancouver, Canada)

The algorithms are implemented in C and Scheme, and the graphics rendering is performed in OpenGL, using only thin platform-specific wrappers to integrate with the underlying operating systems. The benefits are: 1) use of compiled C binaries will ensure that the algorithms maximally utilize the limited processing power of the devices; 2) OpenGL hardware acceleration contained within the devices will greatly reduce the processing overhead associated with the graphical interface; and 3) ubiquity of C and OpenGL software elements in modern computing devices will make the prototype system easily portable to many other current and future platforms, and capable of leveraging a large body of existing and proven signal processing and interfacing algorithms. The developed software application compiles to highly optimized ANSI-C. The application currently compiles on the principal OS like Mac OS X, Windows, Linux, and OpenBSD, and several of the most widely used mobile platforms including iOS, Android, and Maemo (Nokia). Ports to Symbian, currently the most widely used phone OS, and Windows Mobile platforms are in progress.

Novel algorithms have been developed for the computation of respiratory rate, previously not available on pulse oximeters. This relatively low-cost, robust prototype pulse oximeter device conveys the quality and trend of physiological data (oxygen saturation, heart rate, respiratory rate) over time through its intuitive user interface. The ease of use, presentation of warning signals generated by the decision support engine, and reliance on symbols mean that it can aid any clinician, regardless of language, in detecting clinical events and making clinical decisions. This prototype has undergone usability evaluation of the graphical interface in Vancouver, Canada and Kampala, Uganda.

The next generation of the Phone Oximeter will reduce costs further by eliminating 3rd party hardware and incorporating all signal processing onboard the mobile phone. The current decision support engine will be extended to include the integration of clinical expertise and will allow non-specialist healthcare workers to monitor with confidence.

This technology was done by the Electrical and Computer Engineering in Medicine at the University of British Columbia, Vancouver, Canada. For more information, visit http://www.phoneoximeter.org and http://ecem.ece.ubc.ca.