Developing a Portable Wireless Physiology Monitor Using LabVIEW PDA

Author(s):
Thijs Schrama - Faculteit Sociale Wetenschappen

Industry:
Medical/ Medical Instrumentation

Products:
LabVIEW PDA Module, LabVIEW

The Challenge:
Developing a portable device to measure, display, and store physiological parameters, such as electrocardiogram (ECG) signals, and transfer them over a wireless connection.

The Solution:
Using National Instruments LabVIEW software to develop a small ambulant monitoring system capable of meeting all of our requirements and offering significant advantages over the previous system.

Several research groups at the Leiden University Faculty of Social Sciences conduct psychophysiological research. In this type of research, researchers take physiological measurements from participants during psychological research. For example, researchers may measure the participants’ physiological responses during a memory test, during an emotionally stressful task, or during real-life stressful situations at home. During these experiments, researchers may simultaneously measure the following physiological responses, among others:

• ECGs measure the electrical activity of the heart.
• Galvanic Skin Response (GSR) measures the electrical impedance of the skin.
• Breathing or respiratory rate measures the number of breaths a person takes per minute.
• Impedance cardiogram (ICG) measures the chest cavity impedance using external sensors.

These signals represent different ways to measure the physiological state of a person and can be influenced by levels of concentration and arousal. For example, the GSR is associated with activity in the sympathetic nervous system and with emotional arousal, such as fear and anger. If someone is presented with a difficult question, heart rate and breathing rate may go up and skin resistance may change.

Currently, off-the-shelf equipment is available to measure physiological responses. However, this equipment has several drawbacks. Most devices do not have online display capabilities. This may result in unnoticed bad signal integrity if electrodes become disconnected. Moreover, some of these devices transmit signals via an analog radio link to the base station, where it is then digitized. This leaves the signal vulnerable to degradation during the analog radio transmission.

Currently, commercially available personal digital assistants (PDAs) combine processing power, displaying capabilities, and digital wireless connectivity. They provide a perfect platform for taking measurements with on-screen signal visualization and digital wireless data transfer at a low cost.

The system we developed contains an online graphical display and digital transmission of the full signals. This means that researchers can check the signal integrity on the PDA screen and store the entire true waveform of the signals on both the PDA and the base station for later offline analysis. To facilitate offline analysis, researchers can synchronize experiments with data because the system allows them to place and store markers with the data.

The system implementation uses a commercially available HP Pocket PC with NI LabVIEW, the LabVIEW PDA Module, an NI CF-6004 CompactFlash data acquisition card, and an ECG front-end signal conditioning unit that we built in-house.

Being portable and battery supplied, the system is not connected to the 230 V power grid in any way. The signal conditioning unit provides further insulation and prevents participants from being exposed to electrical currents. The signal is then fed to the CF-6004. This card is set up to scan its analog inputs at 200 S/s, at 14 bits, which is sufficiently fast and accurate for all psychophysiological measurements of interest.

The PDA application is written within the LabVIEW PDA Module and consists of three tasks.The first task takes care of data acquisition (producer loop) and stores the data in a queue. The second task sets up a wireless connection to a base station, such as a desktop or laptop computer, and transmits the data to the base station. If the connection is lost, the data is logged to memory until the connection is restored. The third task stores all the data locally to the PDA’s memory, which can be 128 MB on-board memory or 512 MB secure digital memory. Each measurement requires up to 8 MB of memory.

We sped up application development by using a simulated data acquisition function call during the development stage. This considerably improves the compiling time when targeting the PDA. For the final application, we then needed to replace only one function call to measure live signals.

Because PDAs have a small display, we designed a tabbed user interface display. Settings and Markers tabs are available to start and stop measurements and place different timestamps. Four tabs are available to display each of the four analog signal traces of the captured data directly in a graphical view and check signal integrity. An indicator displays wireless connection status, file I/O status, and data acquisition task status.
 
The data is stored on the device and in the base station directly as a binary value (I16). We did this to keep the file size small as well as to enhance compatibility with existing signal analysis tools.

Conclusion and Future Work

The small ambulant physiological monitoring system has several advantages over existing equipment. The portable device features:

• Direct online display of the measured ECG, GSR, breathing rate, and ICG data
• Onboard logging to file
• Wireless digital transmission of the full data stream to a host computer
• Lossless reconnect capability when the wireless link is lost
• Considerable cost saving over comparable systems, which do not contain online graphical displays or digital telemetry of the signals being measured

After initial tests we plan to use multiple devices in different departments of our university. Our future research may focus more on offline data analysis using tools such as LabVIEW.

For more information, contact:

Thijs Schrama
Faculteit Sociale Wetenschappen
Wassenaarseweg 52
2333 AK Leiden
Netherlands
Tel: +31 (0)715271337
Fax: +31 (0)715273712
E-mail: tschrama@fsw.leidenuniv.nl
Web: http://socialsciences.leidenuniv.nl/

 

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