We are investigating the use of a pulse oximeter as one of the main physiological sensors. The pulse oximeter is a simple, off-the-shelf, noninvasive and robust sensor that has been validated in the real world medical domain for two decades. Pulse oximetry relies on the different spectral properties of oxygenated hemoglobin (HbO_2 ) and reduced hemoglobin (Hb) to measure oxygen saturation (SpO_2 ). SpO_2 is derived in standard pulse oximeters from the relative absorption of light from a red and an infrared LED. The photoplethysmogram, commonly referred to as the "pleth waveform", is usually only derived from one of these two signals, and is an indirect measurement of blood volume under the sensor. The temporal behavior of this signal is influenced by the cardiac and respiratory cycle.

Besides providing the standard measurement of blood oxygen saturation, with further signal processing, the pleth waveform has the potential for measuring several other key physiological parameters, including instantaneous heart rate, heart rate variability, respiratory rate, and sympathetic tone. These measurements could then be used to monitor depressed breathing, fatigue, air way obstruction and hemorrhage.

We are in the process of integrating an ear clip pulse oximeter sensor into an earpiece that will combine a wireless transmitter, a bone microphone and speaker for communication purposes in addition to having an "I'm hurt" or "911" button. Data from the sensor would be preprocessed by hardware and software on the ear piece and relevant information would be sent up the chain of command.

Another source of physiological data is the qualitative data entered by the responders themselves. A responder who is tasked with triage can input the results of his or her casualty assessments, adding this qualitative analysis to the data coming from the pulse-oximeter. Alternatively, a person wearing the device may press the "911" button to trigger the assessment algorithm. The information from the human "sensor" is both sent directly to the commander on the scene for his or her evaluation and incorporated into the software assessment algorithm.