Tech Briefs

A reliable hypoxia early detection and warning system would allow aircrew to initiate counter-measures before their performance is significantly degraded.

Symptoms of hypoxia – a deficiency in the amount of oxygen reaching body tissues - have been documented among rotary-wing pilots and aircrew flying at altitudes as low as 8,000 feet. Effective hypoxia-related mishap prevention relies upon rapid recognition of hypoxia symptoms and expeditious execution of emergency procedures. This is particularly challenging in rotary wing aircraft, where the lack of adequate training makes reliance on hypoxia self-detection an ineffective solution. An automated warning would be preferable, but currently no military aviation platform is outfitted with a physiological monitoring system to alert pilots and aircrew of impending hypoxic episodes.

Subjects donned an aviation flight mask connected to the Reduced Oxygen Breathing Device (ROBD), and were then instrumented with a forehead reflectance pulse oximeter.
The monitoring of hemoglobin saturation (SpO2) with a pulse oximeter is the standard of care in clinical settings. Limitations exist, however, for peripherally placed pulse oximeters. In response to the limitations, the commercial market has introduced pulse oximeters that are placed on the forehead, providing a more central measure of blood hemoglobin oxygen saturation. Previous studies have reported that the forehead sensors are as accurate as finger-mounted oximeters with fewer technical disadvantages. These technological advancements should enable forehead-mounted monitors to detect hypoxia more rapidly, accurately, and reliably than finger-mounted oximeters, especially for military applications.

This work compared the sensitivity of, and agreement between, a forehead-mounted pulse oximeter and a finger-mounted pulse oximeter for application in a hypoxia early-warning detection system, and determined if the forehead reflectance sensor could be mounted within an aviation helmet. Nineteen active-duty military personnel participated in the study. After appropriate medical screening, all subjects donned an aviation flight mask connected to the Reduced Oxygen Breathing Device (ROBD), and were then instrumented with a forehead reflectance pulse oximeter, a finger pulse oximeter, a blood pressure cuff, and a skin temperature sensor.

Following instrumentation, subjects breathed ambient air for 10 minutes through the ROBD to allow for acclimation. This was followed with one of two counterbalanced ascent profiles used to model rapid exposures to altitude. Each profile consisted of 8,000-feet and 18,000-feet altitude plateaus for 12 minutes and 30 minutes, respectively, with a recovery period of 12 minutes between exposures. Data were collected at 1 Hz from both sensors for the duration of the protocol. In addition, the viability of mounting the reflectance sensor in the interior of a standard flight helmet was tested. The sensor was successfully integrated inside the helmet; however, after the subject donned the helmet, there was considerable motion artifact due to pressure fluctuations.

Results from the altitude exposure protocol indicate an exceptionally strong agreement between the forehead and finger sensors at all ranges of desaturation. The sensitivity analyses revealed that the forehead sensor was significantly faster, when responding to rapid changes in SpO2, than the finger sensor, especially during the 18,000-foot exposure.

While the data may seem to suggest that the forehead sensor is accurate and sensitive to altitude-induced changes in SpO2, major drawbacks exist for the technology utilized in the study. Motion artifacts significantly limited the ability to collect useful data, and the in-helmet portion of the study had to be discontinued due to sensor incompatibility issues. Reflectance technology remains promising, but significant improvements aimed at diminishing noise, curbing motion artifact, and improving reliability with advancements such as read-through technology are required.

This work was done by Rita G. Simmons, Joseph F. Chandler, and Dain S. Horning of the Naval Aerospace Medical Research Laboratory for the Army Aeromedical Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. NRL-0045

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Forehead-Mounted Sensor Measures Oxygen Saturation for Hypoxia Early Detection and Warning (reference NRL-0045) is currently available for download from the TSP library.

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