This non-contact, non-invasive blood pressure apparatus continuously measures and monitors blood pressure using a laser Doppler vibrometer to create waveforms. The laser-based system may be utilized to continuously provide highly detailed information about the timing characteristics of the blood pressure waveform. The system does not require elaborate adjustments of one or more lasers and laser detectors so it may be quickly utilized. The system takes measurements from a subject when the subject is moving.

The system consists of a laser Doppler vibrometer (LDV) as the remote sensor, a signal processing unit, a graphical user interface, a glint tracker, and a retroreflector target to enhance sensor performance. The LDV directs a single-output laser beam onto a measurement surface to detect the surface vibration velocity at the point where the laser hits the surface. The measurement surface towards which the laser beam is directed is a section of the subject's skin surface orienting the laser beam so that it is perpendicular to the skin surface at a location where the skin surface is moveable.

A low-power (1 mW), continuous, red laser beam is directed onto the measurement surface. By interfering the beam that was reflected by the measurement surface with a reference beam within the LDV, a measure of the surface velocity is obtained. The apparatus also comprises detectors capable of detecting one or more variables related to movement of the skin surface in a direction parallel to the laser beam and/or producing the blood pressure waveform representation by plotting skin surface velocity with respect to time through the use of a signal processor. The signal processing unit provides skin displacement information, which more directly corresponds to the blood pressure waveform than a measured velocity signal. The blood pressure waveform can be obtained by integrating the velocity signal to obtain surface displacement.

The retro-reflector target provides a practical sensor mount scheme and is used in conjunction with the glint tracker to keep the laser Doppler vibrometer on target when a subject is prone to sporadic movement. The system can determine important timing-related parameters such as the left ventricular ejection time (LVET) and pre-ejection period (PEP).

The accuracy and reliability of the laser Doppler vibrometer sensor is dependent upon it receiving laser reflections from the measurement surface. A surface with a poor reflective quality will degrade sensor performance by decreasing the detectable signal level and increase the noise. Reduced optical return and poor laser reflection from the skin surface will diminish the velocity amplitude measured by the laser beam. One solution is to place a strip of retro-reflective tape on the skin surface. The tape can be located above an artery on the subject's neck (carotid artery), leg, or on top of the foot, which does not have breathing modes associated with it.

A glint tracking system is utilized in conjunction with the placement of the retroreflective tape to steer the laser Doppler vibrometer sensor's laser beam automatically onto the retro-reflective tape. The laser beam is continuously steered onto a position where it will receive a direct reflection from the desired position on the skin surface. The glint tracking system uses its own laser beam originating from the tracker light source and directed onto the retro-reflective tape, combined with the laser Doppler vibrometer sensor laser beam. The laser beam from the laser Doppler vibrometer sensor is directed into the glint tracking system, which is designed to align the laser beam onto the tracker beam through a beam combiner so that both the laser beam and tracker beam are superimposed, and both beams take advantage of the motion controlled mirror that provides "mirror steering" of the beams. The frequency bandwidth of the laser Doppler vibrometer sensor is required to be approximately 500 Hz to accommodate various heart rates.

The system can be used on humans in places such as trauma centers and military medical facilities. This is because the optical sensor can be quickly administered to a patient to provide the medical staff with the waveform that provides information on the cardiac physiology of a patient.

This work was done by Lynn T. Antonelli of the Naval Undersea Warfare Center.


Defense Tech Briefs Magazine

This article first appeared in the December, 2008 issue of Defense Tech Briefs Magazine.

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