Bubble Detection for Decompression Sickness
Divers who rise from depths too quickly can experience the “bends,” otherwise known as Decompression Sickness (DCS). DCS describes a condition arising from the precipitation of dissolved gases into bubbles inside the body on depressurization. This disease can also affect high-altitude aviators, astronauts, and other individuals who are exposed to large changes in ambient pressure. Despite the fact that the root cause of DCS has been known for over a century (supersaturation of nitrogen in tissues and bodily fluids), not all aspects of its etiology are understood, including factors that affect a person’s risk for DCS. As a result, operational procedures to prevent DCS are based on large statistical databases and designed to be conservative and consequently inefficient from an operational perspective. But because a DCS incident could be operationally catastrophic, or worse, life threatening, these conservative operational precautions have been necessary.
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| Operational Concept of Creare’s Bubble Detection System. |
Our ability to understand DCS and its causes and effects has been hampered by a lack of non-invasive instrumentation that can detect or characterize the gaseous emboli (bubbles) that are associated with the disease. Standard clinical Doppler and imaging ultrasound machines have a very limited ability to detect or size these bubbles and have no ability to distinguish gaseous emboli from thrombotic emboli, which may also be present. In addition, they cannot detect bubbles smaller than about 80 microns, and Doppler systems cannot detect stationary bubbles that might be lodged in tissues or joints.
To address the limitations of existing DCS instrumentation, Creare has been developing a new ultrasound instrument to detect and characterize DCS bubbles. This work is funded by the Navy, NASA, and the National Space Biomedical Research Institute. Creare’s instrument utilizes two ultrasound frequencies. One frequency (the pump, fp) causes the bubbles to resonate if they are of the correct size. A second frequency, called the image frequency (fi), also interacts with the bubbles. When resonating bubbles are present in the measurement volume, the nonlinear acoustic properties of the bubbles cause the acoustic signal returned from the bubbles to contain side lobes at the image frequency plus and minus the pump frequency (fi±fp). The resonant frequency of bubbles is dependent upon their diameter, and this characteristic provides the ability for the instrument to also size bubbles as well as detect their presence.
Creare’s instrument is unique in that it can detect stationary bubbles, a feature which aids fundamental research on DCS. Many theoretical models of DCS involve the development and evolution of bubbles in tissue. However, the existence of these tissue bubbles was not confirmed until recently, with the aid of our technology. Additionally, tissue bubble models require estimates of the time constants associated with the evolution of these bubbles from their inception, growth, and eventual dissolution. To date, there has been no way to monitor this process and thereby calibrate these models; Creare’s instrument has the potential to provide a revolution in model calibration which can lead to safer and less costly DCS risk mitigation procedures such as adaptive dive tables and/or safer decompression procedures.
Beyond this current application, the Creare instrument has the potential for use in monitoring oxygen pre-breathe procedures that aim to eliminate nitrogen in tissues before hyperbaric stress since it is believed that tissue bubbles are created and exist under normabaric conditions. The instrument may also find applications in the treatment of DCS using standard hyperbaric treatments (i.e., recompression to shrink DCS bubbles) or other non-hyperbaric treatments involving drugs (e.g., surfactants and compounds with high nitrogen solubility) that act to reduce the quantity and/or size of bubbles in blood and tissues.
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