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The term ultrasound refers to very high-frequency sound (usually two megahertz and above) which is well outside the range of normal human hearing. The ultrasound penetrates human tissue with some waves reflected as they pass through layers with differing composition and density. The amount of sound reflected is related to the density of the tissue, and the delay in reflection is related to the depth at which it occurs. Doppler ultrasound is used to detect flow in blood vessels and the heart where some waves are reflected by the moving blood cells. If the cells are moving toward the ultrasound source, the reflected beam will be at a higher frequency, and if the cells are moving away from the ultrasound source, the reflected beam will be at a lower frequency in accordance with the Doppler effect.
Doppler ultrasound can detect bubbles moving in blood vessels since bubbles have markedly different ultrasound reflective properties when compared to blood cells. Thus, when a bubble enters the ultrasound beam it causes a dramatic, instantaneous disturbance of the ultrasound signal. Development of Doppler ultrasound technology has enabled us to monitor flow through blood vessels and to detect the presence of bubbles in those vessels after diving. The most common approach is to monitor the central venous circulation (near the heart) using small portable systems that produce an audible signal. The test normally uses a grading system to quantify the bubbles detected. It is generally accepted that the numbers of these bubbles are an indicator of decompression stress and increased the probability of decompression sickness.
It is important to understand that interpretation of the sounds obtained by portable Doppler devices, and the application of bubble grading systems are subjective. Results are influenced by the experience and expertise of the operator. The timing of monitoring is also important since bubble occurrence peaks sometime after the dive and begins to decline thereafter. There are other problems with interpreting the significance of bubbles detected in the blood. While the risk of developing DCS does appear to be greater following dives that produce high bubble grades, a significant proportion of such dives still do not result in obvious problems. The uncertain relationship between Doppler detected bubbles and DCS may relate to the fact that Doppler detects bubbles in veins, whereas many of the symptoms of DCS almost certainly have nothing to do with bubbles forming in the veins. For example, the typical musculoskeletal pain of DCS, and spinal manifestations are most likely to be related to bubbles forming within the tissues themselves. There are currently no readily available technologies that readily detect such bubbles.
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