MB ChB, PhD, DipDHM, FANZCA
Divers with an interest in medical issues and decompression science will frequently encounter the term âDoppler ultrasoundâ. We hear claims that âdivers were monitored with Dopplerâ, or that a particular dive table or computer has been âDoppler validatedâ. Indeed, claims of such âDoppler validationâ are often mentioned in debates over the relative merits of different decompression strategies.
But what does it all mean?
The term ultrasound refers to very high frequency sound (usually two megahertz and above) which is well outside the range of normal human hearing.
Such sound penetrates a fluid medium very readily and its main application in medicine is in organ imaging technologies. An ultrasound beam is applied at the body surface using a small wand held against the skin by the operator. The ultrasound penetrates inward, and as it passes through layers of tissue with differing composition and density some of it is reflected and this is detected by a transducer back at the wand. 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. These two properties can be exploited to construct a two dimensional image of the tissues through which the sound is passing. It is both convenient and safe since minimal preparation is required, results are instantaneous, and no radiation is involved. However, ultrasound imaging has its limitations: ultrasound is poorly conducted in gas or air, so it cannot image within or through the lungs or other gas containing spaces. Similarly, it is not effective through thick bone.
Doppler ultrasound is a somewhat different technology. It utilises the âDoppler effectâ, named after the person who first discovered it. This is something most of us are familiar with. The classic description is of the train sounding its horn as it comes towards you, and the pitch of the horn suddenly changing as the train passes and heads away from you. The horn itself has not changed, but the sound you are hearing certainly does. This is because the wavelength of the sound emitted from a moving source is effectively shorter (thus the higher pitch) as the source moves towards you, and longer (thus the lower pitch) as it moves away. This is the Doppler effect.
Application of the Doppler effect in medicine is slightly different, because as you will see it is not the sound source that is moving, but the principle is similar. Most commonly, Doppler ultrasound is used to detect flow in blood vessels and the heart. Remember, blood is not like water; it has millions of microscopic particles in the form of red and white blood cells, and platelets. The presence of these cells works to our advantage because they reflect sound. If you aim an ultrasound beam at a segment of tissue with no blood flow through a major vessel, then some of the ultrasound is reflected back in a predictable and uniform manner. In contrast, if you aim an ultrasound beam at a large vessel in which there is flowing blood, then the ultrasound will be reflected by the moving blood cells, and the frequency of the reflected ultrasound beam will be influenced by that movement. 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. Moreover, the speed of the blood cell movement will influence the difference in frequency between the initial and reflected ultrasound beams.
So, for example, if blood is flowing very rapidly toward the ultrasound source, the reflected ultrasound beam will be at a significantly higher frequency. All of this can be summarised by saying that ultrasound is reflected by particles moving in a liquid at a shifted frequency proportional to the velocity of the particles. As you might imagine, this application of ultrasound and the Doppler effect to detect flow in blood vessels has a variety of uses in medicine. Most devices built for this purpose produce an audible flow output that in pulsatile flow (such as produced by the beating heart) produces a whooshing noise, and some provide a depiction of flow velocity versus time in graphical format (see Figure 1).
The link with diving medicine comes from the ability of Doppler ultrasound technology to detect bubbles moving in blood vessels. Bubbles have markedly different ultrasound reflective properties to blood cells. Thus, when a bubble enters the ultrasound beam it is not surprising that it causes a dramatic, instantaneous disturbance of the ultrasound signal. This is manifest as an audible harmonic âchirpâ or âpopâ, and a perturbation of the flow velocity / time display (see Figure 2). Thus, the 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.
There have been numerous attempts to accurately count circulating bubbles using Doppler ultrasound. In non-diving applications, such as the detection of bubbles in the brain circulation during cardiac surgery, devices have been developed that automatically derive a count from the Doppler shifted signal.
In diving applications the most common approach has been to monitor the central venous circulation (near the heart) after diving using small portable systems that produce an audible signal, and to employ grading systems to quantify the bubbles detected As most will be aware, decompression from most dives causes a degree of bubble formation in the veins. It is generally accepted that the numbers of these bubbles are an indicator of both the decompression stress (more decompression stress is likely to produce more bubbles) and the probability of decompression sickness (DCS) developing (higher numbers of bubbles are more likely to result in DCS). This has resulted great significance being attached to the Doppler detection of these bubbles. In part, this is appropriate, but the significance of Doppler detected venous bubbles is often over-called by commentators who do not have a thorough understanding of the confounding issues. These issues relate to the detection technology itself, and also to the significance of these venous bubbles.
With regard to the technology itself, it is important to understand that interpretation of the sounds obtained by portable Doppler devices, and the application of bubble grading systems is quite subjective. Accuracy is highly influenced by the experience and expertise of the operator, as well as their potential bias if they are involved in a project with a âpoint to proveâ. The timing of monitoring is important. If monitoring is conducted either very early or several hours after diving then the peak of venous bubbling may be missed. Indeed, a diver with florid symptoms of DCS who presents late may have no venous bubbles detected at all. Other important influences on bubble detection include the duration of monitoring, and the nature of the monitoring device.
With regard to the significance of the bubbles themselves, there are a few problems. Bubbles are commonly detected in the veins following dives that do not produce DCS. While the risk of developing DCS does appear to be greater following those dives that produce high bubble grades, a significant proportion of such dives still do not result in obvious problems. It follows that Doppler bubble detection is certainly not a valid diagnostic test for DCS, and high bubble grades on Doppler in the absence of DCS symptoms would not be an indication for recompression treatment. 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, and there are currently no readily available technologies that readily detect such bubbles.
To summarise all of this, while Doppler bubble detection is and will continue to be a useful tool in comparing various decompression strategies, the significance of venous bubbles detected by Doppler is not as great as it might intuitively appear. Doppler will continue to be used by the developers of dive tables and computers as a barometer of decompression stress but any claims along the lines of âsafety proven by Dopplerâ need to be interpreted with caution.