Quote: (Originally Posted by
zzzzzzzz)
Everybody who is heavily invested in diving should have one. Dopplers are virtually useless in the hands of hyperbaric personel, since the damage is usually done and the bubbles long gone by the time a victim reaches the chamber.
Validating decompression conservatism and correcting for future dives is an absolutely valid use of these portables.
z
Hello,
I would suggest that to say "everyone should have one" and that their use in evaluating decompression is "absolutely valid" is a significant overstatement of the case. This is, in fact, a very controversial topic; mostly for reasons that others on this thread have already mentioned. Those reasons can be divided up into two broad headings: the technical difficulties of Doppler monitoring, and the meaning of Doppler bubble counts after dives.
Technical difficulties.
As others have mentioned, interpretation of the Doppler-shifted signal is usually not as straight forward as you have portrayed. I would have to agree that training could circumvent this problem to some extent, but it would still be difficult to get everyone "singing from the same songbook". You could argue that this does not matter, and that so long as everyone interprets their own signals consistently (even if inaccurately) that might still be useful. I suppose there is some merit in that, but it remains likely that divers will hear bubbles where they don't exist, or miss bubbles that do.
The meaning of the counts.
This is the biggest problem. As has been pointed out, the correlation between venous bubble counts / grades and clinical DCS is weaker than you might expect. Your implication that these counts are useful in a diagnostic sense is quite wrong. Staff at hyperbaric units would not bother to do bubble counts, even if seeing a diver immediately after a dive, simply because the count would not change what they do. If a diver has symptoms the diagnosis is made irrespective of a bubble count; if the diver didn't have symptoms the diagnosis would not be made even if the bubble count was high.
Despite this, many researchers continue to use Doppler as an imperfect tool to evaluate decompression stress, mainly because the alternative end point (clinical DCS) would not be acceptable to many IRBs / Ethics Committees. However, doing this in large studies with multiple subjects, trials, and using statistical techniques to analyse the results is quite different to the individual trying to make sense of changes in their own counts from dive to dive. There are almost certainly many things that can affect the count other than simply depth / time / ascent protocol. Thus, it is conceivable that you could adjust your profile to be more conservative and yet hear even more bubbles on your next dive because some other factor was at play. You would then be faced with difficult decisions around the interpretation of your numbers.
To summarise, I am definitely not opposed to divers owning Doppler units and self monitoring after diving. With training and care in interpretation of the findings some useful insights might be gained. But I think that your slightly evangelical interpretation of their usefulness needs to be tempered with some realism!
Warm regards,
Simon M