Bubble, Bubble toil and trouble...

DAN often gets questions about bubbles after diving, such as: 
  • Are bubbles always a sign of decompression illness?
  • Where do bubbles form?
  • When do bubbles cause problems?
These are all excellent questions! In fact they have kept decompression researchers preoccupied for more than 100 years! Many aspects of bubbles are not fully answered yet, but here is some of what we know at the moment:

Are bubbles always a sign of decompression illness?
All but the shortest and shallowest compressed gas dives will produce some level of bubbling. So most recreational dives do produce some bubbles. These bubbles typically appear within 2 hours of surfacing and they may be detectable in large venous blood vessels or over the chambers of the heart as they are circulated to the lungs (i.e., the right atrium and right ventricle). This is also why divers are advised not to exercise shortly after diving and to avoid lifting heavy equipment and exerting themselves in the time that bubbles are still buzzing around the body. This does not mean these dives or the associated bubbles produce decompression illness, though. Most bubbles are completely asymptomatic; they are not harmful, as far as we know, even after studying decompression for more than a century. So the question is not whether bubbles are a sign of decompression illness (DCI), but rather, when bubbles produce DCI?

Where do bubbles form?
Bubbles may potentially form in any body tissue or organ if the dissolved inert gas tension exceeds the ambient pressure by a critical amount. This amount seems to vary between tissues based on several factors, including the properties of the tissue itself (e.g., fat or water soluble); the duration of the exposure (i.e., bottom time & inert gas) the amount of circulation the tissue receives (fast or slow tissues); and some tissue-specific motion and pressure dynamics. As an example of tissue-pressure dynamics, consider the difference between arteries an veins: Both arteries and veins contain blood, but arterial blood is pressurised (blood pressure) to a greater extent than veins are. So, irrespective of any circulating inert gas tensions, decompression-related bubbles would appear far more readily in veins than in arteries. Another example has to do with tissues: Organs, like the spinal cord, can stretch as we move and bend; this can predispose it to the formation of bubbles.
Somewhat surprisingly, bubbles that appear in the venous circulation do not actually seem to form within the venous blood itself: They are thought to seep in from adjacent tissues, from where they then enter the venous blood stream and travel towards the lungs. Most recorded bubbles are found in the venous system. However, these are usually not related to DCI symptoms, whereas invisible tissue- and arterial bubbles may be.

When do bubbles cause problems?
Under normal conditions, most of these bubbles are filtered out by the lungs without any difficulty. If the number of bubbles is excessively large, or if showers of bubbles continue over an extended time, the circulation of the lungs can become blocked. Alternatively, gas pockets may eventually form within the heart and interfere with circulation; this is actually quite rare unless there are gross violations of bottom time or ascent rates. The greater concern is that venous bubbles may bypass the lung filtration system (due to heart and lung abnormalities) and then move directly into the arterial circulation and be distributed to the body. As the function of organs like the brain and heart are very sensitive to blood and oxygen supply, they would be easily affected by a bubble-related interruption of circulation. Therefore arterial bubbles entering the circulation may produce symptoms resembling a stroke or heart attack (gas embolism). So arterial bubbles are more of a concern than venous bubbles are. Similarly, within tissues, bubbles may cause distortion of tissue structures or even cause bleeding. For instance, within the spinal cord itself, bubbles may produce compression of, or bleeding within, the cord called spinal decompression illness.

Fortunately, when all is said and done, diving is actually quite a safe sport! One to four out of every 10,000 dives performed within typical scuba diving parameters may produce decompression illness. However, even then, this is usually relatively mild, and responds well to recompression in most cases. What is more important is that divers lower their individual risk by:
  • following the guidelines promulgated by their training agencies;
  • not pushing the limits
  • not over-estimating their abilities; and
  • not ignoring abnormal symptoms after a dive, but start oxygen first aid and call for advice!
When in doubt, remember to call DAN.
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