The science of silent bubbles

After the work of Robert Workman and others in the sixties, we really thought we were getting a handle on the science of diving. But nothing is ever that straight forward, especially when it comes to science, or handles. In 1976 Dr. Merrell Spencer published a report that put a rather significant spanner in the works of decompression theory. The focus of the report was the detection of so-called silent bubbles in the bloodstream of divers. They weren’t bubbles that were causing DCS, the bubbles were found in divers showing no symptoms, who had all dived well within the limits of the dive tables of the time. Prior to this, the idea of asymptomatic bubbles had been discussed (in fact as early as 1951 by Bateman and Behnke), but as it was only considered a theoretical possibility, mainstream decompression research continued to focus on all inert gases interacting with the body whilst in solution. In light of Dr. Spencer’s work the plot had thickened, curdled, boiled over the pan, hardened on the cooker and turned into a bubbly white elephant; time to get the Marigold gloves on.

Subsequent investigations of silent bubbles suggest that they may be permanently present in the bloodstream, even for people who have never been diving. This is due to a combination—predominantly mechanical in nature—of cavitation, micro-nucleation, and tribonucleation. Micro-nucleation relates to impurities in the blood stream that allow bubbles to latch on to (commonly joints and blood vessels), so called “gas-seeds”. This is the reason why weight training is considered unwise before and after diving. The intentional microscopic damage to muscles and ligaments forms ideal micro-nucleation sites. Cavitation is something that we see in boat propellers, mantis shrimps stunning their prey, and the pipes “knocking” in your central heating system. Essentially, fluids undergoing pressure changes create low pressure cavities of vapour, which at high pressures can implode and cause a shockwave. At more intermediate pressures you are left with a bubble. Blood can, under some circumstances produce localised cavitation such as in joints when limbs are contorted, or when the blood is pumping faster, such as during exercise or when thinking about your overdraft. Tribonucleation is the formation of bubbles between two submerged surfaces when those surfaces are suddenly pulled apart. This movement creates negative pressure, which can produce localised supersaturation. Too much of which leads to bubbles. The best non-diving example of this is when you crack your knuckles. People commonly think the cracking sound is caused by a bubble in the joint popping, but it’s actually the shock wave on surrounding synovial fluid as a bubble is formed that makes the sound.

Silent bubbles are now considered to be a completely normal consequence of diving. However, it’s very hard to correlate silent bubble size and frequency with DCS cases. Some people get bent with few or no silent bubbles, and others don’t get symptoms of DCS even though they have what is considered an excess of silent bubbles. Remember, an M-value is a theoretical line through a large, fuzzy grey area. However, as DCS is a bubble of gas in tissue or blood that causes symptoms of varying severity, attempting to reduce the size and amount of any kind of bubbles is obviously going to be a good thing for a diver.

What we do know about silent bubbles is that the body is normally very good at filtering them out. When they arrive at the lungs they get trapped in the fine capillaries, where they then diffuse out through the lung lining. However, if too many silent bubbles get trapped this can reduce the overall efficiency of the lungs to off-gas, which can cause a bottleneck and result in slower off-gassing. The other effect that silent bubbles have is that they can cause excessive tiredness, caused by initiation of the compliment system, which is an immune system response in this case induced by the presence of these “foreign invaders”. This process has no effect on the bubbles as they are inert. So, increased fatigue after a dive may be an indication of a bad dive profile, dehydration, fast ascent rate, or a combination of these.