This section is based on Dan Reinders presentation on The Varying Permeability Model which you can find free on our library or at Decompression.org
- A bubble is a body of gas within a liquid, does the pressure of a gas in a bubble is equal to the surrounding hydrostatic pressure plus a contribution from surface tension.
- Surface tension is the attractive force felt by surface molecules of a liquid from the molecules beneath that tends to draw the surface molecules into the mass of the liquid and makes the liquid take the shape having the least surface area.
How Gas diffuses from bubbles:
- If the pressure inside a bubble is greater than the pressure of the dissolved gas in the surrounding tissue, the bubble will shrink. Otherwise, it will grow, with one exception, after decompression, because dissolved gas pressure is lower than the ambient pressure due to oxygen metabolism. Despite the theory, the reality is, bubbles don’t always dissolve. To explain this phenomenon (why bubbles don’t always dissolve), a lot of ideas have been suggested. One of the best explanations until now is that the tiny bubbles become stabilized by active molecules on the surface. Surface active molecules are molecules that embed themselves in the gas-water interface.
- Just as a water molecule pulls towards each other in surface tension, active molecules push against the others. This countereffects the process of gas diffusion, and no diffusion means no bubble dissolution. We can assume that Surfactants are tiny springs pushing against each other at the interface between water and bubbles.
Look at the process:
- When you descend presents, bubbles are compressed, which means that the available Surfactant area diminish, each spring push more against its neighbor. But at some point, the spring runs out of space for travel, then the springs will start popping off the bubble surface. This means that it’s more advantage for the spring to leave the surface than to compress further countering the effect of the surface tension stabilizing the bubble and making it smaller (the bubble).
- Surfactant definition: compounds that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a tissue.
Understanding how bubbles grow
- Bubbles grow when dissolved gas pressure is greater than the bubble interior pressure, this means that supersaturation is required to stimulate growth, therefore crushed nuclei are better for divers than stabilized bubbles.
- At first bubbles expand thus making the spring loosing contact with each other. Then they surface tension does not get a countereffect.
What other effects does Surfactant have?
- They form a barrier to diffusion
- The more squeezed they are, the stronger the barrier to diffusion.
CHANGING CONCEPTS IN DECOMPRESSION
Jean Pierre Imbert – Solving the mystery of pre-existing micro gas pockets
Our understanding of decompression has changed. Changes are slow with possibly one new good idea every ten years.
We learnt about divers’ pre-conditioning, according to various situations and oxygen breathing before a dive that could reduce decompression bubble grades. Recently, microparticles were detected in decompressing divers’ blood, indicating that oxidative stress and inflammation could be another dimension of decompression sickness.
Both experiments suggest the existence of mysterious gas pockets in the diver prior to the dive.
For years, Dr Arieli has cut pieces of sheep blood vessels and exposed them to air decompression in a small chamber. He could see tiny bubbles forming at the surface of the vessel walls and identify their sites as hydrophobic.
We now believe that these sites can sustain small gas pockets even in ordinary life because of the presence of metabolism. Once involved in a decompression, they will be activated and produce venous bubbles and debris of vessel walls.
Depending on whether you are young or old, active or sedentary, this population will be more or less dense and resistant and will produce different levels of venous bubbles and microparticles.
We have learnt to modify the old Buhlmann algorithm with deep stops and gradient factors. The pre-existing gas pockets concept should further help us to select our gradient factors, not only depending on the dive but also on the type of divers we are.
Dr. Ran Arieli(1):
“As far as we can interpret the wealth of experimental data, it is indeed a tenable proposition that decompression bubbles develop from pre-existing nanobubbles on hydrophobic surfaces within our body. Strong support for this hypothesis may be seen in the explanation it can provide for the numerous and varied features of decompression illness (timing, acclimation, risk on a second dive, endothelial injury, microparticles, taravana, local white matter lesions, spinal and vestibular DCS and joint pain, among others), as we found in our investigations of active hydrophobic spots summarised in the recent overview article.”
- The Israel Naval Medical Institute, Israel Defence Forces Medical Corps, Haifa, Israel; Eliachar Research Laboratory, Western Galilee Medical Centre, Nahariya, Israel