Two questions for diving science nerds only

[Kim]

Thus, whether such a thing as "open glottis barotrauma" due to excessive ascent rates remains speculative.

But speculation makes life more interesting.

Fair enough, but I can't really say it would keep me awake! To have any real sense of how dangerous it *might* be we'd really need to know the maximum upward velocity. Personally I think that that will be quite small. At the end of the day using a single tank people use wings of <40 lbs...mine's about 30 lbs. Add to that my 8 lbs of weight and the maximum lift on my body can only reach about 38lbs with the wing completely full and my weights gone. While I can see that with a stuck inflater or something I might end up with a full wing, but simultaneously losing my weights at the same time?....I can't see that....
Maybe people in drysuits have more to inflate but even then I'd bet terminal upward velocity is still quite a bit slower than the speed that would create such a problem.

Let's face it....people get into uncontrolled ascents very often. While there ARE many accidents due to this, if ascent speed vs exhalation speed was a real factual cause then the incidence of accidents would presumably be close to 100% - we don't see that AFAIK.

Even shooting a lift bag which is virtually all air and no weight it takes some time for the bag to reach the surface. Simple air bubbles also take a while!

 

[shakeybrainsurgeon]

Cool, got it. One questoin, when you say 70%, is the majority of the 30% that's left in air spaces that you can't fully expel? Because, after about a second I really can't expell any more air. And that's after inhaling as much as possible first.

70% is an average --- some people can achieve more also, there is a certain percentage of lung volume, the residual volume, that can't ever be expelled. This is the air that keeps the alveoli and aispaces propped open, so no one can breathe out 100% of their lung air even if they try.

 

[shakeybrainsurgeon]

To put this to bed before I am accused of asking how many angels can fit on the head of a pin:

1) The maximum ascent rate? If everything is ideal --- maximum lift, minimum drag, sufficient depth to accelerate and maybe even a little diver finning tossed in --- might be roughly on the order of 180 fpm, or 3 fps. Three times the PADI maximum, but still not a rocket speed and for most situations, would likely be significantly less.

2) Can barotrauma result from uncontrolled ascents from depth, even without breath holding? Unlikely, given the ascent rates involved and given that such ascents occur without injury. Also, if a diver is ascending at only 2 fps, he/she would have to close the glottis for only 2 to 3 seconds to cause barotrauma --- in a panic situation, this is so likely that any AGE during uncontrolled ascent can easily be explained by breath holding alone, without invoking any other cause.

Thus, we come back to the two rules of diving

1) Never hold your breath and

2) Never forget rule #1

 

[SNorman]

How fast do bubbles ascend?

 

[ClayJar]

You mentioned an exhalation rate and an ascent distance capable of causing barotrauma, however, they must both be used together in order to get a proper potentially-limiting ascent rate. As I am a science nerd (with a degree in chemical engineering -- no c-card, but I got a nice paper), I guess I should give it a try instead of going to lunch... Okay, here goes.

Let's use your numbers and say that full-speed exhalation for a healthy person is 70% of lung volume in one second. We'll write that as 0.7 Lu/s (where Lu is our unit of volume, with 1 Lu equal to a full set of lungs). We'll assume that can be maintained at a constant rate (as we're "adding volume" due to the ascent).

Now, we need to figure out how fast we are adding volume. To do that, we write an equation for volume as a function of depth: V(d) = V₀ * (d + 33 fsw) / 33 fsw. We'll choose V₀ as 1 Lu (at zero depth). Then we differentiate this with respect to depth to get a nice little equation which I won't bother trying to type here, but it is obviously not a constant dV(d)/dd, as the rate of change of volume with respect to depth increases toward the surface. That being the case, the maximum rate of change is what we're looking for, so we evaluate the derivative at d = 0 fsw (the surface). From there, it is trivial to write the time rate of change of volume at the surface as a function of ascent rate.

(Edit: Okay, I won't. You'd fly about 8.2 feet up out of the water.)

We can now put these two halves together to get the limiting ascent rate such that there is zero net change in lung volume for the person exhaling at 0.7 Lu/s (the limiting exhalation rate). This gives 0.7 Lu / s = (1 Lu / 33 fsw) * (ascent rate). Solving for ascent rate, we find the ascent rate to be (0.7 * 33) fsw/s, i.e. 23 fsw/s.

At 23 fsw per second, or almost 1400 fsw per minute, even exhaling 0.7 Lu / s, you would not be able to keep up with the expansion of the air in your lungs by the time you reached the surface. If you use a less forceful 0.33 Lu / s, you're still looking at 10 fsw / second (600 fsw per minute), which is still way up there.

Of course, the rate of exhalation needed to maintain constant lung volume is significantly affected both by additional depth as well as current lung volume. The deeper you are, the lower your exhalation rate need be; the less air in your lungs to expand, the lower your exhalation rate need be. If you have only 0.5 Lu of air in your lungs at the time you break the surface, you could be ascending at twice the ascent rate without overexpansion.

So, to sum up, healthy people can exhale at significantly higher rates than the rates of volumetric expansion of air in their lungs for any achievable ascent rate, so they should be quite safe if they don't hold their breath. I'll leave the physics nerds the additional question of how high out of the water you'd fly, assuming a rigid body with no friction once in air, if you broke the surface at 23 feet per second.

Edit: Okay, I won't. You'd fly about 8.2 feet, or 2.5 meters, up out of the water... which could probably get you right up onto the deck for your emergency O2.

 

[Thalassamania]

According to Professor Adam Curtis (SSAC), "The training requires the use of very fast highly buoyant ascent with speed of 2 metres per second or faster, with buoyancy in excess of 10 kg.&#8221;

A translation of the manual of a Russian rebreather that addressed submarine escapes suggests that the Russians conducted free ascents at up to 3 metres per second.

The flying into the air reminds me of my engineer room mate whom I found one evening staring at a chicken. When I said what's up he muttered something about, "well ... assuming a spherical chicken." He was trying to figure out how long to cook the bird!

 

[shakeybrainsurgeon]

Clayjar

I can't argue with your logic, but you have made one error --- the rate of exhalation is not linear --- exhaled air = 0.7 lu/sec (suggesting that it takes less than 1.5 sec to dump all air from the lungs), but an exponential decline, something like

Residual air = Lu x e to the -exp constant x t --- a half-life sort of equation (half the air exhaled per unit time t) The effect of this change is to prolong total lung emptying from 1.5 seconds in the linear model to about three or four seconds (take a very deep breat, exhale and see when you can no longer produce air --- it's longer than 1.5 seconds)

Fun analysis, for those of us who like this sort of thing (my pre-MD degree was in physics)

 

[TSandM]

Fascinating thread -- one of the things that has puzzled me from the time it was first brought up in OW class was WHY divers who get pulmonary barotrauma get arterial gas embolism.

In my prior practice, we had patients on ventilators requiring VERY high inspiratory pressures, and they blew pneumothoraces routinely -- I have seen a patient with six chest tubes in as a result. Never, ever did we have an arterial gas embolism. The sneaky thought has crossed my mind that AGE may have MUCH more to do with rapid ascents and already extant arterial bubbles -- ones which have either passed the pulmonary filter, or have crossed a PFO -- than with the entry of gas into the arterial circulation within the lung structure itself.

In other words, pulmonary barotrauma and AGE coexist because they share a cause, not because one leads to the other.

 

[shakeybrainsurgeon]

Fascinating thread -- one of the things that has puzzled me from the time it was first brought up in OW class was WHY divers who get pulmonary barotrauma get arterial gas embolism.

In my prior practice, we had patients on ventilators requiring VERY high inspiratory pressures, and they blew pneumothoraces routinely -- I have seen a patient with six chest tubes in as a result. Never, ever did we have an arterial gas embolism. The sneaky thought has crossed my mind that AGE may have MUCH more to do with rapid ascents and already extant arterial bubbles -- ones which have either passed the pulmonary filter, or have crossed a PFO -- than with the entry of gas into the arterial circulation within the lung structure itself.

In other words, pulmonary barotrauma and AGE coexist because they share a cause, not because one leads to the other.

Interesting point --- several issues come to mind: 1) can we achieve the sort of pressure gradients between alveoli and pulmonary veins on land using mechanical ventilation that can be achieved in diving (one atmosphere or greater)? and 2) when holding breath and surfacing, the pressure rises simultaneously in the airways and alveoli --- with mechanical ventilation, the pressure is transmitted from the trachea distally, suggesting airway resistance might moderate alveolar damage somewhat --- in fact, this resistance is sometimes why the increased pressure is needed in the first place?

But you may be right, the true signs of barotrauma may just be pneumothorax and pulmonary hemorrhage/edema --- AGE may just be microbubbles coalescing to macrobubbles.

 

[shakeybrainsurgeon]

I was sent a post concerning submarine escape exercises in which sailors ascended from 100 feet with an average ascent rate of about 15 fps...if Clayjar is in the ballpark, then someone with a reduced exhalation rate ascending at this rate might indeed drop a lung simply because he couldn't empty his lungs fast enough.