Shallow embolism

[Frank O]

Hi Dr. Powell,

This is a follow-up question I meant to ask you at the seminar last weekend.

If my notes are correct, you mentioned that arterial gas embolisms have occurred when people inhale air at depths as shallow as one meter and then ascend to the surface while holding their breath.

In thinking this through, it occurred to me that an ascent from one meter involves a pressure change of only about 10%. Now, it would seem to me that if you're sitting in your chair at your desk, you take a deep breath to fill your lungs, then lock your throat muscles and tighten down on your chest muscles and diaphragm, you might subject your lungs to a pressure change of more than 10%. Yet, as far as I know people do not embolize when they do this at their desks.

I'm therefore trying to get a handle on what is the difference here. In the case of ascending from (even a shallow) depth, the air is expanding from within the lungs -- whereas if you sit at your desk, take a breath and strain down on your lungs I imagine the pressure is external. Is this what makes the difference in why embolism can occur in one scenario and not the other? Thanks for any help in understanding or visualizing this difference more clearly.

 

[cmay]

I'm no expert, but I believe that the difference lies in the change in volume. Using your example of sitting at a desk, the volume of air in your lungs remains constant. But taking a breath of compressed air (even at a shallow depth) and ascending the actual volume increases. You are calculating the ambient pressure change outside your body, not what is actually going on inside your lungs. Hope this helps a little bit.

 

[Diver0001]

Frank O once bubbled...
Hi Dr. Powell,

This is a follow-up question I meant to ask you at the seminar last weekend.

If my notes are correct, you mentioned that arterial gas embolisms have occurred when people inhale air at depths as shallow as one meter and then ascend to the surface while holding their breath.

In thinking this through, it occurred to me that an ascent from one meter involves a pressure change of only about 10%. Now, it would seem to me that if you're sitting in your chair at your desk, you take a deep breath to fill your lungs, then lock your throat muscles and tighten down on your chest muscles and diaphragm, you might subject your lungs to a pressure change of more than 10%. Yet, as far as I know people do not embolize when they do this at their desks.

I'm therefore trying to get a handle on what is the difference here. In the case of ascending from (even a shallow) depth, the air is expanding from within the lungs -- whereas if you sit at your desk, take a breath and strain down on your lungs I imagine the pressure is external. Is this what makes the difference in why embolism can occur in one scenario and not the other? Thanks for any help in understanding or visualizing this difference more clearly.

And Dr. Powell, while you're at it. 1 metre doesn't sound like much. HOw full do your lungs need to be? What is the real risk run by (OW) scuba students for example when taking their first classes when they are the most likely to breath-hold and ascend a metre or two.

Thanks in advance,
R..

 

[rmediver2002]

In the pool training at NDSTC "problem solving" can be pretty agressive at times.

In all cases there is an instructor imposing problems on a student while on a breathhold and another instructor on SCUBA on the bottom as a safety diver.

During the problem phase the student is encouraged to maintain control of thier equipment at all times, as a reenforcement the equipment / air supply can be taken by the imposer.

Sometimes due to this and other reasons the student will panic and attempt to go to the surface, the safety diver will have to grab them and stop / control the ascent.

The students know they are going to lose all or a portion of thier equipment and will skip breathe taking large gulps of air to ensure they are not caught without a good breathhold.

The safety divers are struggling with the student in the event they bolt and may inadvertantly not exhale so there is cause for concern there also.

The training pool is only 15 feet deep but with this situation present, several times a year some type of pulmonary over inflation does occur.

Most of the cases did not involve a complete ascent to the surface, usually from 5 to 6 feet off the bottom while I was there.

It seemed to happen to the instructors nearly as often as the students...

There is a recompression chamber less than 100 feet from the pool and treatment is conducted very rabidly in the event there is a symptom.

So a very wordy way of saying that even very small changes in ambient pressure under the "wrong" circumstance can cause an over inflation injury to occur...


Jeff Lane

One other comment about injuries on the surface...

These over inflation injuries can also be produced on the surface from a variety of reasons... blunt trama and destructive lung disease being primary causes...

 

[Dr Deco]

Hi Frank O:

The difference between lung pressure while in air (e.g., blowing up a balloon) and while in the water lie in the production of the pressure. When you generate this yourself, you are constricting the intercostal muscles of the chest and the diaphragm around the lungs. The pressure [constriction] is inwards.

When you have taken a breath of compressed air and then reduce the pressure, the pressure is larger in the lungs than externally. When this pressure exceeds what the lungs and diaphragm can control, the alveoli will begin to rupture.

If we are describing a problem from a shallow ascent, it is probably true that the lungs were maximally inflated initially. The fact that individuals do not start with a full volume of air in their lungs is probably why we do not see this terrible barotrauma problem more often. If you are deep enough, even lungs with air forced out [down to residual capacity] will expand too much.

Dr Deco :doctor:

 

[Frank O]

Thanks for the explanation, Dr. Deco, that was about what I suspected the issue to be.

I'm always amazed by my more serious freediver acquaintances, who not only perform special stretches to loosen up their intercostals so that they can take a maximum breath, but also then perform a ritual called "packing" using a special tongue/throat/mouth motion to squeeze in even more air (up to several liters) on top of a full inhalation. I have to imagine that the pressure within the lungs is elevated when they conclude this technique just before setting off on the dive. They then descend many tens of meters with their lungs correspondingly shrinking a great deal during descent and expanding on ascent, happily without mishap in almost all cases. Though I'm given to understand that in the case of an obstruction within the lungs, an embolism on a freediving ascent isn't completely unknown.