Hi Scuba,
Can I speak as someone who has actually treated several patients with pneumothoraces during my time as a "resident" on the chest ward of my teaching hospital? While the causes differ the effects of, and the treatment of, a tension pneumothorax is the same.
Forgive me lecturing, but feel this is so important I have to get the message across.
The terms burst lung and lung over expansion are somewhat misleading as this is only a part of the complete picture.
First of all I do agree, the rib cage is a pretty solid pyramidal structure, but had you not noticed that there is a very large flap of soft muscular tissue holding the lungs and mediastinum in place within the chest cavity;- the diaphagm. OK it is muscle, but believe me it is not stong enought to maintain intrathoracic pressure very much above or below ambient pressure. (You cannot snorkel below about a foot of water. This suggests that it cannot vary the intrathoracic pressure more than +- 0.03 bar ambient.). In addition a large proprtion of the chest cavity is filled with blood in the pulmonary circulation which is effectively in direct contact with external pressure through the great veins and arteries in the abdomen (and neck).
The lungs are floating in the chest cavity, in an ambient pressure environment. They are isolated from the chest cavity itself by the pleura (a very, very thin membrane) and the walls of the airways and the alveoli. It is easier to demonstrate than to describe but in effect the airways of the lungs are like lots of little condoms connected together to one opening at the level of the trachea. They are like a baloon suspended in an evacuated solid jar by the nozzle through the top. Because of the near vaccuum in the jar air fills the balloon pressing it against the sides..
To complicate matters the jar contains a piston at the bottom. When the piston goes down the baloon expands as air enters through the nozzle at the top to keep the air within it at ambient pressure and visa versa.
I will not discuss other causes of pneumothorax but this is what happens during an ascent when you hold your breath. (Or you have an asthma attack or other obtruction to exhalation.)
The gas within the airways is a fixed mass and will either increase in volume as the external pressure is reduced or it's RELATIVE pressure will increase if the volume remains constant.
What happens is a bit of both. The increased relative pressure forces the blood within the pumonary circulation into the lower pressure environment of the abdominal cavity as the airways expand against the reducing external pressure. The airways can almost double in size as the blood vessels shrink. Such changes will take place in much less than 1 bar, or from 10 metres to the surface.
In time when the relative pressure increases to more than 0.03 bar the expanding lungs will force the diaphragmatic piston downwards into the abdominal cavity, which remains at ambient pressure at all times.
The walls of the airways and the alveoli are very thin and can stretch to a certain extent. However they are not designed to expand to more than twice their natural size so at the weakest point the lining will tear. This is as if the baloon were to be punctured, and it allows the high pressure gas in the lungs into the space between the lungs and chest wall. The lungs relax as they shrink against their inherant elasticity. Unfortunately since there is no longer a near vaccuum to keep them inflated almost ALL the air within them moves into the pleural cavity.
Once the air is in the space between the lungs and the chest wall it has nowhere to go. It cannot get back into the airways because they have collapsed. In consequence as the ascent continues the relative pressure of the air within the pleural cavity increases.
So scuba
NO!
your body WILL NOT force the air out of your lungs before they could even get to over expanding,
. The increased pressure will simply close the unsupported airways. This is what happens in asthma.
It does not take much to work out that if the pneumothorax occurs at 30 metres the pressure within the chest could exceed 3 bar at the surface. ie 45 psi and more than that of a car tyre! Not only is this sufficent to stop all chest movement and compress the
unaffected lung, this is more than enough to stop all venous return to the heart.
You cannot perform your proposed long slow inhale during the whole of an acent as it is not physically possible. If you are trying to inhale as the ambient pressure rapidly reduces all that happens is there may be little or no air movement at all or it will move
out of the lungs through the open glottis
that's OK then!. The danger, of course, is you will be actively attempting to inhale more and more (as the regulator does nothing) and actively overinflate the lungs to their natural maximum size.
Then what would happen if you were to hold your breath?
When they first attempted submarine escape the, if I remember correctly, the Davy aparatus allowed the excapee to take a beath. Many years ago I ascended from the bottom of the Navy's training tank (60 feet) and was able to forcibly breath out all the way the the surface.
I do not think this drill is done nowadays for obvious reasons.
I hope this explains the danger!