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     Of all the barotrauma discussed, this is the most serious. Lung squeeze and over-expansion can lead to a diver's death. And, if not death, damage to the lungs can lead to serious disabilities. It is most important to heed Boyle's law and never allow a pressure differential to occur between the inside of the lungs and the outside, especially on the ascent. 

LUNG SQUEEZE: Your lungs are designed to go from one size to another as you breathe in and out. The lungs never completely empty. In fact, if they did totally deflate they might not be able to reinflate because the small surfaces might stick together. That is a collapsed lung.  The minimum size the lungs become when one fully exhales is known as the residual volume. If you compressed the chest and forced air to leave the lungs until they were smaller than the residual volume it could cause serious barotrauma. There could be lung rupture due to rib penetration. There might be a fluid build-up in the lungs leading to pneumonia. There could be a total lung collapse possibly resulting in death.

      Taking a full breath and skin diving to 33' would cause the lungs to be 1/2 of their original size. At 66' they would be 1/3, and at 99' they would be 1/4. If your lungs' residual volume was between 1/3 and 1/4 serious damage might result from this breath-hold dive to 99'. Most people do not hold their breath and dive that deep so this is not usually considered a serious problem. The few divers that do dive that deep holding their breath have trained for years to do it, have a changed lung capacity, and probably have their lungs fill with fluid when they are at depth. The fluid prevents further compression. When they ascend and the lungs grow larger the fluid occupies only a small portion of the lungs preventing drowning. Incidentally, the free-diving record, held by a woman on 4/29/07, is 289'. On 12/13/10 William Trubridge did a free-dive to 330' (100 meters) in the Bahamas.

LUNG OVER-EXPANSION: If you were to take a balloon from the surface to 33' it would be 1/2 the size. If you let it go to the surface it would return to its full size. That is what happens to a skin diver's lungs as they go up and down in the water. If you took a balloon down to 33' and blew it up, tied it closed, and then let it go to the surface it would explode. Scuba divers take air into their lungs. If they don't let it out on the way up awful things can happen. Never hold your breath while scuba diving. Don't dive with asthma or pneumonia unless a doctor says it is OK. The practical application of Boyle's law can kill.

     Dwelling on the negative aspects of scuba diving is not the intent. However, this is a very serious matter and it is important to spell out the gory details of what can happen to person that holds their breath as they ascend after breathing underwater. Please make sure to read the words following the gore in order to get a true perspective of the entire issue.

     Following the course from the mouth to the lungs the airways get smaller and smaller. It looks similar to a tree that is upside down. The trunk is the trachea. Put your hand on your Adam's Apple and feel the size of the "trunk." From the trachea there is a branch that travels into each lung. Those 2 branches are known as the bronchi. Just as with a tree, the airways keep decreasing in size until the end of each "twig" is reached. At the end there are alveoli, billions of them. The ends are microscopic but the way it looks resembles broccoli. Each alveolus is very tiny and fragile.

     The following is a model of the airways in the lungs:

     At the end of each branch are the microscopic alveoli. Each alveolus is surrounded with a small blood vessel called a capillary. They are also microscopic. So, the lungs contain both air and blood vessels.

     The upper part of the diagram below shows the mixture of the air and the blood vessels in each lung. The bottom part of the diagram shows what a bunch of alveoli look like. Again, they are similar to broccoli.

     If diver takes a breath and then heads toward the surface without exhaling the lungs will overinflate. Each alveolus overinflates. Air pressure breaks the thin walls of the alveoli and is forced directly into the blood capillaries. The air bubbles in the blood stream will travel out of the lungs and back to the heart. The bubbles may grow larger as the divers ascends. They may join together as well. The bubbles than move out of the heart to the body and some to the brain. In the brain the bubbles travel until they can go no further because the blood vessels get too small. The bubbles are trapped. Clots may form around them. The blood may stop flowing at that point and brain cells will get starved for oxygen. If the brain cells start dying certain functions are lost. The diver has caused a stroke! This can happen in as little as 4' of water! The symptoms for AGE usually appear quickly and within 1 hour.

     The same thing could happen, usually to a lesser degree, if you have asthma, pneumonia, or smoke. If there is mucous in your lungs it might allow air to enter the alveolus easily but not leave on the ascent. The areas of the lungs having these reverse blocks force bubbles into the blood stream. That is why it is important to have a doctor say you can dive with asthma, or if you smoke heavily.

     Any plug in the blood stream is called an "embolus."  The air bubble that might be in the brain is an "embolus." Since it came to the brain in an artery, and since it is made out of gas, the entire problem is called, "Arterial gas embolism", or "AGE" for short.

     In the meantime, the excess pressure in the lungs from holding your breath while ascending could also cause the lung to rupture. This can lead to bleeding which could flood the lungs and make breathing difficult. Also, air might get trapped outside the lung in the chest cavity. If the air expands upon ascent and cannot find its way back into the lung, the lung could collapse and the heart could be pushed to the side. This is a very dangerous condition. Breathe normally.

     Now for the good news! With the exception of the fluid-in-the-lung cases, arterial gas embolism does not occur without warning. Take a deep breath. Now add more air to your lungs. Fill them up as much as you can. That is what a diver would feel like just prior to lung damage. It's uncomfortable and you can feel it. If you are near the surface where the pressure changes are most rapid, and your lungs are full it would take as little as four (4') feet of water to incur damage. But if you are rational, and not in the state of panic, you would get a full-chest feeling prior to this occurring. So, while you are enjoying the scuba experience it is important to stay relaxed and breathe normally.

     We do not want to pass over the word "panic" too lightly. Panic is a condition equaling mentally illness. It is caused by a powerful chemical called adrenaline secreted by the adrenal glands located above the kidneys. When a severe stress is applied to an animal it will react in a variety of ways. Sometimes the reaction is swift and without thought. Thinking you are going to drown could cause you to "claw" your way to the surface, instinctively hold your breath, IGNORE the warning signals, and be unaware of anything that is going on around you. It is easy to say, "Don't panic, stay calm!"but when a problem arises are you really going to be able to maintain your composure?

     First aid for a diver with lung injuries consists of the immediate administration of pure oxygen at no less than 15 liters per minute. If the diver is not breathing the oxygen should be injected into the mouth during the CPR process. This is so vital it's almost nuts to dive without an emergency supply of oxygen at the dive site. Oxygen not only keeps cells alive, but it helps to reduce bubble size as will be explained in the the chapter on decompression sickness.

     At the same time first aid is being administered, it is important to notify the nearest rescue squad so the patient can be transported to the nearest hospital. The hospital should be told about the suspected condition of the patient, and the doctor-in-charge should be told to call the Divers Alert Network (1-919-684-9111). (They accept collect calls in emergencies.) A DAN doctor, an expert in underwater medical problems, is available 24 hours per day to provide advice to the emergency room doctor.

     If a diver develops arterial gas embolism (AGE) they most likely will be rushed to the nearest Emergency Room. From there DAN may recommend to the ER physician to transport the patient to a recompression chamber that is operational. There are recompression chambers all over the world. Some of them are not operational at all times. The DAN doctor will be able to provide the latest information on chambers in the area of the accident that could be used for immediate treatment. (They do not provide this to the general public, but will provide medical personnel chamber information.) Once the diver is in the chamber the air pressure is increased. This, according to Boyle's law, will reduce the size of the arterial gas embolism bubbles. The patient will also breathe pure oxygen for prescribed periods of time while in the chamber. Hopefully, the AGE bubbles will leave the brain before too much damage has been done. The pressure is then reduced gradually to allow the diver to return to one atmosphere.

     The cost of the above might be more than $50,000! The money goes for payment to the rescue squad and hospital, transportation to the chamber (oftentimes via low-flying aircraft to keep the bubbles from growing),  the recompression chamber use, and the personnel operating the chamber.  It's a very expensive venture because one held their breath and ascended.      

     One last point: In the rare instance where an unconscious diver is brought to the surface there is no need to worry about whether the diver is exhaling or not. Unconscious divers do not hold their breath so the chances of lung damage and AGE are slim. As the diver was brought to the surface the expanding air in the lungs would escape from the mouth without doing damage to the lung tissue.

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