This was recently posted on the freedivelist. I've been trying to get permission to repost it but exact ownership is a bit vague, Ron Mullins supplied the article to the FDL. But I thought it might be of interest here, maybe the docs can comment on the details
Ralph
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Pulmonary barotrauma:
In order to understand what happens to the lungs during a breath-hold dive, it is necessary to introduce some basic concepts about the structure of the lungs and how gas-exchange takes place between the air in the lungs and the blood.
Basic lung structure:
Beginning with the main windpipe, the trachea, the air passages divide repeatedly into branches for up to 20 generations (divisions), eventually ending in tiny blind sacs, the alveoli. These alveoli are very small (100-300 µm in diameter) and have very thin walls of about 10 µm , known as the alveolar septa. Blood flows through the the alveolar septa in very thin-walled blood vessels known as capillaries that are 10-14 µm in diameter. This has the effect of creating an enormous surface area (approximately 100 square meters) for gas exchange to take place between the alveoli and the blood (oxygen from alveoli into the blood and carbon dioxide from blood into the alveoli). The heart ensures that the body¹s entire blood volume of about 5 liters flows through the lungs every minute while at rest. During exercise this flow may increase to as much as 15 liters per minute. Air flow in and out of the lungs is generated by the diaphragm and the muscles of the rib cage acting very much like a bellows.
Changes in lung volume during a breathhold dive:
As a diver descends, the surrounding pressure (ambient pressure) increases, exerting pressure on the body, including the chest. The rib cage is compressible, so it decreases in volume, compressing the air in the lungs according to Boyle¹s Law. The volume of the lungs is additionally decreased by ambient pressure compressing the abdomen, forcing the abdominal contents towards the chest. Eventually, at about 25 meters for most people, the chest reaches its limit of compressibility and becomes very stiff. At this stage, the delicate alveoli are OK, because the pressure within the alveoli is the same as the ambient pressure, so that no distortion of the basic alveolar structure occurs.
However if the diver descends further and more pressure is exerted on the body, the air within the lungs is no longer compressed, because of the stiffness of the rib cage. What now happens is that a pressure-difference develops between gas in the alveoli and the rest of the body. Blood is now forced into the blood vessels of the lungs and the thin-walled capillaries within the alveolar septa become swollen with blood, leak and eventually rupture so that blood and fluid escape into the alveoli. This eventually finds its way into the airways and is coughed up. The alveolar septa can also swell with fluid (oedema) resulting in thickening of the septa and impairment of gas exchange (particularly oxygen). This oedema, plus the irritation caused by the bleeding into the lung tissue and airways leads to the discomfort and shortness of breath that follows and which may last for hours or days. In severe cases death ensues. The phenomenon is generally known as "lung squeeze", or in medical terms, pulmonary barotrauma.
Is the depth at which lung damage occurs predicable?
If a person takes a maximum deep breath, the amount of air in the lungs is known as the total lung capacity (TLC). If he/she then exhales to the maximum extent, the amount of air that remains in the lungs is known as the residual volume (RV) . The RV is normally about 25% of TLC. Furthermore, the chest wall usually reaches it¹s limit for compression at RV, so using Boyle¹s law, it is possible calculate the theoretical limit to which a diver can descend without danger of lung squeeze if it is assumed that the diver takes a maximum deep breath at the surface. It works out that the average diver¹s lungs will be compressed to RV at 30 meters in sea water. In fact, in some young men who have very compliant chests and perfectly elastic lungs RV is only 16% of TLC, so that they theoretically, could descend to a depth of 6 atmospheres or 63 meters. In recent years breath-hold divers have descended to more than 100 meters and this could only have been achieved by an additional mechanism that comes into play, namely that blood is shifted from the organs and large veins in the abdomen into the large veins of the chest. This helps to reduce the volume of the thoracic cage, thereby compressing the lungs further. It has been calculated that at such depths about 50% of the surface RV must be filled with blood. Experimental findings indicate that the intrathoracic blood volume is probably increased by a liter at 70 meters.
Diving mammals achieve even greater depths to more than 300 meters. These animals have very flexible rib cages and a huge ability to shift blood into their chests. They also have very large blood volumes as compared to man on a liter-per-kilogram body weight basis.
Implications for ordinary mortals who do breath-hold dives:
If a diver experiences lung squeeze, he should not persist in diving to those depths. If he does, he runs the risk of not only repeated permanent damage to his lungs, but also of an episode of severe generalized lung oedema and haemorrhage that can be fatal.
Always begin the dive with a maximum inspiration (i.e. at TLC). This gives the maximum range of compressibility for the rib cage. Theoretically you will develop lung squeeze at a shallower depth if you take a small breath before diving.
Avoid movements of your diaphragm while under water. Diaphragmatic movements (especially inspiratory movements) can lead to greater pressure-differences between the alveoli and the ambient pressure, resulting in greater engorgement of the capillaries.
Older divers must realise that as their rib cartiledges becom calcified, their rib cages become less compressible and in addition, the aging lung develops a greater RV. The TLC/RV ratio therefore becomes smaller and theoretically they will develop pulmonary barotrauma at shallower depths.
Other than the techniques described above, no amount of training is going to lessen your risk of pulmonary barotrauma. If you have any other pet theories, forget them they are merely wishful thinking.
The average medical general practitioner or even many lung specialists are not aware of the mechanisms or the dangers of pulmonary barotrauma in breath-hold divers, unless they have a special interest or have received special training in diving medicine. (Diving medicine is not included in the pre-graduate medical curriculum).
I surmise that for visible blood to be present in the sputum, probably thousands of alvoeoli have sufferred fluid swelling and bleeding into the alveolar sacs.
Johan Coetzee Feburary 2004.
