By Fred Bove, M.D., Ph.D.
Perhaps the most troublesome part of diving is the continuous need to avoid the consequences of pressure changes associated with depth. The physical effects of pressure are described by Boyle's Law, which defines the relation between pressure and volume. Boyle's Law states that the product of absolute pressure and volume of a gas is constant: P¥V = Constant. If air pressure in body spaces cannot equilibrate with ambient pressure on descent, the forces produced will reduce the volume by squeezing surrounding tissues into the space. On ascent, air in the lungs must be allowed to vent or overinflation will occur. Tissue injury caused by Boyle's Law effects is called barotrauma or squeeze. The accompanying table shows the changes a five liter volume undergoes as it is subjected to a dive of 100 feet. The greatest volume change is in the first 10 feet. Because the greatest volume change is at the shallowest depth, the most common time for squeeze to occur is during the initial portion of the descent or, in the case of lung barotrauma, during the final portion of the ascent.
Ear squeeze is the most common diving related injury. When the Eustachian tubes do not open to allow pressure to equalize across the eardrum, the pressure difference stretches the drum and causes injury. Enough force can tear or rupture the eardrum. The Eustachian tubes can be blocked by mucous or swelling owing to colds or allergies. Middle ear squeeze causes swelling of the lining of the middle ear and fluid accumulation. Often the fluid will not clear until the swelling has subsided and normal Eustachian tube function returns. This frequently requires five or six days and, occasionally, one to two weeks.
Inner ear barotrauma or round window rupture damages the hearing and balance mechanisms. Inner ear barotrauma during descent is caused by forceful efforts to equalize the middle ear. If the ear does not clear, the large pressure difference can lead to rupture of membranes that seal the inner ear fluid from the middle ear. Round window rupture is accompanied by vertigo, hearing loss, nausea, vomiting, noise in the ear and, sometimes, a feeling of weakness. If a round window rupture is suspected, removal of the diver from the water, bed rest, head elevation and medical consultation should be sought. Divers should not be recompressed for treatment of round window rupture, as this will often aggravate the fluid leak. Surgery may be needed to repair the damage.
Reverse squeeze, where the eardrum bulges outward, can occur during descent when a tight fitting hood blocks the external ear canal. The increasing middle ear pressure will not be balanced by external pressure because of the blocked canal and pressure inside the middle ear will exceed the pressure in the occluded canal.
Air in the space between the mask and the face also obeys Boyle's Law. If air is not added to the space during descent, it will be reduced as ambient pressure increases until the tissues of the face and eyes are forced into the collapsing mask. Treatment for mask squeeze is directed toward symptoms. If the eyes are painful, local analgesic drops can be used and some physicians will recommend antibiotic drops to avoid infection in the injured tissue. To prevent mask squeeze, equalize the mask during descent.
If the air passage into a sinus is blocked, the air trapped within will be forced to decrease in volume as pressure increases. The sinuses are air filled chambers within the skull. Although the bony structure will not collapse under the pressure changes, the lower pressure in the blocked sinus will draw blood into it. Blood vessels will engorge and leak. The squeeze results in a blood-filled sinus, which will drain during ascent when the air in the sinus expands. A bloody nose and a sinus headache after diving usually accompany a sinus squeeze.
Air spaces inside the teeth owing to decay, associated with a filling or an abscess, change volume as pressure changes during diving. An air pocket under a filling may expand on ascent and push the filling out of the tooth, while an air pocket may be distorted during descent and cause a severe toothache. If you get a toothache during ascent, there is little you can do but continue your ascent. Pain medication will help on the surface.
Although it is theoretically possible to get a lung squeeze by descending while breath-holding to depths greater than about 150 feet, the freediving record is nearly 300 feet. When the lungs are emptied at the end of exhalation, about 20 percent of the total lung volume remains. One would expect that a depth of five atmospheres absolute (132 feet) would be the limits of lung volume compression during a breath-hold descent. Research has shown that blood shifting into the blood vessels of the lung can displace more volume and allow a diver to withstand a greater volume reduction than the theoretical 5:1 ratio allows. The extreme rarity of this condition indicates it is not a problem with diving.
Air in the stomach at the time of ascent from a dive will expand according to Boyle's Law. Some divers swallow air, which will accumulate in the stomach during a dive. A regulator providing excess pressure at the mouthpiece can force air into the stomach. Ascent with air in the stomach will produce severe abdominal cramps. Further ascent may cause stomach rupture. Prevention is the best management of this problem. Be sure your regulator is functioning properly and avoid swallowing air while diving. If cramps do occur during ascent, stop and try to belch to remove the air from the stomach.
Drysuit users may notice painful red streaks on their skin after diving. These are caused by suit folds lying against the skin during descent. Folds containing air are compressed against the skin, the skin is forced into the folds and local injury occurs. Always leave enough residual air in a drysuit so it is not tight against the skin. Wearing a light garment (thermal underwear) under the suit will prevent the suit from sealing tightly against the skin.
All divers will experience the effects of squeeze. Careful attention to equilibration of air spaces during descent and ascent, good dental hygiene, avoiding air swallowing, maintaining equipment and attending to the health of your nose, throat and sinuses are measures that will help avoid injury owing to squeeze or barotrauma.
Sharm el Sheik Emergency and Trauma Center: A new Emergency and Trauma Center in the well-known Red Sea diving resort area of Sharm el Sheik has recently been opened. The hospital, the first of its kind in southern Sinai, specializes in diving medicine and includes a physical therapy clinic and a pulmonary therapy clinic. Sharm el Sheik Emergency and Trauma Center is close to many hotels and the downtown area.
Further information may be obtained by contacting Dr. A.G. Hamada, 8841 Garden Grove Boulevard, Garden Grove, CA 92844 or call (714) 537-6780, fax (714) 537-6782.