feeling goes away. Swallow, wiggle your jaw and start to ascend slowly. You might need to repeat this technique throughout your ascent. If the block does not equalize, and you must surface, close your nose and mouth and breathe in. If nothing works, ascend as slowly as possible. In some cases, a block might release quickly. If this occurs, the sudden change in your middle-ear pressure might cause some dizziness. The dizziness will pass quickly. Remember to hold on to something if you experience vertigo. Sinuses Your sinuses are air cavities lined with mucous membranes and surrounded by the bones of your head (figure 4-24). Your head has four sets of sinuses. One set is above your eyes, and the other three sets are behind your nose and cheekbones. Each of your sinuses is connected to your nasal airway by an air passage. Under normal conditions, the passages to your sinuses are open. However, when you have a cold or congestion due to allergies or other reasons, the sinuses will clog. Sinus Squeeze and Blockage If air is trapped inside a clogged sinus, and you attempt to dive, you will feel pressure on your sinuses because the air pressure inside the sinus is less than the surrounding pressure. If you continue to descend when the pressure within your sinuses is less than the surrounding pressure, the soft tissue surrounding the sinus will be pushed into the sinus as the tissues react to equalize the pressure (figure 4-25). This is quite painful and can cause blood to flow into the sinus to fill it. If you cannot equalize your sinus normally during descent, it will fill with body fluids. Then, during ascent, the air in the sinus will try to expand to its original volume but it cannot because of the fluid. Pressure builds in the sinus and pain results. Blood can be forced out into your nose, mouth, or mask. A sinus block traps air in a sinus cavity at depth. The air tries to expand on ascent, but cannot, causing pain (figure 4-26). If your sinuses equalized during descent because of medication you had taken to clear your sinuses, and that medication wore off at depth, the openings to your sinuses would close off and trap air inside. As you ascend, the air in the sinus will try to expand, pressure will build, and you will feel pain. You can relieve the pain by descending again, but if you are running low on air, this might not be an option. Try breathing in with a closed mouth and nose. If this does not help, surface as slowly as possible. NAUI Scuba Diver 110 Diving Science FIGURE 4-24. YOUR HEAD CONTAINS FOUR SETS OF SINUSES THAT MUST EQUALIZE WHEN DIVING. Congested sinus Response to a squeeze FIGURE 4-25. THE EFFECT OF A SQUEEZE ON A SINUS IS TO FILL THE VOLUME WITH TISSUE AND BLOOD TO EQUALIZE THE PRESSURE.
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