Pressure is usually referred to in feet of seawater (FSW). At sea level, we live at 14.7 pounds per square inch absolute or 1 atmosphere absolute (1 ATA). As we dive, we sustain another 1 atmosphere of pressure at reaching 33 FSW (or 2 ATA); 2 atmospheres at 66 FSW (or total 3 ATA), etc. If we had a balloon that we inflated at sea level and we submerged with the balloon and dove to 33 FSW (or the equivalent in a monoplace chamber), the balloon would decrease to half its size; at 66 FSW, it would decrease to one-third its size, etc. This is representative of Boyle's Law: at constant temperature, the volume of a gas is inversely proportional to the pressure exerted upon it.
The greatest change in volume is in the first 33 feet of seawater. Specifically, the greatest change occurs within the first 10 feet. The greatest risk for a patient undergoing hyperbaric oxygen therapy (HBOT) or for a scuba diver beginning his dive is ear or sinus barotrauma. The pressure exerted on the eardrum is equalized by releasing pressure via the Eustachian tube. Hence, “clearing” is an important task for both for the patient and for the recreational scuba diver.
As mentioned, the greatest change for the diver is in the top 10 feet of water, or, for the patient in a hyperbaric chamber, the very initial pressurization of the system. On rare occasions, as a patient or diver ascends, the expanding air may have difficulty exiting and a “reverse squeeze” may occur. The treatment for both situations is to “bounce” the pressure. Difficulty on descent would mean that you would ascend and descend and vice versa on ascent. Other options may include removal of cerumen from the external auditory canal and decongestants. If the need for HBOT is urgent, in some cases, a patient may require pressure equalization tubes also known as tympanostomy.