Constantly increasing in number, the recreational and commercial diving community frequently presents with problems poorly understood by the average physician and otolaryngologist unless they have had some training in diving medicine. The consequences of breathing compressed gas mixtures under increasing barometric pressure and subsequent decreasing barometric pressure are confusing unless one understands the physics and physiology of the pressure environment. A well-trained otolaryngologist can be better prepared to treat the conditions that divers encounter by understanding the cause of these conditions.
Living at the sea level, our bodies are surrounded by one atmosphere of pressure (eg, 14.7 psi, 760 mm Hg, and 1 bar). The entire earth's atmosphere exerts this pressure, and it is exerted uniformly against our bodies. Pascal's principle states that any change in pressure in an enclosed fluid is transmitted equally throughout that fluid. The human body, being to a large extent fluid, pushes out against the ambient pressure with the same force as the surrounding media. For this reason, divers can descend in the water to extreme depths with ease. It is only the air-filled spaces in our bodies that are affected by the changes in pressure. For each 33 ft of seawater (ie, 34 ft of fresh water, or 10 m) through which we descend, we add an additional atmosphere of pressure. The pressure is doubled going from sea level to 33 ft of seawater, but is not doubled again until 99 ft of seawater is reached (Figure 57–1).
Ambient Pressure Relative to Depth.
Conversely, as one ascends from depth, the pressure is decreased at the same rate. Boyle's law states that if the absolute temperature remains constant, the volume of a gas varies inversely as the absolute pressure. Because water temperature remains within a small absolute range, as a diver descends in the water, the air-filled spaces decrease in volume proportionately. As the diver ascends, the air-filled spaces increase in volume proportionately. Since we double the pressure from the surface to 33 ft of seawater and not again until 99 ft of seawater, the greatest pressure and volume changes occur closest to the surface. With the exception of decompression sickness, most divers' problems occur in the shallow depths, even as shallow as 4 ft of seawater.
Dalton's law of partial pressure states that in a mixture of gases (eg, air), the total pressure exerted by that mixture is equal to the sum of the partial pressures of each gas in the mixture. Both nitrogen and oxygen, composing most of the air we breathe, increase in partial pressure as the ambient pressure increases. Henry's law of solubility of gases states that as the partial pressure of a gas increases, more of that gas is dissolved in the surrounding liquid until saturation occurs. Since oxygen is utilized in metabolism, nitrogen, which is metabolically inert, is ...