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INTRODUCTION

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Millions of recreational, commercial, and scientific dives are logged annually, and the vast majority of dives are completed without incident. However, there are physiologic effects and injuries relatively unique to the underwater environment. Generally, these effects and injuries are secondary to pressure changes on the submerged human body and the breathing of compressed gas.1 This chapter outlines the most common diving injuries: barotrauma of descent (otic, sinus, and pulmonary), barotrauma of ascent (pulmonary overinflation syndromes and arterial gas embolism), decompression sickness, immersion pulmonary edema, oxygen toxicity, and nitrogen narcosis.

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THE GAS LAWS

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Understanding diving injuries requires familiarity with the three relevant gas laws most pertinent to diving: Boyle's law, Dalton's law, and Henry's law.

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Boyle's law states that given a constant temperature, the pressure and volume of an ideal gas are inversely related. That is, if pressure is doubled, the volume of gas is halved. This law is stated as: P1V1 = P2V2.

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Pressure can be measured in a variety of units. The International System of Units defines pressure using the pascal (Pa). Other commonly used units of pressure include millimeters of mercury (mm Hg), torr, pounds per square inch (psi), bar, or atmosphere (atm): 1 atm = 760 mm Hg = 760 torr = 14.7 psi = 1.013 bar = 101,325 Pa = 101.325 kPa. Additionally, pressure in diving settings is often described using feet of seawater (fsw) or meters of seawater (see below). In this chapter, we use atm, mm Hg, and fsw for pressure units.

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Because of the high density of water, a relatively small change in depth causes a great change in pressure. The weight of seawater produces a change of 1 atm for each 33 ft of depth. For freshwater, pressure increases 1 atm for each 34 ft of depth. Therefore, the pressure exerted on a diver at a depth of 33 ft in seawater = 1 atm for the seawater + 1 atm for the atmosphere above the water = 2 atmospheres absolute (ATA). A diver at 165 ft of seawater would experience 6 ATA of pressure (1 atm for each 33 ft of seawater = 5 atm + 1 atm for atmospheric pressure at sea level).

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Thus, Boyle's law dictates as a diver descends in the water column, the volume of air-containing structures will decrease. For example, if the lungs contain volume V at the surface, a diver who descends to 33 ft of seawater holding his or her breath would have a lung volume of 1/2V. If the diver then breathes compressed air at this depth (from scuba equipment or from a surface-supplied source of gas), lung volume would return to V. If the diver then ascends to the surface without exhaling, lung volume would be 2V at the surface. This ...

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