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, [decompression
sickness (DCS, “the bends”) and arterial gas embolism],
immersion pulmonary edema, oxygen toxicity, and nitrogen narcosis.
Understanding of diving injuries requires familiarity with the
three relevant gas laws most pertinent to diving: Boyle’s
law, Dalton’s law, and Henry’s law.
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.
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.
Because of the high density of water, a relatively small change
in depth causes a great change in the pressure exerted on a body.
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).
Thus, Boyle’s law dictates
that 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 he
or she 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 pressure–volume relationship governed by
Boyle’s law ...