Pulmonary Barotrauma
Pulmonary barotrauma is any damage to the lungs from
unequal air pressures. The greatest danger to the diver on
ascent is from pulmonary barotrauma, a risk greatly increased
if
breath is held. The consequence can be serious, even fatal. The
potential for pulmonary barotrauma is why the first rule of
scuba diving is never hold your breath.
Continuous breathing allows the lungs and communicating
spaces to maintain equilibrium with the increased ambient
pressure. If breath is held on ascent, air in the lungs will try to
expand against an essentially fixed chest volume; depending on
the vertical distance of a breath-hold ascent, the result can be
anywhere from simple over inflation to lung rupture and
passage of air into the blood stream.
A maximum safe rate of ascent in RSD is considered to be 60
feet a minute (the slower the better); at all times the diver
should continuously breathe. Should a diver run out of air and
have to make an emergency ascent, proper technique requires
continuous exhalation under water (by saying "Ahh...") in
order to vent the expanding lung volume. This maneuver keeps
the glottis open and allows continuous exhalation of
expanding
air, so the lung volume does not increase.
For an equivalent change in depth the risk of expansion
barotrauma is greatest near the surface, a fact explainable by
Boyle's law. A breath-holding scuba diver rising from 33-feet
depth to the surface experiences a change in ambient pressure
from two to one atmospheres absolute; if the lungs fully
expand within the chest cavity lung volume will try to double.
By contrast, a 33-foot rise from 99 to 66 feet depth (i.e., from
4 to 3 atmospheres) would maximally increase a
breath-holder's lung volume only 33 percent, posing much
less
risk of barotrauma.
The Greatest Risk of Expansion Barotrauma is Near the Surface.
Barotrauma correlates with both increase in pressure in the
lungs and 'over stretching' of the lung tissue.
Experiments in dogs undergoing rapid ascent in a chamber
showed that the lungs can withstand much higher pressures
(before barotrauma occurs) if the chest cavity is bound and
'over stretching' is prevented (Schaefer 1958).
Although both over stretching of lung tissue and the pressure
of expanding air are factors favoring lung trauma, pressure
seems to be the major one.
The pressure difference across the lungs (from inside to
outside) that is the threshold for experimental barotrauma is
about 80 mm Hg; this can occur with a breath-hold ascent
from only four feet! The pressure difference (and risk of
barotrauma) is obviously much greater with breath-hold from
greater depths.
During a breath-hold ascent from 33 feet the lung volume
would try to double, almost guaranteeing barotrauma if breath
were held at or near the diver's total lung capacity (Figure 3).
If
the lungs could not vent expanding air they would be
subjected
to a distending pressure of nine times the barotrauma
threshold!
Adventure Dominica