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Physiology Workshop
Respiratory Issues in Technical Diving
Thus, normal ventilation is achieved by active movement of the diaphragm and the
intercostal muscles (for inhalation), followed by passive recoil of the lung, chest wall
and diaphragm (for exhalation). In fact at rest, breathing is brought about almost
solely by the action of the diaphragm. During exercise, the intercostal muscles also
become involved, actively expanding the thorax during inhalation and contracting to
help force the air out of the lungs during exhalation.
In respect of quantifying the movement of gas in and out of the lungs, there are some
definitions that may arise later which are worth noting now:
Tidal volume = is the volume of gas moved with each breath.
Functional residual capacity (FRC) = the volume of gas left in the lungs at the end of a
normal “tidal” exhalation.
Residual volume = the volume of gas left in the lungs at the end of a maximal
exhalation.
Vital capacity = the volume of gas that can be exhaled down to residual volume after a
maximal inspiration. Put simply, this represents the biggest breath that can be moved
in and out.
Maximal voluntary ventilation (MVV) = the volume of gas that can be moved in and
out of the lung over one minute when the subject is panting as possible.
Work of breathing
It is important for divers to understand that breathing requires physical work. It is
fairly obvious that if the lung is elastic, then work must be done during inspiration to
overcome that elasticity and stretch the lung. Similarly, there is work involved in
moving the “weight” of the chest wall and other soft tissues during inspirations. Less
obvious is the work that must be performed to overcome resistance to gas flow
through the conducting passages in the lung. Like any other work, the work of
breathing results in the consumption of oxygen and production of CO 2
.
We don’t notice the work of breathing under normal circumstances because we are
well adapted to the normal demands. However, it is important in diving because it can
be increased by immersion, the use of underwater breathing equipment and by
increases in gas density. Though perhaps stating the obvious, the diver’s ability to
respond to increases in work of breathing is not unlimited. The corollary is that
respiratory muscle exhaustion or failure to respond to increased work demand for any
Technical Diving Conference Proceedings 15