P - bei Swiss-Cave-Diving

Physiology Workshop
Respiratory Issues in Technical Diving
seizure. In practice, this is difficult to do accurately, and the results of the test are hard
to interpret.
Increases in work of breathing. As previously mentioned there is a tendency for the
respiratory controller to reduce its sensitivity to CO 2
when work of breathing increases
(3). Put another way, the respiratory controller will tolerate higher levels of CO 2
if an
increase in work would be required to eliminating it. Although there may also be some
individual variation in this tendency, this is relevant to all divers because, as previously
discussed, the work of breathing virtually always increases during diving.
Higher pressures of oxygen and nitrogen. There is some suggestion that the sensitivity of
the respiratory controller to CO 2
falls in the presence of hyperoxia or when high
pressures of nitrogen are breathed (4).
2. Conscious overriding of the drive to breathe.
To a point, divers can consciously override the urge to increase ventilation. This is
sometimes invoked as a strategy to conserve gas and has in the past been referred to as
“skip breathing.” The earlier discussion of gas exchange and dependency of CO 2
elimination on ventilation should make it clear why skip breathing with an elevated
inspired PO 2
would be fine from an oxygenation point of view, but will result in CO 2
retention. This is a dangerous practice and should be discouraged.
3. Adoption of a disadvantageous breathing pattern
There is about 15 ml of dead space in the respiratory tree and this is inevitably
increased by the addition of underwater breathing apparatus, though good equipment
is designed to minimize this. For arguments sake, let’s assume that a diver has about
200 ml of dead space accounting for both the anatomical and equipment dead spaces.
Dead space gas is “last out and first in.” Thus, it is gas from the alveoli that occupies
the dead space at the end of an exhalation, and it is the first gas to be drawn back into
the alveoli during an inhalation. Dead space gas is oxygen-depleted, and CO 2
-rich
when compared to fresh gas, and its re-inhalation with each new breath represents
wasted ventilation.
Under normal circumstances, this should not matter much. The normal tidal volume
is about 10 ml/kg, so for a 70 kg adult it is approximately 700 ml. Assuming 200 ml
of dead space for a diver this means that 500 ml of each breath is fresh gas. However,
problems can arise if a diver adopts a rapid breathing pattern with low tidal volumes.
If the tidal volume were to drop to 400 ml, then dead space gas represents half of each
breath. It is for this reason that divers are encouraged to adopt a pattern of slower
deep breaths in preference to a pattern of fast shallow breaths.
Technical Diving Conference Proceedings 31