P - bei Swiss-Cave-Diving

Physiology Workshop
Respiratory Issues in Technical Diving
The phenomenon illustrated in Figure 13 is “effort-independent exhalation,” named
so because once it occurs, no amount of extra expiratory effort will increase the flow of
gas out of the balloon. This is because any extra pressure created inside the bag is
applied to both the balloon and to the distensible tube leading out of it, thus there is
no net gain. Effort independent exhalation occurs in “real-life.” In fact, it is seen
during a forced exhalation in normal subjects breathing air at 1 ata. But in this setting
it occurs at such high flow rates that it doesn’t really matter. The exercising person can
still shift huge volumes of gas in and out of the lungs despite the presence of effort
independent exhalation.
The problem in diving is that effort-independent exhalation will occur at much lower
flow rates when a denser gas is breathed because the pressure drop along a tube is
much greater. Thus, Wood and Bryan demonstrated that effort independent
exhalation was almost encountered during normal tidal breathing when breathing air
at 10 ata (10). Put in more practical terms, if divers breathing air at 10 ata tried to do
much more than normal quiet breathing, they would have difficulty increasing their
ventilation no matter how hard they tried. While air at 10 ata seems farfetched, it is
not difficult to imagine gas mixes of equivalent density being used at extreme depth
given the rate at which technical diving is progressing. Indeed, as previously
mentioned, David Shaw’s trimix on his 264 mfw dive had an equivalent density to air
at 8 ata (9).
Perhaps most frightening of all, the phenomenon of effort-independent exhalation sets
up the scenario described as a major contributor to David Shaw’s death (9). Thus, a
diver undertakes exercise during a very deep dive, breathing gas at high density.
Various factors (see below) cause an initial rise in arterial CO 2
and the diver starts to
feel breathless because of the consequently increased drive to breathe. Instead of
stopping and resting, the diver tries to work through the problem. If this sounds
implausible, think about the last time you made a descent to a deep wreck into a
current! The attempts to increase ventilation intensify, and this is where the problems
really start. Increased arterial CO 2
is driving the diver to breathe harder, but
exhalation (and therefore ventilation) becomes effort independent and the extra effort
fails to produce the increase in ventilation required to lower the arterial CO 2
. In fact,
the extra effort is just wasted work and only serves to produce more CO 2
. The diver
enters a vicious spiral in which increasing CO 2
drives greater respiratory effort, which
just produces more CO 2
. This will ultimately result in respiratory muscle exhaustion,
rapidly rising CO 2
, and CO 2
narcosis leading to unconsciousness. This scenario was
predicted by Wood and Bryan in 1969 (10), and may well have been demonstrated in
a practical sense by both the Shaw accident and other accidents caused by
hypercapnia.
Technical Diving Conference Proceedings 29