P - bei Swiss-Cave-Diving

Physiology Workshop
Respiratory Issues in Technical Diving
expiratory components, and showed that divers react to an imposed resistance by
prolonging the phase (inspiration or expiration) that is loaded. More importantly,
they showed that expiratory resistance seems better tolerated in terms of both the
divers’ subjective impressions of discomfort and objective respiratory parameters. This
suggests, for example, that rebreather CO 2
scrubbers should be placed on the
expiratory side of the counterlung and not the inspiratory side. Warkander et al. also
published maximum thresholds for inspiratory and expiratory resistance, but these
technical issues are beyond the scope of this paper.
Increasing Gas Density
The density of any given breathing gas increases linearly with depth. Technical divers
substitute helium for nitrogen in gas mixes for deeper diving, which substantially
reduces density. Nevertheless, at the depth targets being set by some extreme
exponents, gas density still increases significantly despite the use of helium. For
example, on David Shaw’s widely reported fatal dive, the use of trimix 4:82 at 264
mfw equated approximately to air at 70 m (8 ata) in terms of gas density (9).
Dense gas impacts significantly on respiratory function primarily by increasing
resistance to flow through airways and thereby limiting ventilatory performance.
Indeed, if you ask a subject to ventilate as hard as they can whilst breathing air at the
modest dept of 30 m (4 ata), the maximum volume they can shift over a minute is
only half of that at the surface.
Work and exercise requires gas exchange, and gas exchange (particularly CO 2
elimination) requires ventilation. The clear implication of progressively limited
ventilation with increasing depth is that as depth increases the diver’s work capacity
decreases. Indeed, it is plausible that the maximum depth, which technical divers can
visit, may ultimately be determined by their ability to cope with the work of
breathing, let alone any other work such as swimming. Even at more modest depths,
there is some evidence that self-perpetuating respiratory failure (inadequate
ventilation) scenarios might be encountered during diving where the work of
breathing is high, and other heavy exercise is also attempted.
Increased gas density will increase the work associated with both inhalation and
exhalation. However, arguably the most dramatic and limiting effects may relate to a
phenomenon seen during expiration called “effort independent exhalation.”
Effort Independent Exhalation and Respiratory Failure.
This complicated physiological phenomenon is explained in a step-wise simple
manner below. Follow the series of diagrams through in sequence, along with the
explanatory notes.
Technical Diving Conference Proceedings 25