P - bei Swiss-Cave-Diving

Physiology Workshop
Respiratory Issues in Technical Diving
In fact, the physiological significance of a SLL is actually quite complex, and we
discuss it here at only a superficial level.
Figure 4. Diagrammatic representation of static lung load (5).
The negative SLL is arguably the most relevant to the majority of diving situations
since it applies to an upright (or slightly head up) open circuit diver and most
rebreather diving scenarios. Even operation of rebreathers with “over the shoulder”
counterlungs is likely to result in a negative SLL because gas will tend to migrate to
the highest point, which will often be above the lungs.
A negative SLL further enhances the redistribution of blood into the very distensible
vessels of the chest cavity (described above in relation to immersion). This
redistribution is simply in response to pressure gradients. It is the same as going below
the water-line of a ship, drilling a hole in the side, and somehow sealing a balloon over
the hole. The balloon would fill with water until its elasticity overcame the pressure
tending to force the water inward. In the case of the lung in the upright diver, the
“water” is the blood of the circulatory system, which is exposed to the surrounding
water pressure, and the “balloon” is the distensible lung blood vessels.
From the heart’s perspective, the significance of this at the degree of SLL encountered
in practical diving situations is uncertain, but extremes can certainly be harmful. An
experiment in which breathing was attempted through a 2m snorkel (representing a
massive negative SLL) resulted in acute heart failure due to dilation of the chambers
by excess volumes of blood (5)!
From the lung’s perspective, even modest and commonly encountered degrees of SLL
are probably “important.” The increased congestion of the lung circulation with blood
Technical Diving Conference Proceedings 23