Thanks all for the good discussion.
It is not clear to me why negative lung loading would be worse then positive lung loading.
I always learned that to avoid getting into a spiral of CO2 intoxycation, you must be able to ventilate your deeper alveoles.
Now it seems that positive lung loading works against a complete full exhale, while negative lung loading makes exhaling easy, but inhaling mork difficult.
so...?
Please note that this is not a pro or contra agains OTS or back-mounted, as both types of counterlungs allow a sweet spot in orientation where the lung loading is neutral, or can even be shifted from positive to negative. Just for correct understaning
Hi Paul,
Same.... my comment is not an argument for avoiding back mounted counterlungs. I have dived them for many years in the 15.5 myself.
Dynamic airway compression happens because the pressure inside non-rigid segments of the airway falls below the pressure inside the chest during an expiration. The airway will then collapse and flow will be limited. This even occurs when breathing air with healthy lungs at 1 ATA during a forced exhalation (the spirometry tests that we all will have done) but it does not matter in this setting because flow limitation only occurs at extremely high flow rates; high enough that we don't even notice it and can exercise as hard as we like... we'll always be able to breathe enough to get rid of the CO2 we produce.
At depth the increased density of the gas contributes to a faster drop in pressure as gas passes along the airway, and so airway collapse occurs with less expiratory effort, and at lower flow rates, thus limiting ventilation much more significantly. Now to the point.... If the exhalation begins with the airways already subjected to a negative pressure (negative static lung load) then collapse will likely occur even more quickly and at even lower flow rates. A slight positive static lung load has the opposite effect of
helping to splint the airway open and prevent collapse as Harry implies. This is almost certainly why studies with heavily exercising divers have demonstrated a slight positive static lung load to be better tolerated from a respiratory point of view than a negative static lung load. Note the emphasis on "slight". A small positive lung load does not impair alveolar emptying. But I am sure you are right to imply that a very large positive static lung load would impair ventilation and be less well tolerated. As you suggest, there is a sweet spot for all of these things.
If you were designing a rebreather with the principal goal of supporting a diver routinely undertaking heavy exercise (eg attack swimming in the horizontal attitude) then you might think seriously about using a front mounted counterlung or at least some arrangement with a slight positive rather than negative static load in most postures. But for 99.9% of technical diving applications I would not be wringing my hands over whether my lungs were OTS or back mounted based on concerns about breathing.
There are references for all of these issues and they can be found in our review of respiratory physiology in hyperbaric conditions. I am happy to send it to anyone who wants it (pms with email addresses please).
Doolette DJ, Mitchell SJ. Hyperbaric conditions. Comprehensive Physiol 2011;1:163-201
Simon M
