Hello again,
First, Gian, although it may be interesting, can we PLEASE not drag this thread into another debate about the relative WOB between rebreathers. You have another massive thread about your spreadsheet somewhere else.
paulraymaekers said:
where we all confuse is mixing up WOB with hydrostatic imbalance: hydrostatic imbalance will directly influence lung loading, more then pure WOB.
I wasn't confusing it Paul, but you are right... there are two very separate issues here:
1. Hydrostatic imbalance, that is, whether you have a negative or positive static lung load. Simply put, when you are connected to a rebreather the pressure in your airways is the same as the pressure in the counterlung. If the counterlung is deeper than your lungs (eg front mounted in a horizontal diver), then the pressure in your airways will be greater than the pressure of the water (and tissues) surrounding them. If the counterlung is shallower than your lungs (eg back mounted in a horizontal diver) then the pressure in your airways will be less than the pressure of the water (and tissues) surrounding them. What is the significance of this?
Forget what you are wearing on your back for a second. A significant part of this thread has been dedicated to discussing expiratory flow limitation. To paraphrase what has gone before (see earlier posts) during expiration as gas passes out along the airway the pressure in the airway drops due to friction. If your lungs are providing extra effort to exhale quickly (eg when you are working and trying increase breathing) then the pressure inside the chest generated by your respiratory muscles pushes on the outside of the airways and can cause them to collapse if it exceeds the falling pressure inside the airways. When breathing air at 1ATA this happens, but only at incredibly high gas flow rates through the airway... so we don't really notice it and can still exercise (and breathe) hard. However, when diving with a dense gas the pressure drop inside the airway happens more quickly, and the airway collapse described above will happen at much lower flow rates. It follows that our ability to ventilate the lungs (and therefore work) is reduced. The greater the density, the less ventilation (and work) we can perform before ventilation is limited. Because CO2 elimination is entirely dependent on lung ventilation, it is possible to get into situations where it is difficult to ventilate enough to keep the CO2 normal, but this would usually only occur when breathing very dense gas.
In this context, the significance of hydrostatic imbalance / static lung load is that a negative static load (see above) further increases the risk of airway collapse during exhalation. However, you would still likely need to be working hard with a pretty dense gas for your ventilation to be limited below what you require to keep CO2 normal. Coughing or grunting at the end of exhalation may be a sign that this is happening, though coughing is a fairly non-specific event on a rebreather. Other things can cause coughing.
2. Work of breathing in the rebreather. This is the work required to move gas around the loop, and is tested by connecting the unit up to a test device (such as an ANSTI machine) and measuring the work required to move a pre-determined amount of gas around the loop under a set of standard conditions. Hydrostatic imbalance is just one of many things that potentially effect this, and the work of breathing of the unit
per se is not a big player in determining whether you experience the expiratory flow limitation I have described above.
Work of breathing is, however, a BIG player in putting a diver at risk of retaining CO2: NOT because of expiratory flow limitation, but rather because when the work of breathing is high, some divers respond to rising CO2 by not trying hard to increasing ventilation in order to lower it again. Put another way, the brain subconsciously chooses to let the CO2 rise rather than perform the extra work to ventilate it off. We sometimes refer to such divers are called CO2 retainers. I can discuss this in more detail if people are interested, but the point I am trying to make here is that work of breathing is important (very), but not so much as a determinant of expiratory flow limitation.... that is mainly related to flow rates and gas density... and it may be worsened by a negative static lung load.
Now, if you have been following this so far and don't want to risk getting confused, then stop reading here.
It is possible that these two processes could be linked in precipitating a CO2 crisis in the following way. Let's say we have a diver who tends to retain CO2 (a CO2 retainer) breathing on a CCR with a high work of breathing at a fairly deep depth with a dense gas. Remember that the reason they retain CO2 is that they don't try hard to breathe it off, so the CO2 level in the blood rises. They actually feel comfortable breathing at a low rate as their CO2 rises (and I reiterate that this is why their CO2 rises). However, even a CO2 retainer will eventually respond to very high levels of CO2, and there is some evidence that it is these divers who are at high risk of developing severe symptoms of CO2 toxicity in a precipitous way. In other words, they transition from comfortable to very uncomfortable very quickly. Thus, our CO2 retaining diver increases their breathing effort from not much to very high over a short space of time, and now, with a CO2 level already very high, they suddenly encounter expiratory flow limitation because they are trying so hard to breathe. This is the sort of scenario where the diver might not be able to get out of trouble simply by sitting and resting. I hope you can see what I am getting at.
Simon M
