Hello,
The grunting or coughing that may be seen at the end of exhalation when breathing a very dense gas is a potentially a symptom that expiratory flow limitation is occurring, and it is an unconscious attempt on the part of the diver to overcome that limitation. The problem is, because of the physiology involved it does not help to increase gas flow. To understand this you could maybe watch the RF3 presentation I did that others have linked to. So in that sense it is "induced" rather than "wanted". It is not a strategy that you can intentionally use to ameliorate the situation. If it is encountered, what you need to do urgently is reduce work (and therefore ventilation requirements) and the density of the gas you are breathing (eg by ascending).
Hi Kev,
It should definitely make you ask yourself whether you struggling to move enough gas in and out of your lungs to match the amount of work you are performing. If you are deep, with dense gas and working hard then the answer is likely to be yes. If this is the case then you are on a slippery slope toward hypercapnia (if not already there), and you need to do something about it (such as the advise in my reply to Naumachos above).
Simon M
The particular incident which comes to mind happened to me while doing a solo-dive at 430 fsw in Indonesia a while back (yes, I know, a "bozo-move", but we're all much smarter today, aren't we? lol...).
I was filming some interesting critters which looked like sea-grass coming out of the sand. I wasn't moving hardly at all - very peaceful, and since I was focused on filming and watching the action, I wasn't really thinking about my kit. Once I was done, I was looking around for anything else interesting, having about 2 minutes of film left, when I started to get the feeling of "impending doom" that others have described prior to some kind of event - it came out of nowhere. I checked my ppO2, and everything was spot-on 1.2 for all 3 cells. Did a Diluent Flush to verify the sensors (had to do the math in my head) and they lined up properly. Since I was on a Mark 16, I did what I'd found myself worked sometimes to clear things up - I knelt on the sand, and leaned backwards (thereby moving the counterlung to a lower position than my lungs, which made the loop go positive-pressure), and inhaled a few breaths deeply.
This seemed to make the "doom" feeling pass, and I then proceeded to ascend up the reef wall in a leisurely fashion and completed my deco without further incident. I've always wondered about that dive, however, if something else was amiss, or if I was just creeping myself out because I was so deep and alone. My unit performed flawlessly throughout the 3 weeks we were diving there, and we did multiple 300+ foot dives thereafter, without incident.
I think the work that Simon and the other authorities on human physiology are doing these days is a huge benefit to divers understanding of what exactly goes on during our diving operations, and I look forward to learning more from them.
Thanks again for all your contributions.
Kevin.
