Clear airway (or central apneas) are where you technically do stop breathing for a spell, as opposed to Obstructive apneas which are caused by your airway closing off. Hypopneas are partial obstructions which are not completely closed, but closed enough to cause your oxygen to drop and/or wake you up.
It sounds like they are using deadspace treatment on you, where they use a non-vented mask and a specific length of hose to force you to rebreathe your exhaled CO2 in order to stimulate the breathing response during the central apneas (the theory being that the body has a CO2 threshold that forces us to take a breath of O2.) It works but people can and do find it uncomfortable. It also needs to be set up and monitored by a qualified technologist in order to make sure it is safe and effective.
sleeptech is correct that ASV (adaptive servo ventilation) is the general standard for treating central or complex sleep apnea. The way ASV works is it monitors your breathing for the first 20 or so minutes and calculates an algorithm which forces the air into your lungs during the times where you have a central apnea. essentially breathing for you. I have seen it work wonders in patients with Hypocapnia, Renal Failure, Congestive Heart Failure and COPD where Central Apneas are quite common in patients. However, your doctor would need to determine if your ejection fraction is above 45 before undergoing a titration in order to ensure its safety.
Hope this helps.
P.S. 3.9 is a normal AHI. so the treatment is working by most evaluations.
What are the pros and cons of the using a Cpap with the dead space vs. the ASV device? Note I did do a sleep study with the mask with the dead space so they could come up with the correct length and pressure (BI in Boston). This is a major teaching hospital in Boston and I wonder why they are messing with this dead space method to increase CO2 rather than use the ASV?
Also is a normal AHI all we are after? If increasing CO2 results in a normal AHI that's great but how healthy is it to breath normally with less O2 (and more CO2)? How do we know its not out of check and the CO2 is to high.
Thank you for the advice
You report waking with a headache, which is a classic symptom of elevated CO2 levels. I'm not sure why they are using the non-vented mask treatment. I have heard of it before, but ASV has been the industry standard in treating respiratory events due to hypocapnia for quite a few years now. It is correct that you would need to have your heart check to see if your left ejection fraction of above 45%, and would hope that your doctor would do this as a matter of course as it is part of the standard procedure. You are also correct in your suggestion that giving you a low AHI but elevated CO2 (above the healthy limit) can also be injurious to your health and so is not a great solution. If you are struggling with your current treatment then I would hope your doctor would at least discuss the idea of ASV and its possible suitability or otherwise.