Well, you shouldn't be on BiPAP for obstructive sleep apnoea for a start. Whether it's more comfortable or not is a matter of individual taste, but it's definitely a more expensive and complicated machine (surely they must also cost more in America?) and you don't need it if that's the only reason you've been given. BiPAP is for treating central apnoeas and respiratory failure, not just OSA. BiPAP can only be properly set in a sleep study by an experienced technician. It is a far more complex device than CPAP, there are way more variables to take into account, and it is important to see how your body is responding as adjustments are made. Also, it may be part of the cause of your onset events. BiPAP makes you breathe more air in and out, which can lower your CO2 levels. It is a tiny rise in your CO2 that tell you when to breathe, and if your CO2 is too low your breathing reflex is nor triggered, causing central events. You should be able to get your BiPAP set in CPAP mode instead of buying a new machine. Also, I'm not sure I'd stick with a doctor who recommends inappropriate treatment like that. See if you can find someone who can recommend a good sleep specialist and get some proper advice and management. Is the AHI of 15.9 when you are using your BiPAP, or in your diagnostic study? Did you ever have a titration study with a CPAP?
I Just noticed that in another post you said that your pressures are 8/4. This is equivalent to a CPAP pressure of 4 (not 8), which is as low as they go, so it's probably not helping your obstruction either.
Are you sure you are on BiPAP and not an AutoPAP or an AutoBiPAP? People on AutoPAP have to go through several apneas/hypopneas before they get to an optimal pressure. Some people (myself included) cannot tolerate the sleep onset apneas and it makes getting to sleep very hard. It is quite common with people with severe apnea, but other people can have it too. What also happens is if the pressure range is set too low it can keep your AHI higher because it is not hitting the correct pressure settings. Some AutoPAP prescribing docs will put a pressure range of 4-20cm H2O or 4-15cm H2O but others set it lower and it can also cause problems. The statement that BiPAP is not for treating OSA is not 100% accurate. In patients were expiratory pressure is intolerable it works wonderfully so comfort is often a deciding factor as to whether to switch, the presence of emergent centrals is also another deciding factor, followed by ASV in the presence of unresolvable central apneas. Respiratory failure we generally do not treat in the sleep lab.
My advice would be to ask your doctor or medical equipment company what you are using, explain to them the issues you are having and see if they can come up with a solution. "I've gone through two sleep doctors who one, never heard of it, and the other one doesn't know what to do about it." Not sure where you are in the world, but there needs to be a sleep doc that can answer you or do something for you to help. You said you turned off your ramp, this is a start. But I suspect you need to start at and stay at your optimal pressure to keep your airway from collapsing at the start of your sleep. To do this you need to get an attended titration study, with a qualified sleep technologist, to monitor your titration and get you to an optimal state. Even people with a 15.9 AHI can require well over 20/16 cm H2O to keep their airways open.
I find it interesting that you do not treat respiratory failure in your sleep lab. I spend A LOT of time treating respiratory failure of various kinds in the sleep lab because we are the ones with the expertise in using BiPAP which is by far the best tool for treating chronic respiratory failure in many instances. Having said that, I know that we treat far more respiratory failure patients than any other lab in the country that I am aware of.
I know that BiPAP is sometimes used in treating OSA simply for improved comfort, or at least has been in the past, but this practice has generally been superseded by use of CPAP with pressure relief which achieves exactly the same thing. BiPAP was not designed for treating OSA, and using it to do so is almost always rather unwieldy and unnecessary. A BiPAP costs several times as much as a CPAP. Using one to do the job of a CPAP is a bit like using a helicopter to go to the local shops in stead of a car - whilst it could actually be a sensible option in a few rare cases, it's almost always going to be an unnecessarily complicated waste of time and money. I have also come to discover over the years that few doctors properly understand how BiPAP works, which further complicates things.
I do agree that the starting point is a trustworthy sleep specialist. They should be able to guide you through the correct process, and be willing to educate you in what they are doing and why.
Could you clarify exactly what make and model machine you have? I'm wondering if you have an APAP machine with two set pressures (minimum and maximum) rather than a true BiPAP? Either one should work for you, but it would be helpful to know exactly what machine you have and what it is set at.