TooNew wrote:
I would be interested in hearing any benefits to ASV over other devices - no matter the disorder.
To answer your question, it helps to understand that ASV was designed with the goal of treating
central sleep apnea and
complex sleep apnea in mind. And hence it helps to understand what the problem in central sleep apnea is and how ASV fixes it.
When a person has real, genuine central sleep apnea (CSA) or complex sleep apnea (CompSA), one of the problems is that the person has a tough time maintaining the proper CO2 balance in their blood. The amount of CO2 is what actually acts as the trigger for the brain to send the Breathe In Now signal to the diaphragm and lungs that starts the inhalation. The level of CO2 needed to trigger inhalations is reset when we transition from wake into sleep; in sleep breathing, the level of CO2 needed to trigger inhalations is a bit higher than it is during wake breathing.
In CSA and CompSA, the complex balance of O2 and CO2 gets messed up. And when that happens, the patient will often have very, very long chains of central apneas that are part of a cyclical CO2 overshoot/undershoot cycle.
During the CO2 overshoot part of the cycle, the person is essentially hyperventilating: They're breathing too deeply and as such, they blow off too much CO2. So the CO2 concentration in their blood drops, and with it, the respiratory drive slows down. And that starts the CO2 undershoot part of the cycle.
As the respiration slows (because the respiratory drive goes down because there's too little CO2 in the blood), eventually it reaches the point where the brain "forgets" to send the Breathe In Now signal to the lungs and diaphragm altogether, and the result is a
central apnea.
As the respiration slows down, and then ceases altogether, the CO2 concentration begins to rise---this is the CO2 undershoot part of the cycle: The CO2 levels are starting to rise because the body is not blowing off enough CO2 for normal respiration. Eventually the CO2 levels in the blood are high enough for the brain to start sending Breathe In Now signals. And respiration typically starts to
slowly rebuild in terms of the size of the inhalations. (This is very different from the typical large, recovery breaths at the end of a typical obstructive apnea or even a typical RERA.)
Eventually, however, the body is once again over breathing and blowing off too much CO2 and the cycle starts over again. And again. And again.
Regular CPAP/APAP/bi-level does nothing to help even out the manner in which the body is blowing off the CO2, and in a small group of OSA/UARS patients, the continuous positive air pressure provided by CPAP/APAP/bi-level encourages over breathing to the point where a CO2 overshoot/undershoot cycle can develop. These are the folks who wind up with what are called
pressure induced centrals. And if the pressure induced centrals don't manage to resolve themselves within a few weeks of starting xPAP therapy, the person winds up with a diagnosis of complex sleep apnea.
As I said earlier, ASV machines are designed specifically to treat CSA and CompSA by stepping in at the first sign of a CO2 overshoot/undershoot cycle developing. ASV does this by using a highly variable IPAP (inhalation pressure) that can be increased substantially in the course of a very small number of breaths. APAPs and Auto bi-levels can increase the pressure, but they do it much slower and they don't make the decision to increase the IPAP on a breath-by-breath basis.
Basically when an ASV machine sees that the volume of the last few inhalations is starting to go down (i.e. is less than 90% of average inhalation during the last few minutes of breathing), the machine steps in and starts increasing the IPAP quite quickly in order to stabilize the amount of air being inhaled in each breath. As the person's respiration settles back down, the machine decreases the IPAP rather quickly so as to not trigger a CO2 overshoot.
For a person with CSA or with CompSA, there's a lot of benefit: ASVs are the only machine that can actually reduce the number of central apneas that occur during the night.
For people without CSA or CompSA, there's no clear benefit of having an ASV. Yes, it will treat ordinary old OSA just as well as any xPAP. But whether there's any additional benefit for a person with regular old OSA or UARS using an ASV is somewhat controversial.
Krakow's claim is that with an appropriate ASV titration (in his lab), the patient's wave form becomes darn near perfect, and Krakow contends that a perfect wave form must mean the sleep is somehow "better" than a decent, but not perfect wave form that a patient is apt to get from a properly set up CPAP/APAP/bi-level. But there used to be a sleep tech who posted around here under a variety of names (Sludge was one of his later names) who put it this way when he was working with people who were having trouble adjusting to their ASV machines: An ASV can be tweaked to fix the flow rate, but that's no guarantee the sleep will be any good.
Sludge would bring up the following things about ASV:
- If a person had CSA or CompSA, then ASV really was the way to go since nothing else eliminates CAs.
- ASV is not the easiest thing to sleep with, particularly if the settings are wrong.
- ASV users can often have serious leak problems that are caused by the sudden increases in IPAP and that can be disruptive to sleep---even when the wave flow looks darn near perfect
On the other hand, as Pugsy will tell you, some people have no trouble adjusting to ASV. And in Pugsy's case, using ASV means that she can get away with using a lower min EPAP setting and a lower min IPAP than she could back when she was using APAP and Auto BiPAP. She's got sleep apnea that is very mild
except in REM, but in REM her pressure needs jump up substantially. The ASV can make those huge jumps in pressure fast enough to break up her REM pressure with lower minimum pressure settings because it is so fast to responde.