robysue wrote:The basic problem in central sleep apnea is what can be described as a Undershoot/Overshoot cycle as far as the nighttime CO2 levels are concerned. That is for whatever reason, you stop breathing as you should. This is the initial UNDERSHOOT side of the cycle. When that happens, the CO2 builds up in the blood stream until your body finally kicks in and restarts respiration. However, since the CO2 has built up abnormally high you start to slightly hyperventilate and blow off too much CO2. This is the OVERSHOOT side of the cycle. You blow off too much CO2, and this reduces the respiratory drive and that sets the stage for another UNDERSHOOT side. On the UNDERSHOOT side of the cycle, your breathing becomes shallower and shallower and may stop altogether---i.e. you have a central apnea. At the end of the central apnea, you start breathing, but you hyperventilate again (the OVERSHOOT), which sets up the next UNDERSHOOT, and the cycle repeats.
The ASV machines are designed to treat central sleep apnea by monitoring how often you breathe and how much air you are inhaling. And it tackles treating the CSA with two tactics.
First, when you have not taken an inhalation in a certain amount of time, the machine will try to trigger an inhalation by rather drastically increasing the IPAP and then (at a set time frame) lower the pressure back down to your EPAP setting. If breathing does not resume, it will continue to try to trigger inhalations at a fixed 'breaths per minute rate' by first increasing the IPAP and then reducing the pressure back down to the EPAP. In other words, the machine will try to make sure that your lungs have enough O2 coming in to maintain your O2 levels while at the same time trying to insure that you will retain enough CO2 to trigger the breathing pattern. Both BiPAP S/T machines and ASV machines have this capability. Regular BiPAPs (including the BiPAP Auto), APAPs, and CPAPs doe NOT have this capability to trigger a patient to inhale.
But the ASV machines go a step further in treating Central Sleep Apnea: An ASV machine will, in essence, try to fix the central sleep apnea by eliminating the repetitive Undershoot/Overshoot cycle that sets up in central sleep apnea. To break this repetitive cycle, the ASV monitors your tidal volume since the tidal volume is a good indicator of how much CO2 you are blowing off with each exhalation. Low tidal volumes indicate that you are starting to drift into the UNDERSHOOT part of the cycle---i.e. you are not blowing off enough CO2. And when the ASV determines that you are not going to meet the target tidal volume on your own, the ASV quickly increases the IPAP pressure over a series of inhalations until it helps ascertain respiration even when you are not breathing as you should. This respiration allows your body to blow off enough CO2 so that you do not kick into the OVERSHOOT side of the cycle. Indeed, the machine is known as Adaptive Servo Ventilation (ASV) because the unit adapts to your breathing style and includes the timed response. They are the only machines designed to treat sleep disordered breathing with this capability.
The downside, of course, to the ASV's ability to break the Undershoot/Overshoot cycle that characterizes central sleep apnea is that the necessary algorithm requires rather dramatic and sudden increases in IPAP pressure. That's why your IPAP is going as high as it is: The machine is trying to do its job and break up the Undershoot/Overshoot cycle that is developing in your breathing pattern while you are asleep. But those sudden increases in pressure also can be very disruptive to sleeping. And it just takes time and hard work---most likely with some help from your sleep doc, to figure out how to sleep with these sudden and drastic pressure swings.
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