(11-22-2013 08:39 AM)me50 Wrote: 1. With BIPAP (BI-LEVEL) or APAP for that matter, how much wear and tear and subsequent problems with the airway from the continuous expansion and deflation of the airway?
First, I am NOT a doctor. But I have been using BiPAP for three years and I've done a lot of reading and as a PhD in mathematics, I understand something about basic math and physics.
I think you're overthinking things and inventing things to worry about here. Here are some of my thoughts about the things you're worried about in particular.
Quote:I wonder if the continuous higher pressure of breathing in and out (as opposed to what it is like during the day when we breath in and out, presuming the natural airflow is at a much less pressure than at night when those with apnea sleep and the airway collapses), it it causes stress or weakening in the airway walls (kind of like when a bulimia patient purges and over time, it weakens the esophagus to where it could and has in some patients, ruptured).
There seem to be three things you're worrying about:
1) Can CPAP/APAP/BiPAP cause some kind of stress on the upper airway similar to the stress induced by bulima on the esphogas?
2) Just how much extra pressure are we subjecting our upper airways to each and every night for eight hours at a time?
3) (Your original question) Is it possible that the constant variation from EPAP to IPAP on very single breath could somehow be doing something stressful to the upper airway? And why is Bi-level ever suggested in the first place???
Let's tackle these three things one at a time.
Can CPAP/APAP/BiPAP cause some kind of stress on the upper airway similar to the stress induced by bulima on the esphogas?
Breathing with a PAP machine is not at all like what bulimia patients do to themselves with purging. In bulimia, patients are subjecting their esophagus to highly corrosive stomach acid over and over, and it's the stomach acid that lead to the weakening of the esophagus, which can indeed lead to rupturing of the esophagus. With PAP, all that's going down the windpipe is plain old (slightly pressurized) air; there's nothing corrosive about what's being pumped down the airway. So IMHO, comparing PAP to bulimia is a very bad case of comparing apples to oranges. Or rather, it's more like comparing good quality, fresh apples to very rotten oranges.
Just how much extra pressure are we subjecting our upper airways to each and every night for eight hours at a time?
Even when set to its maximum settings, a CPAP, APAP, or BiPAP is not adding that much pressure to our upper airway. As you already know, CPAP pressure is measured in cmH2O
, which is not a unit that is very meaningful to most of us. Here's a basic primer for understanding just how little
pressure is needed to correct our OSA.
One cmH2O is the amount of (additional) air pressure needed to raise a column of water by one centimeter. The cmH2O unit is basically a metric version of "inches of mercury" that we use in the US to measure the barometric pressure for weather forecasting. And just how much additional pressure does 1 cmH2O represent in terms that are meaningful to an average American? Well, if we go to Unit Converters
and scroll way far down the list, we can convert 1cm H2O into (about) 0.029 inches of mercury. Since standard CPAP/APAP pressures range from 4 cmH20 to 20 cmH2O, that means they range from 0.116 to 0.58 inches of mercury. A BiPAP can go up to a whopping 25 cmH2O, which is all of 0.72 inches of mercury.
Standard atmospheric pressure varies from day to day, but typical values are in the 28 to 30 inches of mercury range. When a typical storm front comes through, the air pressure drops anywhere from 0.25 to 1.5 inches of mercury. Some people are really sensitive to these changes in atmospheric pressure and get "pressure" or "weather" headaches or lots of joint pain. But most people don't notice them very much. At the maximum BiPAP pressure of 25cmH2O, the machine is ADDING about 7/10 of an "inch" of additional pressure on top of the current atmospheric pressure---about as great of a pressure difference between a typical raining day and a typical sunny day.
Now it's true that when we first start PAPing that little bit of extra air feels like a hurricane. (Heck, even after three years, I still can feel uncomfortably like a balloon being blown up when I first put my mask on each night.) But the fact is, the extra air pressure is just a tiny bit extra that makes it harder for our airways to collapse when we're asleep.
(Your original question) Is it possible that the constant variation from EPAP to IPAP on very single breath could somehow be doing something stressful to the upper airway? And why is Bi-level ever suggested in the first place???
Now, it's true that if you are using straight PAP with NO exhalation relief
, the amount of extra pressure your upper airway is dealing with is constant throughout the breathing cycle. But the actual air pressure inside your airway is not constant.
When we are breathing normally---both during the daytime AND at night, the air pressure in our upper airway is NOT constant. If it were, no air would actually move into or out of the lungs. Air always flows FROM the space with HIGHER relative air pressure to the space with LOWER relative air pressure. When our diaphragm moves down and our lungs expand, that effectively reduces the air pressure in the airway and lungs and allows air to move through the flexible tube we call the upper airway and into our lungs. When our diaphragm moves up, that forces the air out of our lungs and into our airway, which effectively increases the air pressure in the airway ever so slightly so that the air goes out of our noses. The change in the actual air pressure in our upper airway is not very much compared to the ambient atmospheric pressure, but it is not totally insignificant either.
When a person with OSA goes to sleep, the problem occurs when the muscles relax so much that when the diaghragm moves down (and the air pressure in the airway is reduced slightly) the muscles are so lax that they are not capable of keeping the airway open. And hence the airway collapses rather than allowing air to move into the lungs. If you look at your wave flows, apneas tend to start at the end of an exhale/beginning of the inhalation part of the breathing cycle: We're trying to breath, but our throat muscles have let our upper airway collapse when the air pressure inside the airway dropped enough to allow air to go into the lungs.
PAP provides just enough air pressure to keep the airway open at all times. Compared to the ambient air pressure of the room that we regularly are breathing in and out against, this little bit of extra PAP pressure is not much. (More on this in a bit.) But this little bit of extra PAP pressure does keep the flexible tube of our upper airway from collapsing when the diaghragm moves down and the airway pressure drops relative to what it was a few seconds earlier. In general, the bit of PAP-provided pressure is not enough to affect the functioning of the air-pressure changes in the upper airway that occur during the normal breathing cycle---i.e. when the diaghragm moves down, the lungs expand, the air pressure in the airway is reduced and the air moves into the lungs, and when the diaghragm moves up, the lungs contract, the air pressure in the airway is increased, and the air moves out of the nose.
Now when you start breathing with a PAP machine, the muscles that control the breathing have a bit more work to do: They have to find a way to expel the air in the lungs against
the air being blown into the system by the PAP machine---i.e. the diaghragm itself and the chest muscles involved in breathing can notice that it is (a bit) harder
to fully exhale against the positive pressure supplied by the PAP machine. For some people, this can result in a bit of muscle pain until the chest muscles and diaghragm develop the necessary strength to exhale just a wee bit harder (on average) with PAP than pre-PAP. And for some people this discomfort of exhaling against the positive pressure also causes feelings of "suffocation" due to the feeling like they cannot fully exhale all the air in their lungs. In other words, exhaling against the PAP pressure can simply feel very, very "abnormal" (and hence "uncomfortable") to many PAPers.
PAP makers have come up with a couple of different strategies for making PAP machines easier for people to use when "discomfort with exhaling against pressure" is one of their serious adjustment problems.
The most common solution is some form of exhalation relief system. Exhalation relief systems typically reduce the pressure (a bit) at the start of each exhalation, making it easier for the diaghragm and chest muscles to fully contract and expel all the air from the lungs.
Me50, your signature shows that you are using an S9 AutoSet. Do you use the EPR feature? If so, then you are already using a system that varies the pressure based on your breathing pattern. If your EPR setting is 3 and your pressure is set to 10cm, your S9 is gently adjusting through a cycle of pressures that range from maximum of 10cm (at the beginning of your inhalations) to a minimum of 7cm (at the beginning of your exhalations). As soon as your inhalation starts to taper off (the rate the air is going into your lungs starts to drop), the S9 starts dropping the pressure. By the time you are fully exhaling, the pressure should be at 6cm, and when your exhalation starts to taper off (the rate the air is going out of your nose starts to drop), the machine starts to slowly increase the pressure and once it detects the next inhalation, the pressure is increased fairly rapidly back up to 10cm and the cycle starts over.
Resmed refers to this pressure cycle as their "Easy Breathe" wave flow. They claim that it makes adjusting to PAP much easier. Whether there are any scientific papers that back that up is a good question, but there's certainly a lot of anecdotal evidence that a lot of experienced PAPers like the Easy Breathe wave form provided by Resmed's EPR system.
The PR Flex systems are quite different from the Resmed EPR/Easy Breathe system, but the principal is the same: The pressure is reduced at the beginning of the exhalation to make it easier for the diaghragm and chest muscles to fully contract and expel all the air from the lungs, and this in turn should make breathing with the machine feel more comfortable and more "normal." The PR Flex systems provide a variable amount of relief at the beginning of the exhale---how much the pressure is dropped depends upon both the Flex setting and how strong the individual exhalation is. Deeper, stronger exhalations result in greater drops in pressure. When and how the pressure is brought back up to the therapeutic setting varies between A-Flex, C-Flex+, and C-Flex, but all of them result in a pressure curve that is more "squarish" than the Resmed Easy Breathe pressure curve. (See my previous post in this thread
Bi-level machines provide a greater degree of exhalation relief than you can obtain from a CPAP or APAP, but the principle is still the same. The EPAP pressure is enough to keep the airway open while exhaling (when there is additional air pressure in the upper airway
coming up through the lungs) and the IPAP is set higher because more pressure is needed to prevent the airway from collapsing as the diaghragm moves down, the lungs expand, and the air pressure in the airway goes down to allow air to flow into the lungs.
For some of us bi-level users, that greater difference in pressure between IPAP and EPAP provided on a bilevel means its easier to more fully exhale against the pressure, and hence breathing with the machine feels more natural. For some of us using Bi-PAP rather than VPAP machines, the PR's pressure transition between IPAP and EPAP is a bit more comfortable; for some VPAP users, Resmed's pressure transition between IPAP and EPAP is a bit more comfortable.