The Basics of CPAP Machines

This is an excerpt from my forthcoming book, The 7 Day Sleep Apnea Solution: Get the sleep you need and the life you want. Please feel free to make comments and suggestions. If I select and implement your suggestion, I’ll acknowledge you in my book.

CPAP stands for continuous positive airway pressure. A gentle flow of positive air is passed from the machine to a mask that fits over your nose or your nose and mouth. The mask is typically made of a air-cushioned interface, using straps around your head. The pressure level is typically calibrated during a second sleep study once a sleep apnea diagnosis is made. There are a number of different positive airway pressure machines, as well as different types of masks. In most cases, the positive air pressure is delivered mixed with heated humidification to prevent nasal dryness and congestion.
The basic CPAP machine delivers a predetermined pressure level that’s confirmed in an overnight study, called the CPAP titration study. The pressure is measured in cm of water, which is a common scientific standard for measuring air pressure. 
Bilevel PAP
A bi-level machine provides two pressures: A higher level during inhalation and a lower level during exhalation. This makes breathing out easier, especially if your pressure is very high. BiPAP is a trade name from one of the CPAP manufacturers, so from now on I’ll use the term bilevel. These machines are more commonly prescribed for very high pressures, since exhalation can be difficult. This technology tracks and responds to your breathing patterns in and out. Bilevel devices are also used as respirators in hospitals, and also sometimes called respiratory assist  devices (RAD).
An APAP, or automatic PAP machine uses a computer controlled algorithm to continually and automatically adjust your pressure throughout the night. Your doctor will set a range a pressures, with the machine starting off at the lowest pressure but gradually raising the pressure until all your apneas and hypopneas are gone. It constantly monitors your breathing patterns and adjusts your pressure. Your pressure needs many change from hour to hour, or from day to day, whether you have nasal congestion due to allergies, or your your sleep position changes. Some studies have shown that certain models may prevent more apneas and hypopneas and use lower average pressures, which can be more comfortable for the patient. Different manufacturers use different algorithms, so you may have differing experiences. APAP machines can also operate in CPAP mode. Once the optimal pressure is recorded (from data that the machine provides) it can be set at that constant pressure for a while, and re-calibrated every few month or years. 
You may be thinking by now that if APAP is that good, why not give everyone APAP? One reason is cost. Medicare (which insurance companies generally follow) sets reimbursement rates for all PAP devices at only one level. Whether the doctor orders a CPAP, bilevel or APAP machine, it’s paid at one rate which was originally set for straight CPAP. With improving technologies, bilevel and automatic models will cost more, but most insurances won’t allow for any of the more advanced models unless you’ve tried straight CPAP first. This is like
With more and more insurance companies requiring diagnostic home sleep testing, patients who are diagnosed with OSA are given APSP directly, and bypassing the sleep lab entirely. While there are pros and cons to this approach, it’s clearly a cost savings measure. Paying a few hundred dollars more for an APAP machine probably saves more money than the traditional models, despite slightly less effective treatment outcomes.
While some people prefer the automatic models over straight constant pressures, others will prefer the latter. The reason for this may be that the constant rising and lowering of the pressures can cause some people to awaken, especially if they are light sleepers. I’ve had people go from CPAP to APAP, and APAP to CPAP with success. This finding is in line with a study showing that overall, there was no preference between CPAP, bilevel, or automatic devices. Just like any scientific study, it reported overall averages, but didn’t take into account individual variations.
Another potential issue with APAP is that since the pressure starts off low, as the pressure increases gradually, you may get a mask leak if it’s not placed properly. Whereas with CPAP, you’ll know immediately if you have a leak from the beginning. Some APAP models have a mask fit feature, where you can press a button and it’ll give you a certain pressure to check for mask leaks.
These algorithms, though pretty good, are not perfect. They will still miss some apneas. Certain machines will be better at detecting more subtle obstructive events like flow limitation or RERAs. Because it increases pressure incrementally, once you have an apnea, it may take a while to reach your optimal pressure. 
An adaptive servo-ventillator (or ASV) 
An adaptive servo-ventillator (or ASV) device is also a RAD device like the bilevel device, and is often used for people with mixed apneas (obstructive and centrals). Central apneas happen when there’s no physical obstruction to breathing but the brain doesn’t send a signal to the diaphragm to breathe. To be eligible, your ratio of central to obstructive events has to be more than 50%. Sometimes central apneas can happen during CPAP titration, especially when the pressure gets very high. This can also be an indication for ASV therapy. 

Next: Reading your CPAP data

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