When your mouth is open while using a nasal mask, all your therapy pressure is escaping out of your mouth, so your OSA is not being treated, you are not get any benefit from your CPAP, and your machine is unable to report anything useful to you during that time your mouth was open.
That is not necessarily correct. It very much depends on the size of the leak. In many instances the machine will compensate and treatment pressure does not alter a lot. It is very individual. I have never seen a mouth leak so large that there is no pressure.
Actually, it IS correct. An open mouth, which is what the OP asked about, is as severe a leak as can possibly occur for any user of any mask other than a ffm. An OPEN mouth is a problem on a much higher level than mere occasional mask leak around the edges of the mask. A dry mouth reported by a ffm user may not be an issue beyond humidification. But a dry mouth reported by a *nasal *mask user should never be ignored as a nonissue, in my opinion, because of the potential for getting little or no benefit from the pressure. Same is true for a proper diagnostic or titration PSG. A patient with an OPEN mouth cannot evaluated or titrated using a nasal mask.
As I said before, It is not necessarily correct.
I have watched thousands of people using CPAP with nasal masks and mouth leaking. Sometimes it is a problem and sometimes not. Sometimes there is no loss of pressure, sometimes a small loss of pressure and sometimes a large one. I have never, in almost 2 decades, seen anything like a total loss of pressure due to mouth leak. It may be possible but I have never seen it.
While it is not ideal to perform a titration PSG with mouth leak it is certainly possible and in some cases necessary. Some people cannot use a full face mask for various reasons and have no option but to use a nasal mask even though they mouth leak, and in these cases they must be titrated with the mouth leak.
A diagnostic study cannot be performed with a mouth leak because to have a mouth leak one must be using CPAP and if the patient is using CPAP it is not a diagnostic study.
If a patient using CPAP at home had such a large mouth leak that it was causing a major loss of pressure it would show up very clearly on their download data.
These are the simple facts.
If most (all) of the air was escaping through my mouth, wouldn't that show up as a leak? My leak % is usually very close to 0
Yes. Leak is reported in full-data machines. If your mouth is open to your airway, your machine will report leak if it is on. However, as to the importance of stopping mouth leak for effective titration and effective PAP therapy . . .
"Air leakage may occasionally negate the efficacy of nasal ventilation . . . consider excessive air leakage through the mouth as a cause of failure to improve gas exchange and hypersomnolence. . . . The association between air leakages and arousals was very clear . . . arousals consistently occurring soon after the onset of leaking . . . preventing entry into deeper sleep stages." -- Sleep, Vol. 20, No.7, 1997.
"When a patient sleeps with their mouth open, the air from the CPAP entering the nose will escape out of the mouth. This problem causes the CPAP pressure going into the airway to be less than is actually required to keep the airway open, thus reducing the efficiency of the CPAP therapy." -- https://www.sleepresolutions.com/blog/cpap-masks-and-problems-with-mouth-breathing
"How Can Mouth Breathing Affect Your CPAP Therapy? The PAP in CPAP stands for Positive Airway Pressure — and you need the pressure! For those with Sleep Apnea, this constant airstream is necessary to keep your airways open. Whether it’s a mask leak or an open mouth, reduced pressure works against CPAP therapy. If air is escaping, pressure is escaping; it’s like sleeping with a big hole in your mask. If that’s the case, you’re not getting the airflow that keeps your airways open all night long." -- https://www.cpap.com/blog/mouth-breathing-solutions/
"Mouth breathing in newly diagnosed sleep apnea patients is so prevalent that many sleep doctors start all of their patients on CPAP therapy with a full face mask. Most patients that weren’t born mouth breathers quickly relearn how to sleep with their mouths closed within a few weeks or months of CPAP treatment. They can then exchange a full face mask for a traditional nasal mask or nasal pillow mask if they so choose." -- https://www.sleepapnea.org/treat/cpap-therapy/troubleshooting-guide-for-cpap-problems/mouth-breathing-on-cpap/
Why leak can be a problem. . . . The effectiveness of your therapy can be compromised if the leak is excessive, this is because the effective pressure to maintain your open airway will be lost as the air escapes around the mask or out through the mouth. This can cause breakthrough symptoms during the day such as increased tiredness and snoring at night. . . . Air leak from the mouth Mouth breathing often causes an excessive dry mouth and sore throat. . . . If you experience mouth breathing a full face mask will certainly work for you and would allow you the option of breathing through your nose and/or mouth. If your mouth is excessively dry in the morning and your throat is frequently sore this is an indication that you are probably breathing through your mouth. Patients who find it difficult to use a full face mask may find some benefit from use of a chin strap which helps to hold the lower jaw to remain closed, but a full face mask is the preferred option. -- http://osahub.resmed.com/do-you-have-air-leaking-from-your-mask-or-your-mouth-realsleep-through-better-care/
But getting back to the direct question of the OP, ResMed says about high and varying leak, on p. 15, under "Leak":
"High or varying leak rates may affect the accuracy of other measurements." -- https://airview.resmed.com/resources/welcome-page/pdf/1018991_airview-report-guide_usa_eng.pdf