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Do mask leaks reduce minute vent levels?
#1
Do mask leaks reduce minute vent levels?
I normally use an F20 full face mask but I have been trying out a n30 nasal cradle mask.  I find this far more comfortable especially in the hot weather we have been experiencing here in Melbourne. I do, however, have to do something about the leaks (probably mostly mouth breathing) and I am experimenting with using a soft neck collar.  

But one thing I am worried about - I note that I have been getting very low median Minute Vent readings, below 5 Lpm (on the nights I have been using this mask they range from 3.8 to 4.8).  With my f20 I am normally in the 6-7 range.

Could these low results be arisings because of the leakage I am experiencing, or do they suggest that the mask is just not suitable for me?  If the it is caused by the leaks, I will experiment further.  Otherwise I may just give up with this very comfortable mask.   (It is probably worth noting that with either mask my AHI is good - normally less than 1).
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#2
RE: Do mask leaks reduce minute vent levels?
I definitely think that the data being recorded during leaks has wonkiness in it. There is one particular pattern that I only see with leaks where the inhales barely come off zero, while the exhales are pretty clearly there (heading down to at least -10 or so.) Everything goes to hell as far as the data -- respiration rate and tidal volume get recorded as zero, minute vent disappears, mask volume shows that the EPR/PS stops changing pressure.

And the machine turns itself off! (Even without SmartStart turned on!)

[Image: p.png]
https://www.dropbox.com/s/13l52i8ecr3ubo...f.png?dl=0

I think that what might be going on is that the ResMed algorithm is set so that it only uses the inspiration to decide what's a breath, and it's set to ignore cardiobalistic effects, and so it decides that you aren't breathing because it can't detect your inhale.

Quite frankly it's enough to make me go back to my full face mask!
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#3
RE: Do mask leaks reduce minute vent levels?
Hi Elliot,

I don't know how the algorithm works, but: Yep, if air escapes from the system, it seems logical to assume that there is less for the machine to use to calculate MV and Vt.

With the FFM the volume of air in the mask is larger so the discrepancy may not be so noticeable.

In the attached chart from last night (P30i nasal mask) see how the Tidal Volume (Vt) falls with each large leak and when the leak ends, the Vt spikes up immediately (grey vertical line and blue box). The Minute Ventilation = RR x Vt so your resp rate should compensate for that.

In the screengrab I include data from my spreadsheet from last year showing high negative correlation between Leaks and Vt and MV. The higher the leaks the lower the "recorded" Vt and MV.

You don't mention your leak rates, but unless either the leaks disturb you or you are symptomatic from the lower numbers, or you have evidence of low pO2 it should be OK.

Just another reason to control the leaks. Good luck with the Neck Collar: I find mine indispensable. Most of all, aim for comfort.

   
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#4
RE: Do mask leaks reduce minute vent levels?
Very interesting.  I could not see from your traces just how low your MVs go.  My medial figure for the whole night can be close to 5, but some of the high leak periods have it below 4.

My time with high (>25) leaks is generally low, but have almost continuous leaks the rest of the time, and so my Med leaks are around 12 (sometimes considerably lower). This is what I feel needs working on.  With the FF mask I can also get occasionally multiple >25 leaks (and can even be way higher) but the Med leaks is almost always 0! So it is a case of a few very bad patches on a flat background. 

But the point is that I actually feel better in the morning when I don't use the FF mask.  I have to use it with the straps pretty tight with the F20 and I don't think this does me any good. That is why I am trying  to get lower leaks out of my various nasal masks. I have never tried the pillow masks.  Maybe I will add the P30 to my collection!
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#5
RE: Do mask leaks reduce minute vent levels?
Hi Cathy,

Like you I have noticed that leaks seem to reduce inspiratory peaks more than expiratory troughs.  Not to the extent you have reported, but still noticeably. I have no idea  why this is so!
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#6
RE: Do mask leaks reduce minute vent levels?
In theory if leaks were calculated accurately they would not affect minute ventilation calculations, in reality I assume they can have some effect but I don't know how significant it would be.

Leaks from a nasal mask can affect minute ventilation in a different way though. Mouth leaks often occur during mouth breathing and if you are breathing in air from your mouth you will be breathing in less air from the machine. The machine only records what you breath through it so if mouth breathing regularly I would expect you should see a drop in MV.
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