Tidal Volume and mask type

[elliotg]

Some time ago in answer to a question on a different matter it was suggested that my Tidal Volume results in the Oscar attachments were too low. I was getting a median reading of around 320 ml and a 95% of around 450 ml. I have calculated my "Ideal Body Weight" to be 61.4 kg.  I am not clear about how you go from there to Ideal Tidal Volume as I have seen references to multiplying the IDL by 8 but also by 12. But either way my measured results are definitely lower than they should be! 

These results arose when I was using a Resmed Activa LT nasal mask.  More recently I have been trying out a full face mask - the Resmed F20.  I am observing significantly higher Tidal Volumes when using this larger mask.  While there is some small day to day variation, i have mostly been getting a median of 400 ml and a 95% of 600 ml.

Is it usual for the Tidal Volume measurements to be so dependent on mask type? And does this increase suggest that I probably should be sticking to the full face mask?

[Sleeprider]

Tough question becaue tidal volume is never considered by itself. The product of tidal volume (Vt) and respiration rate (BPM) is the minute vent (L/min). By only looking at tidal volume, we get only half the equation. A more meaningful metric is the minute vent. I don't usually comment on tidal volume unless it is clearly low, and yours is certainly not impressive, but not at the level of concern, and some people just have shallow breathing as they sleep. Over the years, I have found it very difficult to manipulate Vt and Mv in a menaingful way with CPAP or BiPAP pressure, however we do see an increase with more CO2 in the rebreathing circuit as with EERS, and that may be what is going on with the full face mask.

[jaswilliams]

I personally have low tidal volume when asleep a MED value of 360 but its also coupled with a fairly high RR of 17 so my minute vent is 6.38 which is reasonable for my size. I have been unable to vary the tidal volume much with machine settings if I increase Pressure support then my Central Apnoea increases I just don’t breath . It easy to find when I am asleep buy just looking at the Tidal Volume graph

[elliotg]

I personally have low tidal volume when asleep a MED value of 360 but its also coupled with a fairly high RR of 17 so my minute vent is 6.38 which is reasonable for my size. I have been unable to vary the tidal volume much with machine settings if I increase Pressure support then my Central Apnoea increases I just don’t breath . It easy to find when I am asleep buy just looking at the Tidal Volume graph

Looking back over the recent history, the respiratory rate does not change dramatically.  So mu minute vent with the nasal mask is around 5 (sometimes less) but with the full face mask goes up to at least 7.  I am not sure which mask I should be using as my ongoing one!

[sosotired]

Hi jaswilliams,

Very Interesting. A lot of people claim the F20 Quiet Air diffuser/elbow doen't vent correctly and causes CO2 headaches from re-breathing CO2. I personally woke up with headaches using the F20 with the Quiet Air diffuser/elbow. The headaches felt just like the de-saturation headaches I used to wake up with every morning before treatment. Once I replaced the Quiet Air diffuser/elbow with the old model "Loud Air" elbow, the headaches went away. I think you stumbled onto an accidental EERS device and added a little more credence to the augment that the Quiet Air elbow has less than optimal venting. Thanks for sharing.

Below is one of the discussions on the Quiet Air diffuser/elbow:

http://www.apneaboard.com/forums/Thread-...r-Vent-F30

[elliotg]

Hi jaswilliams,

Very Interesting. A lot of people claim the F20 Quiet Air diffuser/elbow doen't vent correctly and causes CO2 headaches from re-breathing CO2. I personally woke up with headaches using the F20 with the Quiet Air diffuser/elbow. The headaches felt just like the de-saturation headaches I used to wake up with every morning before treatment. Once I replaced the Quiet Air diffuser/elbow with the old model "Loud Air" elbow, the headaches went away. I think you stumbled onto an accidental EERS device and added a little more credence to the augment that the Quiet Air elbow has less than optimal venting. Thanks for sharing.

Below is one of the discussions on the Quiet Air diffuser/elbow:

http://www.apneaboard.com/forums/Thread-...r-Vent-F30

I have seen this discussion and would like to try one of the older "Loud Air" elbows.  However, I am at a loss as to where I can get one.  The various outlets that sell CPAP equipment here in Australia only seem to advertise the current version!

[GuyScharf]

I personally have low tidal volume when asleep a MED value of 360 but its also coupled with a fairly high RR of 17 so my minute vent is 6.38 which is reasonable for my size.

How do you determine what minute vent rate is "reasonable" for one's size, and what measures of size?

[jaswilliams]

Minute vent over 6 is considered reasonable why I have a higher RR when asleep I have no idea

[sleepyp]

You were likely mouth leaking with the nasal mask, so the measured tidal volume is low. All of the air volume is not returning through the system so it measures low.
I'll wager that you'll see a inverse correlation between your leaks and tidal volume trends in OSCAR while wearing the nasal mask.
There should be no equivalent losses from the full face mask.

Tidal volume can only be calculated accurately when there are 0 leaks present. Simple control system engineering. :-)

[drmaestro]

Do you know how the CPAP calculates tidal volume? Technically, the volume during inspiration should be equal to the volume during expiration, so the device can calculate one or the other (or maybe it calculates both and the difference is the leak?)