Is my Tidal Volume low?

[cojp18]

           


Hello all,

I just started CPAP in late May of this year (2024), so I have been using it for about 1.5 months. My main symptom without CPAP was daytime sleepiness, often falling asleep in class even while taking notes and drinking water. I was usually unable to sit down on my couch because I would fall asleep sitting up. This was all while sleeping 10+ hours every night. I ended up asking for a sleep test after I fell asleep with a scalpel in hand during a lab.

During my initial (home) sleep study that got me my diagnosis of sleep apnea, my AHI was around 2, but my pRDI was 17.2. My understanding is that this is likely UARS and I do have an upcoming appointment with an ENT to see if anything in my anatomy could be making things worse. My flow limit seems to be low/normal though, so I'm not sure if this is needed.

My question now is about my Oscar data. I just put a SIM card in my machine recently, so I only have four nights of data, but I was noticing that my tidal volume seems lower than expected. Would you all be able to take a look at these numbers and let me know what you think?

For reference, I am 20 years old, female, 5'8" (68 inches), and 135 lbs (61.2 kg).

Thank you so much!

[cojp18]

   
Adding this info--here's what my graphs look like on a given night. The graphs for the other nights look very similar.

[Jay51]

Tidal Volume Calculator - Endotracheal Tube (ETT) Depth (mdapp.co)

I plugged in your height and got this:

Volume (IF target is 6 mL/kg, IBW 64 kg) = 384 mL

■ Tidal Volume (IF target is 8 mL/kg, IBW 64 kg) = 512 mL

Minute ventilation is the product of tidal volume (amount of air volume of each breath) x respiratory rate (how many breaths a person takes per minute).  It is a more important number IMO.  Some people have a little lower tidal volume, but a little higher respiratory rate, so minute ventilation is fine.  Some people have a little lower respiratory rate, but a little higher tidal volume.  And minute ventilation is still ok.  Your minute ventilation is either at the low end or normal or slightly below normal also.  Tough call here.  

Congestion, the altitude a person lives at (higher can produce a lower tidal volume due to less O2 concentration in the atmospheric air) can decrease tidal volume.  The type of mask a person uses can slightly affect it also.  Pap pressure obviously can, too.    

Minute Ventilation Equation Calculator - MDApp

 Minute Ventilation (VE) = 6000 mL/min

Normal range for adults:

Variable
Lower limit
Higher limit
Tidal volume (Vt)
400 mL
600 mL
Respiratory rate (RR)
10 resp/min
16 resp/min
Minute Ventilation (VE)
4000 mL/min
8000 mL/min
Disclaimer: This tool should NOT be considered as a substitute for any professional medical service, NOR as a substitute for clinical judgement. Please read the full disclaimer.

*4000 to 8000ml/min = 400L/min to 800L/min.  

It is normal for tidal volume to vary some nightly also.  Looking at your charts, you are most likely at the low end of normal to slightly below normal IMO.  How do you feel after a night's sleep?  That answer may tell you were you stand in regards to tidal volume also - if you need to do more to help your therapy.  A good plan would be to show this to whoever prescribes for your therapy (PCP or Sleep DR., etc.).  Also, keep looking at your charts over time and see if a trend develops of lower tidal volume.  This could be indicative of some respiratory problem.  

Your upcoming ENT appointment could shed some light on the situation also.  

[cojp18]

Thanks so much for your response! That makes sense. I'm still feeling pretty tired during the day, although not as bad as before CPAP, so I might bring it up with my doctor. I appreciate the info and advice!!

[PeaceLoveAndPizza]

Note that the tidal volume calculators are meant for a mechanical ventilator, not related to sleep. Don’t get too worried about it, but it would be good to discuss it with your GP if concerned.

[BoxcarPete]

I had a fairly extensive nasal surgery earlier this year (septoplasty, submucosal turbinate resection, adenoidectomy) and my CPAP tidal volume and minute vent stats (which were a little low for size like yours) went up around 10-15% after the surgery. I have since benched my CPAP after recording a squeaky clean sleep study in lab. I'm one of the lucky ones to have everything clear up with surgery, but for those who still need PAP most state that it's easier to work with post-op. In my opinion it's a good thing that you are going to see an ENT, as the younger and overall healthier you are (and more UARS vs. apnea), the more likely surgery will be a more effective option than CPAP. Stick to the basics at first: tonsils, septum, and certain turbinate procedures have very good risk:reward and success rate. Save anything more extreme for after all other options have been eliminated.

[Deborah K.]

I too have low tidal volume, sometimes as low as 220.  I learned many years ago that I have tiny, child-sized sinuses and never developed the forehead sinuses that usually appear at about 8 years old.  (My ENT said it was probably a good thing, as they would no doubt be full of infection too.   )  I had surgery to fix lots of sinus problems, but mine are still tiny.  I think that's why my tidal volume is low, and I just don't worry about it.  

I say all this just to tell you that there can be lots of reasons for low tidal volume and that you are not alone.