SpO2 decreases when lying down - feeling of getting not enough air

[MyronH]

When I initially lay down to sleep I'm ok. I have BI-LEVEL machine set to 5min ramp, starting EPAP 4, IPAP 8, PS4. Then it goes to EPAP 4.4, IPAP 8.4 to start for rest of night. 

If I get up during the night, then lay back down and put my mask on, the pressure it starts with again doesn't seem as high as when I first turned it on at the start of the night. I'm not sure if it's due to me being used to the pressure after a few hours of using it, or not.

Anyway, I feel like I'm not getting enough air. I check my SpO2 with Contec device, and an oximeter, and it reads 91-92% when I first lay down again. Then it goes to 94-95% for rest of the night. My pulmonologist and hematologist ran tests and didn't find anything wrong. Cardiologist says nothing wrong either. Although they may do a VQ Scan to test lung function and check for remaining blood clots from last years DVT/PE.

Is it normal for SpO2 to drop when first laying down? What could cause this initial shortness of breath?

[mesenteria]

About your SPO2 when first hooked up, that's an indication, maybe, that you 'exist' near that general level during waking/working when you are relaxed and not very active.  Perhaps your breathing slows as the night wears on prior to bed, or it becomes more shallow...dunno.  However, if I read you correctly, you seem to improve as you sleep, and that's salutary in my books.

[mesenteria]

Your EPAP level of 4-ish is considered too low by the heavy majority of users...like about 90%.  They feel oxygen starved.  If it works for you, and your sat is okay, …..

About your SPO2 when first hooked up, that's an indication, maybe, that you 'exist' near that general level during waking/working when you are relaxed and not very active.  Perhaps your breathing slows as the night wears on prior to bed, or it becomes more shallow...dunno.  However, if I read you correctly, you seem to improve as you sleep, and that's salutary in my books.

[MyronH]

Thanks. Yeah I'm not sure. I could raise EPAP to 5, as long as it doesnt cause centrals. I've had two DVT's/PE's, last one was in 2018 and on Xarelto blood thinner now. My problem is my normal SpO2 is never above 95%, even in the day. Before my DVT/PE it was much higher 98-99%. I guess those blood clots messed up my lung and heart function, although all tests so far are negative.

[Matt00926]

It could just be that when changing positions it takes a good while to get a good and steady reading from the pulse oximeter. But your starting pressures are likely low enough to create a feeling of being starved for air as well. Just like when people use CPAP of 4cmh2o. Maybe start at 9/5, PS 4, and see if that helps.

If lying down before you masked up causes you to feel short of breath because a physical condition with your body, it would show on the tests. It would be either the heart, lungs, or diaphragm muscle strength that most likely would be the culprit. Sounds like your doctor is on the case. If the heart and lungs themselves check out, I would have them test your diaphragm using a peak flow meter having you use it while both sitting up and lying down, and see if there is a large discrepancy in the measured forced vital capacity (FVC) which would likely indicate muscle weakness. Again this is highly unlikely but it is indeed comprehensive.

Do you have any obvious swelling/water retention in your feet/ankles?