Attached are charts arranged as requested from Friday (15th), Saturday (16th) and Monday (18th). The following settings are the same for all of them:
- Mode = iVAPS
- EPAP = 5
- PS min = 4 (IPAP min = 9)
- PS max = 9 (IPAP max = 14)
- Ti min = 1.0 s
- Ti max = 2.5 s
- T rise = 900 ms
- Trigger = Med
- Cycle = Med
- Height = 175 cm
- Mask = Full (F&P Evora Full)
The following settings were changed:
Friday (15th):
- Patient Target Rate = 12 bpm
- Va = 5.5 L/min (MV = 6.9 L/min, Vt = 578 ml)
Saturday (16th):
- Patient Target Rate = 10 bpm
- Va = 4.8 L/min (MV = 6.0 L/min, Vt = 600 ml)
- Switched to Vitera mask for second period of sleep
Monday (18th):
- Patient Target Rate = 11 bpm
- Va = 5.0 L/min (MV = 6.3 L/min, Vt = 574 ml)
I have overridden the Y-axis settings on all graphs. No the pressure graph is not clipped, PS max is limited. As indicated earlier in the post, I have experienced aerophagia previously and did not want to allow the machine to shoot up to pressures that I might not initially tolerate, especially during NREM, when I do not need pressures that high.
My mind was obviously working well last night as I slept. One thing I realized was that your explanation of the machine taking over breathing because CO2 drops too low could explain the difference between the two periods of sleep on Saturday with the change of masks being the key difference - if the Vitera mask is more effective at venting CO2 than the Evora Full. No, I can't be sure, but it is a possibility.
The other thing I realized is that I have missed a few things that should have been obvious to me because of my previous career as a control systems engineer, when I used to tune control loops in chemical plants. VAPS uses feedback control to achieve a volume set point by controlling pressure and, if I am over ventilating, it appears to be over controlling. Frequently this occurs in control loops because the feedback loop is being asked to do too much. The best approach to rectify this is to establish reasonable open loop control and then use feedback control to make smaller control actions to only achieve set point accuracy and deal with disturbances.
To get reasonable open loop control in PAP therapy, I would need to plot volume (take your pick of tidal, minute or alveolar) as a function of pressure support. If I set a particular pressure support value, what volume would be delivered? Once I establish the characteristic curve, I should be able to choose an appropriate PS value to become the PS min in VAPS mode. This should let me breathe freely in NREM with the controller only altering IPAP to the (hopefully small) extent that the volume set point has not yet been achieved and to deal with whatever disturbance REM throws my way. PS max needs to be high enough to allow sufficient change of IPAP to deal with REM without allowing it to overshoot.
So, how do I establish my relationship between pressure support and volume? The manual way is to switch back to ST mode, fix EPAP at 5 and and progressively increase IPAP (over several nights), recording the resultant volumes (and respiratory rates) as I do so. ST mode is better than S mode as it will take over breathing if I set IPAP too high and over ventilate. That point, where the percentage of spontaneous breaths starts to decrease, is also useful knowledge because it sets the maximum value for PS min.
When I started with the machine, I ran for a few days in S mode with PS between 3 and 5. Tidal volumes were still generally less than 500 ml. Following the logic of the process above, PS min probably needs to be higher than 5. Interestingly, there was a suggestion in the sleep study from the sleep tech, but not the specialist, that IPAP min should be 11 (PS min = 6). Maybe.
I could feel this last night. When I was awake enough to control breathing, I breathed deeply and IPAP stayed at IPAP min. But as I started to drift off and my tidal volume decreased, the pressure controller stepped in to increase IPAP. It was noticeable, I could feel it, it roused me before I got to sleep and the cycle started again. I even worked out how to inhale a little sooner to reset the action of the controller and bring IPAP down to IPAP min.
In the meantime, I will try Learn Targets this afternoon and get back to you with what it comes up with.
Final thing. The appointment that I had this morning provided one piece of information, which you might find relevant. I had a basic lung function test done a while ago. The results were summarized as "normal spirometry, elevated gas transfer". Full report will be mailed to me.