I have noticed several posts reporting that "CFlex doesn't work at pressures over 15 cmH2O."  I kinda scratched my head, because I know it did, so I went and did a brief run at various pressures to see what happened.
Here are a couple of histograms showing the effect of CFlex at 5, 10, 15, and 20 cmH2O CPAP, and a CFlex setting of 3.





There are 3 phases in a respiratory cycle-- inspiration, expiration, and the period between the two, where no air movement takes place, which I'll call the static period.  Each of these areas has it's own issues as they relate to CPAP.
CFlex is an Expiratory Pressure Relief (EPR) adjunct.  In other words, it will only apply to the expiratory phase of the respiratory cycle.  As you can see in the histograms, the pressure drops about 3 cmH2O during the expiratory phase of breathing, at least at trough during the moderate to higher pressures.  It therefore should make exhalation easier.  At CPAP 10 cmH2O, this can be a noticeable difference.  At CPAP 20 cmH2O, maybe not so much.  It isn't that it doesn't work, it may be just that exhaling against 17 cmH2O isn't like exhaling against 7 cmH2O.
At the completion of exhalation, or nearly so, the pressure returns to baseline.  You can also see that the return to this level occurs gradually, not abruptly, coincidental with flow reduction.  So although the peak benefit of CFlex is relatively short, since flow is reduced, this may be academic.
During the static period, however, you are back at your therapeutic pressure.  This needs to be so to insure your that your airway remains open to allow the next inhalation.  At high pressures, it may be this that is the major source of discomfort.  And CFlex or any EPR can't help that.
You may notice at CPAP 20 cmH2O, there are several breaths where the EPR ceases during somewhat active exhalation.  You may also notice that there is a glitch in the expiratory limb of the waveform at that point, and a noticeable increase in resistance felt by the patient.  This may be another source of the perceived lack of benefit at high pressures, it would seem that this glitch could be significant enough to cause an arousal.



I would solicit comments from people using CFlex about their experiences.  This is only one brief test on one patient, and on a single CFlex setting, so I don't know if this is what occurs in everyone.
But it does show that there may be benefit at the higher pressures.  Certainly, at the low to moderate levels, there is little doubt that this is quite an effective modality.
It would be great to get an adjunct that would allow the EPR mode to extend deep into the static period, but that would entail reducing CPAP during the critical period and altering the entire therapy.  The reduction in flow is the trigger to stop EPR, and baseline pressure must be present through the static period to prepare for inspiration.  When the Resmed EPF version comes out, 2 areas I'll be looking at are the behavior of EPR at high pressures and if it looks at the static period at all.  I'll try to get some similar waveforms to do a comparison as they become available, or I'm sure there should be some accompanying material with the device.
sleepydave



Last edited by sleepydave on Thu Oct 06, 2005 5:51 am; edited 2 times in total

Dave,

That is a great graph!  Respironics refuses to quantify the relief amount, more than likely to avoid being grouped into the bipap category.  Looking at the patent for Cflex, it appears that there is also a component of cflex that takes into consideration the expiratory effort/volume.  The greater the effort/volume the greater the relief.  Wonder if you could catch that on a graph.

I think the reason it has been stated by a number of folks that cflex doesn't work above 15, is due to the minimal relief it does provide at that pressure.  I have personally tried setting cflex on at 20 to see, and I can say that it it barely perceived as working to me.  That said, there was a slight difference between cflex on and cflex off, so it was working, but 3cm/h2o drop isn't as significant at the higher pressure.

Quote:

Looking at the patent for Cflex, it appears that there is also a component of cflex that takes into consideration the expiratory effort/volume.  The greater the effort/volume the greater the relief.  Wonder if you could catch that on a graph.

I think Mike hit the nail on the head as to why C-Flex "doesn't work" at high pressures for some people.  Of course, what's "high" for one person to try to exhale out against might be a piece of cake for someone else, regardless of whether C-Flex is enabled.

I'm one who has done a lot of experimenting with my Respironics REMstar Auto with C-Flex.  Using it at a lot of different pressure settings, in auto mode but with the Low/High range set for one number....in effect, using it in auto but at one single pressure, like 12/12  or  15/15  and even up to 20/20.    With C-flex set at "3" for maximum pressure reduction upon exhaling.

I'm probably one who has used the phrase "doesn't work" at or above x pressure in some of my previous posts on several message boards.  I should have been saying, "doesn't work well" or "doesn't work well enough".  I think (hope!) I've been qualifying it each time with "for me".

I don't doubt that C-Flex "works" at high pressures.  Nor do I doubt that there are many people out there, particularly large men, whose volume and force of exhalation causes the pressure reduction during C-Flex to keep going and going and going...right to almost the very tail end of their exhalation...even at pressures of 19 or 20.

C-Flex doesn't work well for me at any pressure 15 and above.  At a straight pressure of 17 or 18, C-Flex barely makes an initial dent in the incoming air --  for a split second -- at the moment my exhalation starts.  In other words, I might as well not even have C-Flex turned on, given that the reduction can't  last more than a split second at the beginning of one of my exhalations at "higher" pressures.  Mike's examination of the C-Flex patent reveals why that's the case for me.

Obviously the effort/volume of my normal exhalations are not enough to make the C-Flex feature sustain its magic reduction long enough throughout my exhalation for it to be useful at all to me.... IF I really had to use a single pressure of 15 or more.  Thankfully, I don't have to.

It would be interesting, Dave, if you could find a willing lab rat who had normal, but somewhat rather light, shallow breathing and see how far up the pressure ladder they could go before C-Flex produces only a blip of reduced pressure for an initial instant of exhalation, and then opens the door again wayyyyyy too prematurely for the hurricane to come through.  That graph would be interesting to compare with the graph you posted.

I think the comfort benefit of C-Flex is extremely dependent on exactly what Mike points out.  YMMV, for sure (imho), when it comes to how forceful or how light a person's natural exhalation is.   Above 15 -- C-Flex can't make a dent in the pressure for me.  Wait!  Let me phrase that a different way:

Above a cpap pressure of 15 cm H20,  the force and/or volume of my exhalation is inadequate for C-Flex to sustain a reduction in pressure long enough for that feature to be of any comfort benefit for me.    

I agree with Mike...great graph!  Thanks for posting it!  And thanks for causing me to realize I need to be much more precise in making statements that apply only....to me.      

Hi guys!
Thanks for your kind comments!  I think the jury is still out on the use of CFlex at high pressures.  There appears to be flow patterns that will not initiate the premature termination of CFlex at the higher pressure and create the glitch, but we are talking about sleeping here, where the flow patterns cannot be controlled.  That's a significant "bump" that could lead to arousal, and even if it's only 1 out of 10 breaths, that gives you RDI 60, and now you've got a big problem.
Perhaps another question would be "Is high pressure CFlex a waking adjunct or a sleep maintenance adjunct?"  Based on what I've heard so far, not too many people are satisfied with the effect as they settle in for sleep, and that bump might need closer scrutiny under PSG to see if its a help or a hinderance.
At this point I think our position has to be to hold the manufacturers' feet to the fire and keep users informed.  EPR is a great idea, a proven product, but has shortcomings in its present condition.  I hope Resmed can address this in a big way, and if so I hope it's something that the other manufacturers can duplicate.
RG, your points are well-made.  And if 4 out of 5 people say they are having significant issues trying to use CFlex at pressures >15 cmH2O, then frankly I don't have a problem if you say it doesn't work cause that's a pretty poor track record for the group that needs this modality the most.  I'd toss the whole thing back into the manufacturer's lap and say "Do better!"
sleepydave

Had an appointment with my Resmed rep today, and he brought along an S8 Elite with EPR, their answer to the Respironics CFlex.
I had about an hour to try a few things and do some pressure readings to get a rough idea how it works.  I didn't get all the information to do a good head to head comparison, but hopefully I should be able to get either a diagnostic unit which will allow for better waveform analysis or at least a prolonged loaner so I can set up my own transducers.  At any rate, I wanted to at least get back with some preliminary information.
From a straight put-it-on-how's-it-feel, I absolutely must say this thing is phenomenal.  I ran it at 5, 10, 15, and 20 cmH2O, it was very comfortable throughout, and none of the glitches associated with CFlex at the high pressures.  We're really talking night and day here.
The EPR is maintained virtually to the point of inhalation.  Doing this really addresses high pressure problems in a big way.
I did inquire as to the trigger that terminates the EPR function, the rep didn't know but said he'd get back to me.
Since the model I tested did not have outputs to monitor the transducer activity, I used a nasal pressure transducer to gather waveform information, but couldn't set up a second unit to monitor the actual CPAP pressure at critical moments, and I think that has to be looked at closely, because there may be a couple of potential issues.
Here's the waveform I generated at 20 cmH2O:



The first two arrows represent the termination of EPR (the first tiny waveform), and you can see how close they are to inspiration.  This suggests that the trigger might be inspiratory flow, which could create a problem if you're trying to inspire from a sub-therapeutic pressure level.  Or, it could be a very low expiratory flow rate.  Try as I might, I could not verify this either way, so we'll have to wait for the manufacturer's response.  But you really need to be at therapeutic CPAP level prior to inspiration, so we'll have to look at this carefully.
The second two arrows show where the EPR did not terminate, and inspiration occurred at well under therapeutic levels.  I was able to generate this situation at all pressure levels.  The obvious problem here would be if the EPR level was below the apnea threshold, therapy would not be effective.  Again, I don't know why this occurred, and passed this information along to the rep.
This situation might be overcome by raising the baseline CPAP slightly, but the point is that using this EPR might change your CPAP requirements slightly.
I should also point out that there is a time function for EPR, if no breath is detected after a period of time, EPR is immediately suspended, so it seems that only a single breath would miss detection.
These issues aside, and I'm not yet sure if they are in fact issues, I can say that I was thoroughly impressed with the feel of this modality.  It's definitely worth a look.
sleepydave

Mike wrote:

it appears that there is also a component of cflex that takes into consideration the expiratory effort/volume.

I also called up Respironic tech support to ask them that question and about the termination trigger for CFlex.  Their response was that yes, it was "effort-related", that it would respond with up to a 5 cmH2O drop depending on the expiratory effort.  They could not tell me what the trigger was, however.
Even though everybody says, "reduced expiratory flow", nobody actually measures flow, they all measure pressure and extrapolate that out.  You take the pressure waveform, invert it and amplify it, and lo and behold, there's a "flow" waveform.
What?  Don't believe that? OK,
fine.

Here's the CFlex pressure waveform, inverted, amplified, and matched up against the "flow" waveform:



As Maximus would say, "Are you not amused?"
So it's a pressure trigger.  Actually, big deal.
At high CPAP pressures, exhalation is rapid and short, and it's tough to pick a point on the exhalation side to decide to terminate EPR.  They could probably fix this glitch by simply making the pressure termination value equal to baseline CPAP at pressures >15 cmH2O.  Why don't they?  Got me.  At CPAP 20cmH2O, exhalation is pretty dynamic.
OK, so if pressure is the trigger, how does ResMed EPR work?  Is it negative inspiratory force?  Almost have to be, and that makes me the tiniest bit nervous.  Hopefully find out in the next couple of days.
sleepydave

I've had my c-flex machine for five nights.

I find it easier to exhale if I exhale long and slow, and after inhaling, pause momentarily before exhaling.  I wonder if this is in my imagination or if it has something to do with the way that Cflex works.