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When Zero is not Zero
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PaytonA Online
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Post: #1
When Zero is not Zero
I have just caused myself a lot of confusion and I am hoping someone can clear things up for me. On the flow rate charts the median value is marked as zero but the flow rate is not zero at this line.

Please help me to understand this phenomenon.

Best regards,

PaytonA
(This post was last modified: 02-03-2015 12:50 PM by PaytonA.)
02-02-2015 10:23 PM
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eseedhouse Online

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Post: #2
RE: When Zero is not Zero
(02-02-2015 10:23 PM)PaytonA Wrote:  I have just caused myself a lot of confusion and I am hoping someone can clear things up for me. On the flow rate charts the median value is marked as zero but the flow rate is not zero at this line.

The "median" is the middle value. The median of 2, 0, and -2, for example, is zero. In your graph the peaks values above the zero line appear to be the same as the peak values below, so the median would indeed be zero. The arithmetic mean would not be zero if you spent more time above the zero line than below for instance, but the median would still be zero.

Ed Seedhouse
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Actually you know, it is what it isn't.
02-02-2015 10:30 PM
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Ghost1958 Offline

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Post: #3
RE: When Zero is not Zero
Sorry cant help on that. I could show you a flow graph that shows only the top half above 0 and nothing below on the exhale.
Ive put it down to i must be for a half hour or so rarely breathing in thru my nose in the nasal mask so it picks up the inhale, then exhaling thru my mouth into open air where the machine cant pick it up.

Doesnt show any additional leak rate and if I was only inhaling and not exhaling for half an hour I think I would flll up like a Macys day parade float lol.
02-02-2015 11:53 PM
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justMongo Offline

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Post: #4
RE: When Zero is not Zero
(02-02-2015 10:23 PM)PaytonA Wrote:  I have just caused myself a lot of confusion and I am hoping someone can clear things up for me. On the flow rate charts the median value is marked as zero but the flow rate is not zero at this line.



Please see the attachment for a clearer picture of my confusion. Please help me to understand this phenomenon.

Best regards,

PaytonA

Why would you mark the inspiration and exhalation peak as zero flow?
Clearly it's not zero. Only df/dt is zero there.

The flow meter sees the total flow as a function of time. Your in/out wave is added to the vent rate. Which for a VPAP different during inspiration and exhalation because of the switch from IPAP to EPAP.

The machine will put the zero where the area under the inspiration and exhalation flow are equal. These areas must be equal or you would either blowup like a balloon -- or be a Patriot's football.

INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED AS MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEB SITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.
02-03-2015 11:34 AM
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PaytonA Online
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Post: #5
RE: When Zero is not Zero
(02-03-2015 11:34 AM)justMongo Wrote:  Why would you mark the inspiration and exhalation peak as zero flow?
Clearly it's not zero. Only df/dt is zero there.

I marked the inhalation and exhalation peak as zero flow because of the physical reality. You can not go from inhale to exhale and vice versa without there being at least a moment of no flow which would be right at the peak. Also, to my knowledge, I do not get zero flow in the middle of my exhale.

(02-03-2015 11:34 AM)justMongo Wrote:  The flow meter sees the total flow as a function of time. Your in/out wave is added to the vent rate. Which for a VPAP different during inspiration and exhalation because of the switch from IPAP to EPAP.

The machine will put the zero where the area under the inspiration and exhalation flow are equal. These areas must be equal or you would either blowup like a balloon -- or be a Patriot's football.

It would make some sense to me to put a baseline at a point where the area above and below is equal. One thing that I have just noticed going back over some of these charts is that neither Rescan noor Sleepyhead puts a 0 at the baseline. In fact Sleepyhead does not display a baseline on the version that I am using anyway.

Best Regards,

PaytonA
02-03-2015 12:30 PM
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PaytonA Online
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Machine: ResMed S9 VPAP Auto
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CPAP Software: ResScan SleepyHead

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Location: Orange County,California

Post: #6
RE: When Zero is not Zero
OK, Thanks folks. I think that I have gotten myself straightened out now. I was looking at the graph bass ackwards. Embarrassing!

Best Regards,

PaytonA
02-03-2015 12:48 PM
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justMongo Offline

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Post: #7
RE: When Zero is not Zero
(02-03-2015 12:48 PM)PaytonA Wrote:  OK, Thanks folks. I think that I have gotten myself straightened out now. I was looking at the graph bass ackwards. Embarrassing!

Best Regards,

PaytonA

You mean like when you said:
(02-03-2015 12:30 PM)PaytonA Wrote:  I marked the inhalation and exhalation peak as zero flow because of the physical reality. You can not go from inhale to exhale and vice versa without there being at least a moment of no flow which would be right at the peak...

OK, back to math class for you. Cypress J.C. would be good.

INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED AS MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEB SITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.
02-03-2015 05:55 PM
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