In a PM with me50 asking for what kind of information he was looking for, me50 wrote:
me50 Wrote:flow rate, resp rate, tital volume, minute vent, flow limitation. I also would like info on insp. time, exp time
The links posted by zonk are very useful for all the standard stuff (leak, AHI, and a nice tutorial for getting around ResScan, Encore, and SleepyHead), they don't actually answer me50's questions. Neither does Pugsy's Tutorial over at the other forum.
So here's what I sent back to me50 in a pm. I thought I'd post it here as well since it is something that others may be interested in.
Most of these things (flow rate, resp rate, tidal volume, minute vent, flow limitation, insp. time, exp time) are highly variable from person to person and their "normal values" depend on the person's:
size (both height and weight)
AND level of physical activity
They also depend strongly on whether you're actually awake or asleep. And if you're awake, you can control them: Indeed, the very fact that you try
to observe your own respiration patterns will change them. That's why when the nurse in a doctor's office is counting your respiratory rate, she's still typically hanging onto your wrist acting like she's counting your pulse: If you knew she's counting your breaths, you're likely to start breathing slower and deeper.
All that said, every single one of these graphs depends on the flow rate graph. All the rest of them are computed from the flow rate graph: It's the master graph.
Flow Rate Graph
The flow rate graph
is the trace of the actual rate of air flow into and out of your lungs as measured by the back pressure at the machine end of the blower unit. The inspiration part of the flow wave is the part of the curve above
the 0-line. The expiration part of the flow wave is the part of the curve below
the 0-line. (Note however that SH has some bugs in it and occasionally the vertical axis of the wave flow graph is noticeably off and the humps in the wave flow are clearly NOT centered around the 0-line. If you see that on breath after breath all night long, that's a bug in SH, not a problem in your breathing.)
The units for this graph are "Liters per minute." And the tops of the humps occur when you are most actively inhaling and the bottoms of the valleys occur when you are most actively exhaling. The higher the inhalation bumps, the deeper you are inhaling. The lower the exhalation valleys, the more forcefully you are exhaling.
There are no "normal" values for this graph. "Normal" depends on the gender, the size (both height and weight), and the physical shape of the person doing the breathing. It also depends on whether the person is awake or asleep. And it depends on the sleep stage. What is important in this graph is that you can see when the flow rate drops to 0 for more than 10 seconds: That's the basic definition of an apnea for the machine's scoring algorithm. You can also see the reduction in airflow needed to trigger a hypopnea. (The actual definition of a hypopnea depends on which machine you are using.) When you go through each and every event, you can tell that some events are likely "false" events because the wave flow is in a clear transition between WIDE AWAKE and more relaxed or sleep breathing, which is naturally much more shallow than WIDE AWAKE breathing.
Flow Limitation Curve
A flow limitation
is a change in the inspiration part of the wave flow that is thought to indicate that the patency of upper airway is compromised. In other words, when the upper airway is just beginning to show signs that it is starting to collapse, the shape of the inspirations in the wave flow change in a mathematically measurable way. Sometimes this change is visible to the human eye and sometimes it's subtle enough where it's hard to spot. At any rate, both the Resmed S9 AutoSet and the PR System One Auto will increase the pressure in response to flow limitations since flow limitations indicate that the airway is in danger of collapsing.
The flow limitation data, however, is recorded in very different ways on the two different machines. The PR System One Flow Limitation Data is flagged as tick marks in the event table and there is no Flow Limitation graph. (A note to PR System One users: Flow Limitation data is recorded ONLY if the machine is running in AUTO mode.) The Resmed S9 machines record flow limitation data as a function of time, so you get a graph rather than tick marks.
This next paragraph applies ONLY to Resmed S9 data: On the flow limitation graph, the vertical scale in ResScan is made up of three crypic icons that indicate "an uncompromised airway", "a partially compromised airway", and "a fully compromised airway." Jedimark's vertical scale uses numbers instead of icons. You can roughly think of the numbers as "percents" since a 1 on Jedimark's scale corresponds to "a fully compromised airway" and a 0 on Jedimark's scale corresponds to "an uncompromised airway" and a 0.5 on Jedimark's scale corresponds to the "partially compromised airway" icon in the ResScan graphs. Since flow limitations are thought to indicate that the airway is potentially in danger of collapsing, in theory, you'd like that flow limitation graph to stay relatively close to the 0 line. In practice, however, you may need to find a working compromise between the pressure needed to smooth out the flow limitation line to an acceptable amount and a pressure level that you can still tolerate in terms of mask leaks and aerophagia. In other words, if the flow limitation line is still really, really ragged and you're still dealing with OSA symptoms, increasing the pressure a bit may help smooth out the flow limitation line and you might feel better. But you don't want to just keep increasing the pressure in an attempt to get a perfect flow limitation line if creates problems with mask leaks or aerophagia or other pressure related problems. Flow limitations don't bother everybody equally and if you are more bothered by the increased pressure than the flow limitations, it may be better to accept a more ragged flow limitation line if that allows you to actually get some quality sleep with the mask on your nose.
Respiratory Rate, Inspiration Time, and Expiration Time
One full breath is made up of one inhalation plus one exhalation. Respiratory rate
is just the number of breaths per minute. The machine may be doing a running average rather than simply recording the number of breaths each minute. Average at rest respiratory rates are usually given in the 10-20 breaths per minute range. Some sources give much narrower ranges than others do. I think the most typical range I've seen is something like 12-18. Sleep respiratory rates are usually similar to "wake at rest respiratory rates", but they can spike when you enter REM. They'll often drop during an apnea or hypopnea (for obvious reasons) and then increase (sometimes rather noticeably) during the mini-arousal with the associated recovery breaths immediately following an apnea or hypopnea. But the RR can also increase with the effort to turn over in bed so if you're thrashing around in bed a lot, it can go up. Plain old nervousness and anxiety will make it go up temporarily too. And simply the transition to full waking causes a brief spike in RR in some people. (On a side note: one definition of hyperventilation uses a Respiratory Rate of 40 or more lasting for several minutes. The most common reason for hyperventilation is plain old anxiety.)
and expiration time
are nothing more than the length of time (measured in seconds) of the inspiratory and expiratory parts of the wave flow for each breath. In most normal respiration patterns, the inspiration time and the expiration time are very roughly equal. They're not going to be exactly
equal, and I don't think there's any clinical significance connected to which is typically longer. And in the calculated values, it's going to depend on whether the "flat" part of the wave flow has a barely positive (inspiratory) air flow or a barely negative (expiratory) air flow. The "typical" number for the inspriation time and expiration time will depend on the respiratory rate. Very, very loosely speaking you'd expect these numbers to be about
- inspiration time roughly = (60/respiratory rate)/2
- expiration time roughly = (60/respiratory rate)/2
since (inspiration time + expiration time) = time for one breath
and respiratory rate = breaths per minute
So "normal numbers" for inspiration and expiration time are again going to vary widely from person to person and depend on the same factors that "normal" respiratory rates do. Moreover, in wake breathing, things like deep inhalations and sighs affect the (local) inspiratory and expiratory times significantly. And that can really affect the maximum
values of the numbers in table on the left side of daily data in SH.
Typically OAs tend to happen at the end of an exhalation: The airway collapses as we finish exhaling and just before we start to inhale. (That's why it's the EPAP pressure on an auto BiPAP that is adjusted in response to clusters of OAs). Hence if you tend to have a bunch of long OAs, that may wind up affecting the statistical calculation of the 90/95% expiratory time number as well as the maximum expiratory time number.
So for these three graphs, its really a matter of looking at enough of your own data to figure out what's normal for you. In general you're really looking for a graph that is mostly flat at where ever it happens to be. In my data, for instance, the RR usually bounces between 11 and 13 or 14 (so it's "mostly flat") with occasional short peaks, sometimes as high as 20-30 (or even higher) that usually correspond with known wakes. Or suspected REM periods. I usually ignore the inspiration and expiration graphs completely, but when I do look at them, they both spend most of their time between 2 and 3-4 seconds.
Tidal Volume and Minute Ventilation
Tidal Volume (TV)
and minute ventilation (MV)
go hand-in-hand in many ways. Tidal Volume is how much air is inhaled (or exhaled) in one inhalation (or one exhalation). Minute ventilation is the total amount of air that was inhaled (or exhaled) in a one minute period. If the breathing is super regular (like normal sleep breathing is supposed to be), then we have the equation:
- Minute ventilation = Respiratory rate * Tidal Volume
SleepyHead measures MV in liters and TV in milliliters, and its important to remember that 1000 milliliters = 1 liter.
TV varies from breath to breath and a "normal" value depends strongly on what you're doing. If you're not doing anything (you're sleeping soundly or you're at rest), you're not breathing anywhere nearly as deeply as you theoretically can breathe. If you're focusing on doing deep, relaxing yoga-type breaths, you're taking in a lot more air. Very, very loosely speaking an "average" tidal volume is about 500-600 ml for a typical adult in the waking state who is not paying too much attention to how they are breathing. But there's a huge variation on what's "normal" and "normal" really does depend on gender, height, weight, physical shape, and even the altitude that you grew up at. When we're asleep, the tidal volume and minute ventiation typically drop quite a bit. One study that I've found shows that these things can decrease by as much as 73% in REM sleep for example.
Now for the average person with OSA, the TV and MV numbers are not really all that important. The default setup for ResScan suppresses these graphs (and Encore Pro) does not show them at all. Where TV and MV do become important are when the patient has another lung condition such as asthma or COPD. In general if a patient has a serious lung condition where his/her docs are monitoring the TV and MV numbers, the patient is already going to know it. And daytime TV and MV will have been measured through a test known as a Spirometry test. And oddly enough, in many chronic lung conditions, the resting TV and MV are elevated
compared to normal numbers rather than lower than expected.
Finally, one interesting paper to find out more about how sleep affects respiration patterns can be found at: http://www.ncbi.nlm.nih.gov/pubmed/7164002