reading the flow chart

[staceyburke]

I’m confused on the flow limit chart…  Flow limits are defined as 50% blockage or less. (if I’m wrong let me know)  The chart in Oscar goes to 1 ( I know I can change it).  So if the flow line is at .50 that means that the flow is really .25?  and if the flow shows .25 it is really 12.5?  If that right why not just label it the correct amount of limitation.

Like I said I’m confused – someone give me a little help understanding what the chart shows.. Thanks.

[Crimson Nape]

Good question! That is the way Resmed reports them. Right or wrong, I've always regarded this value a percent, with 1.0 meaning 100%. If you wish to go one step further, Philips can and does report the Flow Limits higher than a 1.0 value. To date, I haven't found what the actual limit is or how to correlate its value to my perception of a Resmed value.

[Sleeprider]

To confuse matters more, the Resmed flow limit index varies by individual. Someone with high chronic flow limitation can have flattened inspiration waves and not have an elevated FL index, while someone with normal respiration, may have slightly flattened inspiratory flow, and have a higher index. Flow limitation appears to be indexed to the individual's baseline, rather than to a fixed reduction in inspiratory flow rate. The flow limit chart goes to 1 which suggests a near cessation of flow. The best way to see what that means is to zoom in on any spike in FL that shows very high flow limit. During apnea, flow limit may be near 1.

[multicast]

Flow limits are defined as 50% blockage or less. (if I’m wrong let me know) 

Well, in a way you are.  The notion "flow limitation" is somewhat misleading.  It's not primarily   a "degree of obstruction" in the sense that a flow limitation of 0.5 means that the airway is "closed" by 50%.   It is rather a grade how "good" your breathing is: 0 corresponding to an A and 1 to an F.  In that sense these values should not be interpreted as percentages as they are ordinally scaled (like school grades).

The quality of your breathing is continuously evaluated by analysing the shape of your flow curve an other parameters, see the attached file, please.  Assumed there is a flow limitation of class 3 *no obstruction does occur* but your breathing does not earn an A-grade.  Determine flow limitation is a delicate process, quote from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4629962/

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In order to evaluate flow limitations, Respironics determines roundness, flatness, skewness, and WPF to rate the most recent four breaths as better, worse, or the same compared to baseline. Roundness is determined by the similarity of the WPF between 5% and 95% values to a sine wave. Flatness is determined by the absolute value of the variance between 20% and 80% of inspiratory flow from the average of all the values in the same period, divided by 80% volume point. Skewness is determined by dividing the average of the highest 5% of flows in the mid third of the breath by the average of the highest 5% of flows in the first third of the breath.17

ResMed also determines flow limitation. S8 AutoSet defines flow limitation using flatness of an inspiratory breath. The flatness index is calculated by the RMS deviation from unit scaled flow calculated over the middle 50% of a normalized inspiratory breath.4 From the S9 onward, flow limitation is calculated using a combination of flatness index, breath shape index, ventilation change, and breath duty cycle. Ventilation change is the ratio of the current breath ventilation to recent 3-minute ventilation. Breath duty cycle is the ratio of current breath time of inspiration to total breath time of recent 5 minutes. If a breath is severely flow limited, the flow limitation index will be closer to one and when the breath is normal or round, the flow limitation index will be zero.8

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Strange enough, there may be obstructions where the "flow limitation" is zero -- that's no contradiction.  Anyhow, "flow limitation" isn't a percentage at all.

[Sleeprider]

Multicast, thanks for the post. Can you add some attributes to this and send it to me by PM? I want to add it to the Flow Limitation wiki.

[multicast]

Ok, please let me know what I should add.  Maybe interesting is also that ResMeds miscount respiration rate in presence of certain flow limitations.  Hence a high respiration rate may be an indication of flow limitations: 

   

Clearly flow limitations are visible, but not indicated as such.  Instead these are miscounted as additional breathes (it nearly doubles) as seen in the respiration rate graph.

(Please note also that the flow rate's zero line is always only *estimated* as it can't be calculated exactly.)

[Sleeprider]

I have seen similar respiration where the inspiration is so limited it is overwhelmed by cardioballistic artifacts in the flow rate that the machine cannot even detect the respiration rate accurately http://www.apneaboard.com/wiki/index.php...cillations We have sometimes seen bilevel and even ASV used to mitigate this. I honestly think that may be the direction you are headed. In theory, the pressure support not only combats the flow limitation, but stabilizes inspiration to suppress these artifacts. You are a wealth of interesting problems. Have we tried using some EPR?

[multicast]

To be honest, I'm not regarding those irregularities as cardioballistic artifacts -- I don't own a lion's heart, unfortunately.  Furthermore we can't rely on the zero line: as there was no leakage we should have equal ares above und under the zero line.  This is clearly not the case at 5:11:15 and between 5:11:20 and 5:11:25.  Hence we may assume a class 3 at 5:11:15 and at 5:11:55, a class 2 at 5:12:10, a class 4 between 5:12:00 and 5:12:05 and so on.

But in the end we're all looking in more or less polished crystal balls, don't we?

And regarding "flow limitation" not as a partial collapse of the airway but as an evaluation of how "healthy" our breathing is the machine doesn't really combats flow limitation -- how to combat an evaluation? It rather takes an appropriate action, namely an adjustment of the pressure, trying to prevent possible upcoming obstructions. (How this is done by different machine is described in the article I've cited.)