Clarification on Graph Titles and axis

[Wondering2]

I am contacting you folks because you are the acknowledged "experts" on all things OSCAR.

On another board, it is contended that the OSCAR graphic chart of titled "Flow Rate" is mislabeled
That chart is NOT really FLOW RATE  versus time of day,

 but should properly be labeled "FLOW", furthermore the y-axis units for that graph should be ml  and not ivolume/time (L/Min)

So the chart should be depicting ml versus time of day.

For my own edification, is the sensor reporting flow (as in volume) or is it reporting flow per min (as in rate)?

Thanks for your help.

[Sleeprider]

The flow rate graphs displays the respiratory flow in mL/second (flow rate) for the time shown in the graph. It is not an instantaneous volume, but is a volume over time, or rate. It is graphed based on the 25 Hertz (25 data points per second) in Resmed or 5-Hertz frequency data from Philips, or whatever measure is used by the manufacturer. Any given point on the graph is the flow-rate per second, but the graph represents the flow-rate in mL/second over time. It is properly named. Other flow rates in Oscar include the tidal volume (milliliters/second) and minute vent (Liters/minute), both presented as minimum, median, 95th percentile and 99th percentile statistical measures. Measured flow is always based on a time basis or flow rate. We almost always refer to flow in a quantity per time, or rate. This is true of water treatment plants at MGD (million gallons per day) or a river flow rate in cfs (cubic feet per second) or even stack-flow in meters per second. Other measures are velocity (distance per time). I'm not aware of a "flow" measurement that is not a rate, velocity or based on some increment of time.

If you isolate just one point in the graph of flow rate, you will see the flow in mL/sec for that data point, however the graph is the flow rate, and changes over time and with the respiratory cycle. To be exact, the Y-axis is flow-rate per second and the X-axis is time. The resulting graph is flow rate per time or flow rate.

To fully refute the contention that the graph represents "Flow", the flow being measured is a volume (milliliters). Without time, or zero, we have a divide by zero error. Give that a try and you are left with a meaningless static measurement.

[Wondering2]

@Sleeprider, for context,
 
I've been trying to suggest (over there) that the OSCAR tidal volume graph is a critical tool to be considered when evaluating a hypopneas, in light of the fundamental definition of a hypopnea as being a reduction in volume (aka less air inhaled compared to prior breaths).

Thanks for the clarity.

[Sleeprider]

I have a longer history on the CPAP Talk forum dating back to 2008. I don't participate over there for reasons you are learning. I really don't want to get dragged into that conversation.

Tidal volume is a simple function of inspiration or expiration flow rate over a single breath cycle. Oscar reports the tidal volume, minute vent and breath rate as reported by the manufacturer. As far as I know these statistics are identical to what your should see in Airview or Orchestrator. Hypopnea events are also flagged by the CPAP software and reported by Oscar. Remember that Oscar is a reporting software and does not actually flag events and each manufacturer has its own definition of what a hypopnea is in terms of a short duration flow rate deviation from a longer-term average. In clinical PSG, hypopnea is defined by flow rate reduction and SpO2 desaturation of 3 to 4%. Be sure when you debate over there, that you are all on the same page as there are differences between machines and sleep studies. Oscar is only reporting and graphing what is in the data generated by the machine. I would argue tidal volume cannot be considered without also looking at breath-rate, which gives the total air exchange over time.

[Wondering2]

I do appreciate, and understand, your hesitation about the other...... (nuf said) 

The thought that I'm trying to promote is that the tidal volume data should not be ignored when assessing the questionable hypopneas flags.

For my benefit and understanding, is the OSCAR graph for tidal volume taken directly from the EDF files. What I mean to say is that OSCAR software does NOT calculate tidal volume, but rather just presents the EDF data in graphic form. 
 
Am I correct in that assumption?
Thanks

[pholynyk]

You are correct; the Autoset, vAuto, and ASV machines (ResMed) all report tidal volume every two seconds.

[Wondering2]

You are correct; the Autoset, vAuto, and ASV machines (ResMed) all report tidal volume every two seconds.

for additional clarification, please.

The tidal volume determination (that I have assumed is calculated rather than measured), can NOT be prior to the completion of an inhalation. 


What I'm suggesting is that the limiting factor (for tidal volume determinations) is not the cycle time of a sensor, but rather is the number of inhalations per unit time. 

Is this a correct way of looking at it?

Thanks again for your patience.

[pholynyk]

I have no idea how the machine calculates anything. The code in the machine is a black box.

OSCAR takes the data in the edf files and displays it. I have no interest in going any deeper than that.

[Wondering2]

I have no idea how the machine calculates anything. The code in the machine is a black box.

OSCAR takes the data in the edf files and displays it. I have no interest in going any deeper than that.

OK, lets try it this way, does the EDF file containing what OSCAR plots on the flow rate graphic have a heading and dimensions?
Like "flow rate, ml/sec "?

[pholynyk]

Of course it does... read up on the edf file format: https://www.edfplus.info/

From the header:

Code:

Label      >Flow.40ms       <
Transducer >                                                                                <
Units      >L/s     <
Min        >-2.000000<
Max        >3.000000<
Dmin       >-1000<
Dmax       >1500<
PreFilter  >                                                                                <
Smp/Rec    >1500<
Reserved   >                                <

OSCAR scales the values to ml/sec for convenience