Definition of Flow Limitation and Flow Limit

[Gideon]

ResMed uses a "Flattening Index" as a means of identifying flow limitations that require action (raising pressure).  Look at the Flow Limits and the Pressure charts and see a flow limit correlates with a pressure increase.  Think of flow limits as part of a continuum

Apnea (80-100% restriction)(min 10 sec)
Hypopnea (50-80% restriction)(min 10 sec)
Flow Limit(<50% restriction)(not timed),

Technically flow limits would include <10 second events or that Apnea and Hypnea are more significant subclasses of flow limits.

Think of Flow Limit as the basic event and the others as more significant.

Since I'm suffering incredibly from Flow Limit issues, does this mean that a flow limit is a reduction in oxygen to the lungs but under 10 seconds long which would then become a Hypopnea?  

Could this explain someone having issues with daytime wheezing in the lungs?  And brain fog during the day?

No. Because You may be breathing harder (not good either) to compensate for the restricted breathing.   Oxygen reduction is something that may or may not happen from any obstructive or central event, including apnea and hypopnea. Oxygen Desats would be more common with longer events.  A flow limit may be only as long as a single breath.  Should DeSats be a concern, including just knowing that you do not have them, get a recording Oximeter compatible with OSCAR so O2 Sats can be plotted along with your other charts.  

Any CPAP will help with Oxygen Desats, Different models, such as ResMed with EPR, or BiLevel with Pressure Support, will help more than other models.

Flow Limits concern "upper Airway",  Lungs would be considered "Lower Airway" so no.  Wheezing frequently occurs from a narrowing of lung passages, think Ashama and COPD.  
Brain fog, Three main causes, Neurological, Drug-induced, Oxygen deprivation.  So yes, to the degree that OSA and CSA impact Oxygen uptake, yes they can cause brain fog.
My wife was diagnosed as depressed and given (in)appropriate meds.  The problem was her left largeneal nerve was cut and paralyzed her Vocal Cord. The 'fix' was a teflon injection into her vocal cord. That fixed a multitude of issues.  A granuloma formed at her vocal cord gradually reducing the size of her airway resulting in undiagnosed reduced oxygen.  Later the same problem was diagnosed as Asthma.  That caused Brain Fog in the worst way.  Do not diagnose Oxygen deprivation with anything other than an Oximeter.

[Sleeprider]

If you are experiencing wheezing, this is a strong sigh of COPD, which would explain flow limitation. Only your doctor can diagnose this, and it is a comorbidity that may qualify your for higher bilevel pressure support. This can also be seasonal allergies, but we cannot diagnose this on the forum. Discuss your symptoms and ask your doctor for a pulmonary function test, or referral to pulmonology. Increased pressure support can overcome the flow limitations and allow for adequate expiratory time if you have non-compliant lung tissue that is making it difficult to breathe.

[SarcasticDave94]

Wheezing in your breathing may be a pulmonary issue. I'd mention this to your doctor as you don't want to take chances with your lung health.