Resperonics Flow Limitation Index

[RWhipkey]

Does anyone have a good understanding of Resperonic's Flow Limitation Index?  I know its calculated as the marked flow limitations per hour just like AHI.  But what does the number actually mean?    In other words, what is a number that one needs to be concerned about and what is a good number?  Do we even pay much attention to this number?  Any insight would be much appreciated.

[Sleeprider]

According to Respironics EncoreBasic Help Glossary:

Flow Limitation
Changes in flow limitation are recorded as events. The Flow Limitation Index is calculated by the total number of flow limitation events per night divided by the hours of use. Note: The average is calculated by taking the total number of events divided by the number of therapy days. This can be used to indicate if there has been a significant degradation in the flow signal, resulting in a pressure increase.

The flow limitation algorithm analyzes the peak and shape of the inspiratory airflow waveform. The algorithm looks for relative changes in the peak, flatness, roundness, or skewness of the inspiratory portion of the airflow waveform. These changes are observed both over a short period of time (groups of 4 breaths) and over a long period of time ( several minutes). Statistical measures are used to help minimize false events while allowing the devise to be sensitive to even small changes.These measures are disabled during unstable breathing conditions, such as a snore, apneas, hypopneas, high leaks, and variable breathing.

Apnea
An apnea is indicated if there is an 80% reduction in airflow for 10 seconds compared to the average airflow over an extended period of several minutes or if there is no airflow detected for 10 seconds.

Hypopnea

An hypopnea is indicated if there is approximately 40% reduction in airflow for a duration of between 10 and 60 seconds, compared to the average airflow over an extended period of several minutes. Following a reduction in airflow, the therapy device must see two recovery breaths in order to label the event as a potential hypopnea.

[Sleeprider]

To take this discussion a bit further, there are differences between how a tech will score a PSG vs what the machine reports. For example, an 80% reduction in flow during PSG is not an apnea, but a hypopnea. Auto CPAP machines monitor flow limitation as a part of the treatment algorithm, and will normally increase pressure in response to flow limits and snores as indicator of increasing obstruction. There are limits on how this is done. Also, it's important to keep in mind that some people with chronic flow limitation as indicated by the flow rate graph may not register flow limitation events if the flow restriction does not meet the event criteria from earlier breaths. So we may see someone with extensive flow limits, but no event flags. These are much easier to spot with a Resmed graph, or by zooming in on the flow rate.