Saturday, February 28, 2009

Q. We have doctors who want to use the respiratory disturbance index (RDI) instead of the apnea hypopnea index (AHI) to qualify patients for PAP therapy. What’s the difference?

A. These two metrics have been causing confusion for quite some time. When the recently released local coverage determination (LCD) included a definition of RDI that was different from what’s outlined in the national coverage determination (NCD) and used by the American Association of Sleep Medicine, that confusion increased exponentially.

To understand the differences, let’s return to the bed and the patient that’s in it. Realize that even if the patient is in the bed with eyes closed, that does not prove “sleep” is occurring. You may be “recording” the event, but the patient may not be truly asleep. Hence, the difference between “recording time” vs. “sleep time.” The only way to prove sleep and to stage it is to have several channels monitoring the patient, including EEG or brain wave.

RDI, up until the LCD was released, was understood to include respiratory effort related arousals (RERA), which means the patient begins to wake up in an effort to breathe. AHI is the total number of apneas (complete ceasing of air flow) plus hypopneas (shallow breathing, but some flow still present) during sleep time.

Here is an easy way to help keep it straight (at least until the next change): The LCD definition of RDI is different in that it does not include RERAs. AHI=Apneas+Hypopneas/“sleep time,” thus requiring more channels. RDI=Apneas + Hypopneas/“recorded time.” CMS generally views RDI as the index for HST (portable monitoring) and AHI as the index for polysomnogram (PSG).

To get paid, our industry must focus on the intent of the LCDs. Often, however, it must also explain to a physician why the definition that is accepted by the sleep community is now not applicable.

Kelly Riley is director of The MED Group’s National Respiratory Network. Reach her at