Understanding My Sleep Apnea: Part One "Sleep Studies"

Sleep Better

I get asked every day, “How bad is my sleep apnea?”

Many doctors and most patients don't really understand the information on their sleep study. AHI, RDI, T90, T88, Oxygen Nadir ... What the heck! "Doctor told me I wake up 40 times an hour, but I don't ever wake up." Here's the answer, and it's actually pretty simple.

Apnea: No breaths for 10 seconds or longer & oxygen saturation drops by 3% or more

Hypopnea: Shallow breaths for 10 seconds or longer & oxygen saturation drops by 3% or more

Apnea Hypopnea Index (AHI): Total number of apneas and hypopneas divided by the number of hours slept.

Respiratory Disturbance Index (RDI):   measures not only apneas and hypopneas, but a third category, RERAs, or respiratory effort related arousals. RERAs measure disruptions in sleep that are not classifiable as apneas or hypopneas.  This is less commonly used due to the difficulty in being certain that the arousal is directly due to a breathing problem with many sleep tests.

The idea of “waking up” isn’t an event where you are fully awake.   The body moves to a lighter sleep stage so that you are more awake.  When this happens, you take a deeper breath and the event ends.

AHI is the measurement that government agencies and insurance agencies use to determine the SEVERITY of your sleep apnea.

But wait- does it matter how long my events are and how low my oxygen goes???

Absolutely- most recent studies show that the longer the events and the lower the oxygen, the more significantly your health is negatively impacted.

T90 and T88: These measurements are the total number of minutes that are spent during the night with oxygen saturation below 90% or 88% respectively.

These measurements are much stronger predictors of your risk of long term complications from sleep apnea such as cardiac arrhythmias, pulmonary hypertension, stroke and others.  Also, they are really easy for a sleep test to measure.  But they do have limitations.  First, they are total time.  So, they don’t account for how long someone slept.  Second, they group anything lower than 90% or 88% together, so it runs into the same problem of lumping together things that really shouldn’t be.

The best test for predicting how big an impact your sleep apnea has on your health is called hypoxic. burden.  This calculates how long and how low your oxygen is during the night then creates a composite number.  This is a complex set of information for a sleep study to capture, so many sleep studies don’t have this incorporated.  In a later article, we’ll dive deeper into hypoxic burden.

FOR NOW:  If you’ve been told you have sleep apnea, don’t ask how severe is it (AHI).  Ask how low did my oxygen go and what was my total time of low oxygen. 

Upcoming:  Part 2 in Understanding My Sleep Apnea:  Sleep Study Examples and Comparisons