oxygen desaturation index

The oxygen desaturation index, or ODI, counts how many times per hour the blood oxygen level dips during sleep. Where the apnea–hypopnea index tallies breathing events, the ODI tallies their oxygen consequences: each time the saturation drops by a defined amount and then recovers, that counts as one desaturation. It is, in effect, the hourly rate of oxygen dips.

It is measured from the pulse oximeter trace by finding falls in saturation of a set size — commonly 3 or 4 percent below baseline — and dividing the total count by hours of recording. Because it needs only a finger oximeter, the ODI can be obtained from very simple devices, making it useful in screening and in home or overnight oximetry when a full study is not available.

The ODI usually tracks the apnea–hypopnea index closely, since most breathing events cause an oxygen dip, and it gives a direct sense of the oxygen stress the body endures overnight. Deep and frequent desaturations are particularly relevant to cardiovascular and metabolic strain, so the ODI adds physiologically meaningful information beyond a simple event count.

But the two indices are not interchangeable. Hypopneas scored on arousal alone, without a qualifying oxygen drop, raise the apnea–hypopnea index without raising the ODI, so the ODI can be lower. Conversely, in people with lung disease who start sleep with low oxygen, even small reductions in breathing produce large dips, inflating the ODI. The desaturation threshold used should always be reported.