What Does An Oxygen Desaturation Index (ODI) Mean?

The oxygen desaturation index (ODI) is a measure of insufficient blood oxygen (low levels) during sleep. If you have a diagnostic sleep study, this measure can be one of the components in the report that you receive describing your results.

The ODI scores may be one part of a pattern of symptoms and findings that suggest obstructive sleep apnea (OSA), or certain heart and lung conditions, are a cause of low oxygen levels. Other factors that contribute to OSA risk include age over 50, a body mass index considered overweight or obese, daytime sleepiness, and certain facial features associated with a more narrow upper airway.

This article explains what the ODI is and how it's measured. It discusses health conditions that can cause insufficient oxygen levels, as well as those that are caused by low oxygen levels, and how they may be diagnosed and treated.

What Is the ODI?

The ODI is the number of times per hour of sleep that your blood oxygen level drops by a certain degree from baseline. Oxygen saturation is a measure of the oxygen in your blood. Drops in blood oxygen level are called desaturations.

Desaturations can occur due to illnesses (like lung disease) while you are awake, but they are even more likely to occur while you are asleep. Sleep apnea is the most common cause of oxygen desaturation during sleep.

The ODI is typically measured as part of standard sleep studies, such as:

• Polysomnogram, a sleep study while wearing electrodes to record breathing, heart rate, and other body changes
• Home sleep apnea testing, with devices used to detect apnea (but not other sleep disorders)
• Overnight oximetry, which uses a pulse oximeter to record oxygen levels while sleeping. A pulse oximeter clip can be used with any finger.

Breathing and Oxygen Levels

Several other aspects of your breathing can affect your oxygen saturation and your ODI results, and they are each separately reported as part of a comprehensive sleep study. These factors include:

• Respiratory rate: How fast you breathe
• Apnea: Episodes of paused breathing lasting for at least 10 seconds
• Hypopnea: Inadequate or shallow breathing episodes lasting for at least 10 seconds with at least 4% oxygen desaturation and at least 30% reduction in airflow
• Absolute minimum blood oxygen: The minimum oxygen saturation (the oxygen nadir) recorded in the study
  

While it can cause low oxygen levels at night, sleep apnea does not cause low oxygen levels during the day. Still, your low daytime levels may lower if you have a sleep-related disorder. One study of people with COPD found that those with low daytime oxygen levels were more likely to have problems breathing while sleeping. Whether low daytime oxygen can predict desaturation at night is still being studied, however.

Scoring Guidelines

Normal oxygen saturation when you're sleeping or awake should be 96% to 97%. A drop below 90% is considered mildly abnormal, while 80% to 89% is considered moderately abnormal. A drop below 80% is considered severely abnormal.

A dangerously low oxygen level at night or during the day is anything below 88% and requires immediate medical attention.

Generally, your oxygen levels will drop slightly during sleep. This is because your respiratory rate and volume tend to be reduced when you're sleeping.

Your ODI score is based on how much and how often your oxygen level drops during the test. The degree of change from baseline can be measured in two different ways with the following criteria:

• According to guidelines from the American Academy of Sleep Medicine, any respiratory event during sleep with a 3% drop in blood oxygen levels is counted towards the total. For example, a change from 95% to 92% would be an event that is counted toward the index's total.
• According to Medicare (and some other insurances that still rely on older scoring rules), they require a 4% change for an event to be counted toward the index.

How low oxygen drops with sleep apnea varies depending on other factors. For example, anemia, hypotension, peripheral vascular disorders, obesity, COPD, and frequent movement during sleep can all affect the score when a pulse oximeter is used.

An ODI score that indicates frequent or severe desaturations correlates with substantially reduced oxygen levels, but it does not identify the cause of the problem. Your healthcare provider would determine the cause of your desaturation by considering your medical history, physical examination, and possibly other diagnostic tests as well.

Generally, longer and deeper desaturations are associated with worse obstructive sleep apnea (OSA), and scoring criteria are designed to reflect these components.

What Causes Worsened ODI?

During sleep, muscle tone diminishes a little. This doesn't have a noticeable or substantial effect on most people.

However, the normal decline in muscle tone during sleep means that illnesses that affect breathing or blood oxygen can make low oxygen levels (and ODI) worse.

Apnea and Hypopnea

When breathing becomes disrupted during sleep, the oxygen levels of the blood can fall repeatedly. With OSA and other conditions that affect ODI, these drops are typically associated with apnea or hypopnea.

• Obstructive apnea is a complete interruption of breathing. It is the most common symptom of OSA, and it occurs due to blockage of the upper airway. This is usually due to the increased tendency of the throat muscles to relax during sleep.
• Central sleep apnea is far less common than OSA. It is a pause in breathing during sleep that is caused by impaired control of breathing from the brain or from heart failure or poor heart function.
• Hypopnea is an episode of impaired breathing that represents a partial collapse of the airway.
• Oxygen drops can occur due to snoring or upper airway resistance syndrome (UARS), two conditions in which breathing is disturbed, yet to a lesser degree than OSA.

The ODI differs from the apnea-hypopnea index (AHI), which is another measurement. The AHI also includes events that may cause arousal or awakenings from sleep without affecting oxygen levels.

Other Causes

Other conditions can cause problems with breathing and/or oxygenation during sleep and affect your ODI.

For example:

• Obesity hypoventilation syndrome makes it hard to breathe adequately and can affect your oxygen levels during waking or sleep, but especially during sleep.
• Neuromuscular diseases can impair your ability to move your muscles for breathing and can cause or contribute to ODI.
• Heart diseases, like congestive heart failure or mitral valve disease, or lung diseases, including chronic obstructive pulmonary disease (COPD), often coexist with OSA but can cause ODI on their own.

Several of these conditions contribute to each other, and OSA often coexists with other medical issues that affect breathing, such as COPD. With diminished lung capacity and/or heart function, the collapse of the upper airway may cause your blood's oxygen levels to drop more quickly and to a larger degree than they would if you had only OSA.

Health Consequences

Recurrent episodes of oxygen desaturation can lead to hypoxemia (diminished blood oxygen) and hypoxia (diminished oxygen in the body tissues).

Hypoxia and hypoxemia can be harmful to your body, especially if the problem is prolonged and recurrent. ODI may also be associated with:

• An increased level of carbon dioxide, which can be harmful to your health
• Oxidative stress and free radical formation in the body
• Associated bursts of cortisol due to oxygen deficiency
• Insulin resistance and increased risk of diabetes
• Daytime sleepiness and difficulty with concentration
• Altered metabolism and obesity

These factors may contribute to the risk of long-term health problems, including:

• Hypertension (high blood pressure)
• High cholesterol
• Heart disease, heart attack, and chronic heart failure
• Stroke
• Stress and depression

Cancer, heart arrhythmias like atrial fibrillation, and memory loss associated with dementia also are active areas of sleep research.

Treatment

The treatment for ODI abnormalities during sleep depends on the cause. Generally, OSA, the most common cause of ODI problems, is treated with continuous positive airway pressure (CPAP). It can normalize breathing and reduce the long-term risks associated with untreated sleep apnea.

Sometimes other sleep apnea treatments, including surgery, are considered as a treatment for OSA that doesn't improve with more conservative measures.

Lifestyle changes that can help include:

• Smoking cessation
• Exercise and weight loss
• Limiting or stopping alcohol use

Sometimes treatment of lung disease or heart disease can help improve ODI. For some people who have OSA or other causes of ODI problems, home oxygen supplementation can be helpful.

Summary

The oxygen desaturation index (ODI) measures your oxygen levels while you are sleeping and is an important tool for diagnosing and managing obstructive sleep apnea. Your ODI score is an assessment of how much and how often your oxygen level drops during the test.

The scores may be interpreted differently, and they are used alongside other factors that contribute to sleep apnea, or heart and lung diseases that can cause breathing disruptions while you sleep.

If you think you may be experiencing sleep disruptions, talk to your healthcare provider about whether a sleep study would be useful in diagnosing your condition and exploring treatment options. Resolving sleep-disordered breathing may benefit both sleep quality and long-term health.