What is pulse oximetry?
Pulse oximetry is a noninvasive method for monitoring a person's oxygen saturation. It is used to measure the oxygen level in your blood and your heart rate.
How does pulse oximetry work?
A finger pulse oximeter functions by shining light through your finger. The sensors detect how much oxygen is in your blood based on the way the light passes through your finger. Pulse oximetry is the technology calculating the results to display a number on the oximeter’s screen that tells you the percent of oxygen in your blood. A finger pulse oximeter also measures your pulse rate.
Oxygen Desaturation Index (ODI)
When you record your pulse oximetry, you can obtain your ODI (Oxygen Desaturation Index). ODI is the number of times per hour of sleep that your blood's oxygen level dropped from the baseline. It may not be as accurate as other ways of monitoring sleep unless the stages of sleep are also recorded.
Any respiratory event during sleep with a 3-4 percent drop in blood oxygen level, lasting 10 seconds or more, is an ODI event. An example of an oxygen desaturation event would be a change from 95 to 92 percent. (source)
Checking for sleep apnea using pulse oximetry
This video shows you how you can check if you have sleep apnea, using a rough estimate based on oxygen desaturation events, using pulse oximetry. It is important to note that this will not tell you if you definitely have sleep apnea and is no substitute for a doctor. Please see a doctor if you feel you have sleep apnea.
This video uses the CMS50E pulse oximetry.
Limitations of oximetry
Pulse oximetry has many well known limitations. The amount of ambient light, skin pigmentations, nail polish usage, the temperature of the hand, interference and other factors can impact its reading. A pulse oximeter also does not know if you are asleep so the readings cannot be considered as accurate as AHI, which needs to be obtained using other technology.
Limitating factors (source):
- Carbon monoxide
- Cabon monoxide molecules, even in small amount, can attach to the patient's hemoglobin replacing oxygen molecules. A pulse oximeter cannot distinguish differences and will show total saturation level of oxygen and cabon monoxide.
- Hemoglovin deficiency (anemia)
- Blood volume deficiency
- Irregular signals such as irregular heartbeats
- External interference
- Exposure to strong external light while taking a reading can give inaccurate readings
- Fingernail polish and pressed nails
- Nail polish and pressed-on nails may interfere with readings and should be removed
- Skin pigmentation
- Dark skin can give over-estimated SpO2 readings when it is below 80%
- Intraveneous dyes
- Intraveneous dyes (such as methylene blue, indigo carmine and indocyanine green) can cause inaccurate readings
- Methemoglobin
- Methemoglobin is a form of hemoglobin that does not carry oxygen. The higher the percentrage of methemoglobin can be genetic or caused by exposure to certain chemicals and medications.
Pulse oximetry and sleep apnea
Sleep apnea is often measured in terms of AHI (apnea hypopnea index) per hour. This is the amount of times someone stops breathing while they sleep per hour.
A pulse oximeter cannot give AHI but it can give ODI (oxygen desaturation index). ODI is an approximation of AHI and is considered a good predictor (source).
Sleep apnea AHI levels:
- Normal - less than 5 AHI/ hour
- Mild sleep apnea - 5 to 15 AHI/ hour
- Moderate sleep apnea - 15 - 30 AHI/ hour
- Severe sleep apnea - 30 or more AHI/ hour
By recording your oxygen desaturation events at night per hour and relating them to AHI, you can have an estimate for the presence of sleep apnea.
A pulse oximeter is not as accurate in diagnosing sleep apnea as a polysomnograph. Find out more about screening sleep apnea using pulse oximetry on this page.
What is abnormal in pulse oximetry measurement?
In general, it's considered abnormal if the oxygen level falls below 88% for an adult or below 90% in a child. If the levels are below 88% for longer than 5 minutes a night, a condition known as hypoxemia may be diagnosed.
Pros and cons of overnight oximetry
Pros:
- Easy and inexpensive
- Devices can be purchased in many places
- Can be useful to see if someone needs further testing with a polysomnograph
Cons:
- Oximetry alone is not adequate to diagnose sleep apnea
- Cannot be used for insurance purposes
- Is not as accurate as AHI
- Cannot detect central sleep apnea
Oximetry is no substitute for a doctor
You should see a doctor if you believe you may have sleep apnea. Oximetry is merely a low cost, at home way to monitor an approximatation of sleep apnea.
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