Pulse Oximeter as a Sleep Apnea Monitor

Sleep apnea or sleep apnoea is a sleep problem of having excessive pauses in breathing or abnormally shallow breathing during sleep. Each pause in breathing is called an apnea and can last a few minutes. Each shallow breathing is called a hypopnea. In a overnight sleep study, the objective is to detect and count the number of apneas occurred during the study. This study (polysomnography) is normally performed in a lab and many different types of data are collected, such as oxygen saturation level, respiration rate and flow, heartbeat, EEG and EKG... The sleep normally costs thousands of dollars.

Instead of doing a full-blown study, the recommendation now is to use a pulse oximeter to do a preliminary study to determine if a polysomnogram is required. In many cases, using the pulse oximeter is sufficient except for the borderline cases. Many studies performed have shown the effectiveness of using pulse oximeter alone in sleep study.

When an apnea occurs, the insufficient air flow into the lung would cause a drop in the blood oxygen saturation level. By using an oximeter to record the oxygen saturation throughout the night, a software program can analyze the data, identify when an apnea occurs and generate a summary report. One can determine his severity of his apnea by looking the frequency of apnea events.

AHI (Apnea-hypopnea index) is an index for measuring the severity of sleep apnea. It is defined as the total number of apnea and hyponea occurred per hour of sleep. Each apnea must last for at least 10 seconds and affect the blood oxygenation. The following table shows the classifications.

AHIClassification
5 - 15Mild
15 - 30Moderate
>30Severe

Besides using an oximeter as a screening tool, one can use the oximeter to monitor the effect of treatment and to see how one is doing.

Here shows some of the criterions used in various studies on detecting an apnea event.

  • SpO2 drops by 4% from the base line and is at or below 90%
  • SpO2 drops by a set amount (2%, 3% or 4%) for at least 10s and the rate is > 0.1%/s. Also, within one minute, SpO2 should return to within 1% of normal or recover by >150% of the set amount of the dip.
  • The delta index is the average of absolute differences of SpO2 between consecutive 12-seconds interval. The delta index is compared against a set amount (2%, 3% or 4%). This approach also tracks the total amount of time spent below various levels (90%, 88%, 86%, 84%, 82%, and 80%)