Abstract
I tested several sensors that claim to measure sleep apnea through optical measurement of blood oxygen content.
Background
Blood oxygen arterial saturation (Sa02) is approximated by Sp02. At 5,000 feet elevation, Sp02 should be above 92%. Values under 90% are considered low. Hypoxemia is dangerously low blood oxygen, defined as Sp02 <88% for more than 5 minutes. (Hypoxia is low tissue oxygen levels)
An oxygen saturation level over 95 percent is considered normal. Anything below 92 percent oxygen may be a sign of breathing problems during sleep, such as sleep apnea or another disorder like severe snoring, COPD or asthma. (Low levels can also be caused by anemia.)
Sleep apnea is characterized by a cessation of airflow lasting 10 seconds or more. Hypopnea (shallow breathing resulting in desaturation) is a decrease in airflow lasting 10 seconds or more with a 30% oxygen reduction and a 3 to 4% desaturation from the baseline. It is not uncommonpatients with sleep apnea to desaturate below 88%.
Note: low Sp02 is an indicator of sleep apnea and is not necessarily the mechanism of the bad effects. It may be possible to not have hypoxemia, but still have interrupted sleep.
Sleep apnea and hypopnea can cause small awakenings that disrupt normal sleep cycles. This can cause irregular heart beats, high blood pressure, blood sugar excursions, strokes, heart attacks, etc. Most common effect is to create chronic, elevated stress response.
Results
- Fitbit Charge 2 never showed any apnea episodes
- Biostrap showed several, including some that my wife didn't remember
- Wellue 02 ring showed several mild episodes per night. Based on other testing, this sensor seemed potentially the most accurate, but location on finger wasn't comfortable. Also ended up not being accurate because I sometimes sleep on my arm, cutting off circulation, which reduces Sp02.
Conclusion
I have been diagnosed with mild sleep apnea and my wife has no known apnea. We both tried the Sp02 wearable sensors and they consistently showed the same level of apneas between us, either none (Fitbit) or mild (Wellue 02 Ring). Based on the lack of a clear and actionable signal from these sensors, I no longer use them to track apnea events. Instead I focus on trying to minimize possible breathing issues and promote general good sleep hygiene to try to improve restfulness.
