Blood Oxygen (SpO2)
What Is SpO2?
Peripheral Capillary Oxygen Saturation (SpO2) measures the percentage of hemoglobin in your red blood cells that is carrying oxygen from the lungs to the rest of the body.
- 95-100%: Normal saturation.
- 90-95%: Mild hypoxemia (may be normal during sleep or high altitude).
- < 90%: Clinically significant hypoxemia (often requires supplemental oxygen).
How HealthKit Measures It
The Apple Watch uses a specialized Blood Oxygen sensor: 1. Red & Infrared LEDs shine into your wrist. 2. Photodiodes measures the reflected light. 3. Color Analysis: Oxygen-rich blood is bright red (absorbs infrared); oxygen-poor blood is dark red (absorbs red light). The watch calculates saturation based on this ratio.
Scientific Background
Oxygen Transport
Oxygen is the fuel for every cell. Without it, cellular metabolism switches to anaerobic pathways, producing lactic acid and eventually failing (cell death). * Hemoglobin: The "taxi caps" for oxygen molecules. * Saturation: Tells you how full those taxis are. 100% means every binding site is full.
Silent Hypoxia
Conditions like COVID-19 brought attention to "Silent Hypoxia," where patients have dangerously low SpO2 (<80%) but do not feel short of breath. Regular monitoring can catch this decline early.
Clinical Significance
Factors Affecting Readings
Consumer pulse oximeters (including Apple Watch) can be affected by: * Perfusion (Blood Flow): Cold hands or vasoconstriction cause low readings. * Motion: You must be perfectly still. * Skin Pigmentation: High levels of melanin can sometimes reduce signal strength, though algorithms try to correct for this. * Tattoos: Ink can block the light path.
High Altitude
At higher elevations, the air is thinner (less partial pressure of oxygen). * Sea Level: SpO2 98-100% * Denver (5,280 ft): SpO2 ~95-98% * Aspen (8,000 ft): SpO2 ~91-94%
This drop is physiological and expected. It triggers the body to produce more red blood cells (acclimatization).
Recommendations
Getting a Good Measure
- Placement: Watch should be snug but not tight.
- Position: Rest your arm on a table with the watch face up.
- Stillness: Do not move for the full 15-second countdown.
When to Seek Help
If you repeatedly get readings < 90% while at rest and at sea level, especially if accompanied by shortness of breath, confusion, or chest pain, consult a healthcare provider immediately.
References
- Luks AM, Swenson ER. (2011) Pulse Oximetry at High Altitude. High Altitude Medicine & Biology.
- Jubran A. (2015) Pulse oximetry. Critical Care.
- Apple Inc. (2020) Blood Oxygen App Feature Paper.