Apple Watch's Blood Oxygen app measures the oxygen level of your blood directly using an optical heart sensor that shines red and green LEDs and infrared light onto your wrist. Then, photodiodes—small devices that turn light into an electrical current—measure the amount of light reflected back to the sensor to determine your blood oxygen level. This takes place in the course of just 15 seconds, with the majority of people will have a blood oxygen level of 95 to 100 percent. Researchers believe the metric to be a huge indicator of your overall health—but Dr. Ronaghi says that it's particularly meaningful for your respiratory well-being.
"[Blood oxygen] is a great feature because we normally have our patients with lung disease buy a pulse oximeter where they have to put it on their finger and generally check their oxygen levels throughout the day," says Dr. Ronaghi. "Now, the fact that this could be combined into a watch setting is great—especially for a lot of our patients who have lung disease can just monitor their oxygen levels throughout the day." The Apple Watch keeps a log of how a user's blood oxygen levels ebb and flow, information that their doctors can use to observe what factors (like exercise, sleep, and nutrition) affect their levels throughout the day. And this isn't just useful for people with pre-existing respiratory conditions; it's great for anyone who's curious about their body.
Dr. Ronaghi says that the utility of the new Apple Watch feature doesn't stop there. It's a metric that means even more when you look at it alongside your heart rate. "Taken in context together, [blood oxygen and heart rate] allow us to know how our patients are doing. The general correlation there is that, once your oxygen levels drop below a certain level—typically 90 percent—the body's heart rate typically starts to increase in order to be able to meet that demand of the body requiring more oxygen," says Dr. Ronaghi. In other words, doctors and technology are learning to work together to put patients' needs first.
Smartwatches don't replace medical professionals, but they can help to keep a conversation going with your medical provider. "Until we can see truly how accurate these metrics are, if you do see one reading that's below that 90 percent, keep track and see if it happens again. Again, these are really based on that infrared and it being able to contact the skin. If you have one reading that's abnormal, I would say look at it again and make sure that it was an accurate reading," says Dr. Ronaghi. "However, if this is a consistent thing, or you're noticing that pattern happening with certain activities, or certain times of the day, or throughout the day, or every few days, then the next and most important thing is to make an appointment and see your provider as soon as possible."
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