Thursday, January 14, 2016
January 14, 2016 at 11:03PM
Today I Learned: 1) Let's talk pulse oxymetry! Pulse oxymetry is the measurement of blood oxygen content by light (it's those little finger-clamp things for blood O2 concentration measurement). Today I learned that pulse oxymetry works because oxygenated blood and unoxygenated blood have different absorption spectra. Oxygenated blood absorbs red light more efficiently, and deoxygenated blood absorbs IR better. By measuring the absorption of both and taking the ratio between the two, you can account for and remove most of the noise. ...and oh boy, there are a lot of sources of noise in pulse oxymetry. For starters, there are differences in background absorption from skin and bone between people; differences in finger thickness; differences in light source intensity and sensor sensitivity. All of these should be corrected by the ratio-of-absorptions measure. The bigger problem is that venous blood is often less oxygenated in patients (it's actually pretty close to arterial blood oxygenation in healthy adults, most of the time). When a patient, say, moves their finger, venous blood sloshes around and messes up the reading. There are algorithmic ways to deal with this noise, which I won't go into here -- suffice it to say that finding the arterial blood signal in the wash of different noises of pulse oxymetry is one of the big algorithmic challenges of modern medicine. 2) Some fun numbers to know -- all estimates, taken from a talk, check before using these for anything important. All numbers are annual, for US only: 30 million people hospitalized 3 million hospital-acquired infections 200,000-400,000 preventable deaths in hospitals. These are deaths in people who were successfully recovering from whatever they went to the hospital for, but ended up dying from something else, like misapplication or misdiagnosis of drugs (or hospital-acquired infection). ~800,000 deaths in hospitals 3) Baby eyes are pretty sensitive to oxygen pressure. Pulse oxymeters, see, are often used with babies in ICUs to make sure they're getting enough oxygen. When the sensors detect a drop in oxygen, whoever's attending will usually raise the O2 pressure in the baby's chamber. However, the increased pressure can damage or even disable baby eyes. That's worth the risk if the baby would otherwise, you know, die, but for most of the history of pulse oxymetry, more than 90% of alarms were false alarms, so a lot of babies have been unnecessarily blinded . That's one reason it's really important to keep the false positives low with pulse oxymeters (the biggest reason is so that you don't waste all of the nurses' time with false positives and potentially) 4) Bonus fact: Carbon monoxide (CO) poisoning has chronic effects as well as acute effects. CO is an acute toxin because it binds ridiculously strongly to hemoglobin, displacing oxygen in your blood and suffocating you if you're exposed to too much. Turns out even if you *aren't* exposed to enough to suffocate you, it will stick in your system, making you more vulnerable to future CO poisoning and possibly cause nice little symptoms like increased risk of heart disease, neurological damage, and severe flu-like symptoms. Today's facts brought to you by Masimo Corporation. Not, like, in any sort of corporate sponsorship way. Just that the president told me all these facts. In a talk.
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