I recently bought a smartwatch, I won’t say which model because what I found is now available on several makes of watch. I discovered when playing around with the Apps that there was one which claimed to measure blood oxygen levels. After clicking “go” I received a measurement about 30 seconds later, which was probably inaccurate, I usually have a saturation of about 98% at sea-level, and it only read 92%.
Which made me wonder what is the purpose of this?
It also made me realize that I have been doing neonatology a long time; I was around when pulse oximeters were invented and I published one of the first studies to evaluate their use in the NICU (Barrington KJ, et al. Evaluation of pulse oximetry as a continuous monitoring technique in the neonatal intensive care unit. Crit Care Med. 1988;16(11):1147-53). I even studied using them in rabbits! (Barrington KJ, et al. Pulse oximetry during hemorrhagic hypotension and cardiopulmonary resuscitation. J Crit Care. 1986;1:241-6). I wondered if they would be useful during low perfusion states, and I thought that a pulse oximeter would be great during cardiac massage, as you would be able to tell if you were achieving pulsation at the site it was placed, and also what the saturation of the blood being delivered would be.
The low perfusion part of the rabbit study was interesting, as the oximeter remained accurate until there was very little perfusion, then it just stopped, which was an improvement in the previous technology of Transcutaneous PO2 monitoring, which become progressively inaccurate as perfusion falls.
But when performing CPR on the rabbits after inducing cardiac arrest I was initially very excited when it seemed to work well, and routinely gave a saturation of about 85% with the same frequency as the cardiac massage! Wow, publication in Nature on the way, I thought. Then I realized that when you do cardiac massage on an adult rabbit, the front legs, where I had placed the probe, move, a lot. You’ll probably all have to take my word for that unless you happen to have tried to resuscitate a rabbit. So I then stopped doing the massage and just rhythmically shook the rabbit’s paw; it continued to get a nice beeping sound and a saturation around 85%. I then put the probe on a piece of red rubber tubing that was lying around, and shook that rhythmically, and found the same thing.
That was my introduction to movement artefact in pulse oximetry. It also got me thinking about how oximeters function and details of their design (I’ll get back to the watch soon).
Pulse oximeters work by shining light of different wavelengths onto a tissue and measuring the relative absorption of the light at those 2 wavelengths. Clinical pulse oximeters do this with transmitted light, whereas my watch is obviously doing this with reflected light. Most clinical pulse oximeters use 2 wavelengths of near infra-red light which are on either side of an isobestic point. That is a point at which the absorption spectra of haemoglobin and oxyhaemoglobin cross. As long as you have 2 wavelengths with different relative absorptions by oxygenated and de-oxygenated blood it will work, but by using 2 wavelengths that have inverted relative absorptions you can make the calculations more accurate.
It was a Japanese engineer Takuo Aoyagi (who died earlier this year aged 84) who realized in the 1970s that the pulsations he was seeing in his signals were entirely from arterial blood, and so if he screened out the constant part of the signal, and only analyzed the pulsatile part of the signal, he could calculate the proportion of pulsatile haemoglobin that was oxygenated or de-oxygenated.
That is why movements will give you an apparent signal, because there are fluctuations in the light absorption, it also explains why the specific pulse oximeter I was using read 85%, because at the 2 wavelengths that the company used (which are all slightly different because of patent issues) when the absorption of light at the 2 wavelengths was 1:1 that corresponded to 85% of the haemoglobin being oxygenated and 15% being de-oxygenated.
When pulse oximeters were used under anaesthesia, movement artefact was not a big deal, but for continuous monitoring on moving patients, it required progressive improvements in technology to reduce the artefacts, which are still a problem for very active patients.
Also, it is worth remembering that it is the pulsatile part of the signal which is being analyzed, so if you have venous pulsation that will interfere with the result. I had a patient recently with severe pulmonary hypertension and the pre-ductal saturation was often 5-8% lower than the postductal, the patient had a closed ductus. On the echocardiogram, there was tricuspid regurgitation, which I think was causing venous pulsation and erroneously low oximeter readings in the upper limb, but wasn’t severe enough to be transmitted to the foot. In the past, we sometimes had an oximeter integrated into the monitor in one place, and a stand-alone monitor for the second site. Because the technologies differ between machines, sometimes you could change the gradient just by switching the probes around!
To get back to the watch, I am not really sure that this is a good idea; I also don’t know if it is accurate. Using reflected rather than transmitted light, and not having wavelengths that are chosen specifically for their use in oximetry, not having any idea if they can account for methaemoglobin or carboxy- or fetal haemoglobin etc etc.
I can imagine many people finding a saturation a bit low, like mine, will freak out and phone their doctor or go to the Emergency Room, and not just shrug it off like me as being probably inaccurate. We don’t need extra pressure on medical services right now! I read something about it perhaps being useful to detect sleep apnea, but for that, you would have to have it in continuous mode (if that exists) and wear it while you are asleep, which my watch battery would have a problem with, it would be very low the next morning.
The manufacturers, of course, come up with some weasel words about ‘not being intended for medical use, including self-diagnosis or consultation with a doctor, and are only designed for general fitness and wellness purposes’ but that just sounds like the usual get-out-of-jail free statements that health supplements use.
Now, how can I get my saturation higher? Maybe if I take high dose vitamin D, or find somewhere to insert a jade egg… hmmm.