Methemoglobinemia and Pulse Oximeters

Topic created by The Professor
on Thu, 26 May 2022 at 17:11

The Professor said on Thu, 26 May 2022 at 17:11...

There have been quite a few topic lately with members reporting their experiences with heart rate, oxygen saturation and poppers.

It's a bit evident that some people are relying on their PulseOximeters to be accurate, some know it is inaccurate but hope that it reads low (it doesn't) and some are outright lying about their stats (reporting mild heart rates with accompanying low O2 SAT).

I've tried to explain the importance of the situation, but have evidently failed, so i'll try again.

The type of Oximeter a consumer buys at Walgreens is a two band device. It works by shining two beams of light into the finger, where it reflects back into the device. When the light beams bounce off hemoglobin in the blood, if that hemoglbin is carrying oxygen, it will be more reflective, if not carrying oxygen, the hemoglobin is darker, and less reflective.

The rest is just math, EXCEPT in the case of methemoglobin (a damaged red blood cell that can no longer carry oxygen). Alkyl nitrites turn normal hemoglobin into damaged methemoglobin.

Methemoglobin appears darker than oxygenated cells, and brighter than un-oxygenated cells. The light sensor, therefore, is now getting erroneous data from the beams, and in practical use, tends to give a higher saturation level than a true reading.

MUCH higher. In fact, the confusion between 'is it oxygenated or not' tends to leave these devices reading 85% when TRUE sats are from 10 to 40%.

For this reason, general guidance is to NOT trust PulseOX readings below 90%.

We've discussed cardio pulmonary response earlier TL;DR a 25 yr old can keep sats above 90, bit with a BPM that's near their aerobic target (about 155 ), while a 55 yr old may drop to low 90s or below once his BPM gets to 130 or so (a nominal human body loses a heart-beat every year, aerobic target is 80% of max heartrate (220-your age).

People reporting low 90 O2 sat with low heart rate are either:

1. inhaling Amyl and in their twenties (which doesn't fit because Amyl wouldn't drop a twenty year old down to the low 90s).

2. have a major heart problem, pacemaker level stuff (a human heart would be beating like hell to keep sats above 90% if it could)

3. Lying outright

A human with sats below 90% is in crisis, and will pump its heart as fast as it can in an attempt to increase those sats to a healthy range.

Back when we were all immortal, this type of challenge wouldn't kill us, but into advanced age one needs to be careful.

from MedScape:

"
Pulse oximetry

Pulse oximetry is used extensively in the evaluation of patients with cyanosis and respiratory distress. However, findings of bedside pulse oximetry in the presence of methemoglobinemia may be misleading. Pulse oximetry measurements with low-levels of methemoglobinemia often result in falsely low values for oxygen saturation and are often falsely high in those with high-level methemoglobinemia. The reason for these inaccuracies is as follows.

The pulse oximeter only measures the relative absorbance of 2 wavelengths of light (660 nm and 940 nm) to differentiate oxyhemoglobin from deoxyhemoglobin. The ratio of absorption of light at each of these wavelengths is converted into oxygen saturation by using calibration curves. Methemoglobin increases absorption of light at both wavelengths (more at 940 nm) and therefore offers optical interference to pulse oximetry by falsely absorbing light.

As a result, oxygen saturations by pulse oximetry in methemoglobinemia plateau at about 85%; therefore, a patient with a methemoglobin level of 5% and a patient with a level of 40% have approximately the same saturation values on pulse oximetry (~85%). The severity of the cyanosis does not correspond to the pulse oximetry reading: a patient may appear extremely cyanotic but still have a pulse oximetry reading in the high 80s.
"

Billy said on Fri, 27 May 2022 at 18:30...

Can't find the other picture right now but have a photo where I was much lower in the 80s. Pulse is elevated as 55-60 is normal for me. This is on GoldenCock Poppers or ArtisanalCleaners, butyl or n-butyl I think.

https://imgur.com/a/22iAiiI

Billy said on Fri, 27 May 2022 at 18:30...

Taken mid-session just after the Nth hit.

The Professor said on Fri, 27 May 2022 at 18:34...

Yeah, that's dangerous

Billy said on Wed, 22 Jun 2022 at 08:54...

I had tinnitus for a long period afterwards. A sign of my body having gone through a period of acute stress.