Why hasn't someone modified a Pulse-ox meter

[Windwalker]

Why hasn't someone modified a pulse-ox meter to affix to a heads up to notify you when your O2 Saturation drops? Wouldn't that be an easy "Gauge" that could give you real time feedback that your not getting enough Oxygen? Also, Unlike the O2 Sensors- They don't go bad since they read with infrared. There are even pulse-ox sensors that are patches you can place on any thin skin and can be submersed...

Thanks..

 

[medic001918]

Pulse oximeter's tend to be about five minutes behind a person's (patient's) true saturation. They're not that great of an indicator of perfusion and are effected by a number of factors such as carbon monoxide and temperature.

I use one all the time at work because I'm required to, but I don't put much faith in it's accuracy.

All of that being said, I don't know enough about rebreather diving to know what numbers matter to you and how important their accuracy is.

Shane

 

[Windwalker]

Very interesting, Don't they measure the real time oxygen saturation? I know there are several units like the "SPO Pulseox Check Mate Oximeter (Heart Rate & Oxygen Saturation)" Which is designed to give real time analysis for people like pilots and what not. 5 minute lag could mean that they are dead before they will notice their saturation is too low.

I know the Medical ones average out several readings over x amount of time, I used to work in a ER, but the Triage nurses had units that could get an accurate reading in under 30 seconds (and that was over 15 years ago)

I can see how what is currently being breathed would lag behind what the body saturation actually is, the duration of the lag would greatly depend on where the measurement is being taken wouldn't it? Saturation from the forehead would be much more accurate then the finger tips, and be less prone to cold due to the increased circulation in the cranium..

Co2 being a problem, isn't that directly related to the ability to off-gass the Co2, and doesn't your body have a detection mechanism for high Co2 levels but not low O2 levels? In other words. Are most problems with Rebreathers under or over-exposure to O2 versus over exposure to Co2?

 

[medic001918]

It's listed as an accurate reading...and it is accurate for the given minute. But once you start having problems, it takes about five minutes for the physiology to catch up and the changes to reflect.

Example: A patients starts having a respiratory distress. The distress will often occur before the change in the pulse oximeter since the cells being measures haven't desaturated yet and had difficulty in resaturating. That process takes a few minutes before the machine will start to read low.

Hope that makes some sense.

Shane

 

[Windwalker]

Yes, Excellent sense, but in terms of rebreathing, wouldn't the change in your body O2 level be commiserate with the intake of O2? In other words, Changes won't happen suddenly but could be easier tracked as the body metabolizes oxygen. So in addition of being able to watch your O2 Sensor, you can also track your bodies metabolism of Oxygen?

For your Respiratory arrest example, a diver is going to know if he has an cataclysmic air supply problem.. I am thinking about the sneaky ones that claim most rebreather divers.

It would in effect give a computer another datapoint if it was integrated. If it sees the O2 saturation start dropping it could throw an indicator on the HUD. It could be the diver is passed out and is unable to add O2, Where having an actual datapoint of where the body is could allow more automation to keep the exposure to oxygen at a safe level or allow better management of set points by the eCCR.

Also, I know Uwatec has started using a Heart Monitor to track Microbubble creation and tissue loading. Having the heart rate on pulse-ox would also allow similar tracking for a rebreather computers, would it not?

 

[medic001918]

I'm not familiar enough with rebreathers to say for sure if they would benefit in that application or not. I'll have to let someone else step in and answer that one.

Shane

 

[scubanimal]

Windwalker,
First, good on you to be thinking of such an idea. Please do not take what I'm about to say as a distractor for continuing to think of ideas for RB diving.

Ok, let's do an example of a RB dive when things go bad. Diver is cruising along with a setpoint (SP) of 1.2 and everything is fine, and at a steady state. For the purpose of this example let's stay at a constant depth. At this point the oximeter is reporting everything is ok. Now something goes wrong and O2 is not being feed into the loop (several things could cause this). With each breath the Loop O2 content is going to be reduced, but the O2 the diver is consuming will remain the same as long as sufficient supply exists (loop content). So with time the loop partial pressure of O2 (PPO2) will dimish down to .21 and the body will not know anything is wrong. At .21 PPO2 is where trouble begins. With each breath ~.04 (in PPO2) will be reduced. At roughly .1 PPO2 most likely the diver is unconscious and beyond self-help. So that leaves about 3 breathes from the time everything is ok, to too-late. The body of course lags a bit behind, but not enough to provide a significant amount of time to self-help.

The numbers above are generalized as each diver is unique in both consumption, and at which point they would become unconscious. Plus there are other factors, depth, change in depth, temperature that will affect this to a degree. But in general this should outline the problem and thus the requirement that low O2 be discovered quickly and resolved quickly.

But continue to think of how we might solve this issue. High CO2 and low O2 appear to be the biggest causes leading to RB deaths.

 

[vtach]

Windwalker, I think you are on the right track for ideas but like Medic alluded to, there are many factors which contribute to the inability to rely on something like pulse oximetry for a consistent reliable measure of physiologic pO2/fO2. Shunting for instance, most pulse oximetry probes are placed peripherally (finger, earlobe, etc..) if you are diving in a cold environment and your core temp begins to drop slightly, your body will shunt more blood to your core and less peripherally. That in itself is a good reason to consider pulse oximetry inaccurate.

I think the more appropriate device would be End tidal or "pre" tidal capnography (exhaled or inspired gas CO2 monitoring). I wouldn't be surprised if future CCR's include this form of technology.

Just my .02$