# SpO2 "lag"



## zzyzx (Oct 19, 2012)

Can anyone provide an explanation for why the reading on a pulse ox may be a minute or too behind the true value? In other words, the pulse ox shows 90% but the patient may already be profoundly hypoxic?


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## SicilianMedic07 (Oct 19, 2012)

Cold fingers, nail polish, CO poisoning?  Probe not working correctly?


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## Achilles (Oct 19, 2012)

http://www.nda.ox.ac.uk/wfsa/html/u11/u1104_02.htm


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## NYMedic828 (Oct 19, 2012)

The device does not monitor every single passing red blood cell independently. It measures them as sample groups passing through the probe and then reports back its findings on how saturated the sample it measures was. So instead of giving you exact beat for beat measurements on the blood passing the probe at that exact moment it is giving you an averaged measurement that it acquired over a few seconds based on whatever programmed algorithm your device follows. The pleth wave on the other hand should theoretically match up with the radial pulse of the measured limb as that is a visual of what the probe is seeing rather than the number it is giving to you.


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## Brandon O (Oct 19, 2012)

NYMedic points out one important factor -- signal averaging. If you DID see an instantaneous change in SpO2, it would be a good indication that it was artifact; the number calculated is averaged from readings over several seconds. So that's one factor.

Another big one is simply physiology. You mention the sat may read normal while someone is "hypoxic," but I think that's not actually what you mean. What you mean is, there's been a change in the patient's pulmonary/respiratory status, and you want to know about it, but the sat doesn't reveal it yet.

Suppose I stop breathing, for instance. My sat may not drop for a few minutes. Is that a sensor error? No, it's accurately depicting my arterial hemoglobin saturation. But my sat simply hasn't dropped yet due to my circulating (mainly venous) and pulmonary oxygen reserves. No doubt it will eventually, and hopefully by that point you've noticed that I've been apneic. But strictly speaking, that reserve is a good thing. You just have to remember that oxygen saturation is an end-point, and your "early warnings" should never be the end-points, but rather the compensatory mechanisms (respiratory rate and volume, etc.) that maintain them.

There may be a couple other factors, such as hardware and software delay, but in most cases I believe they should be negligible. The above two are the main factors to time delay in an otherwise accurate sat.


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## VFlutter (Oct 19, 2012)

Brandon Oto said:


> Suppose I stop breathing, for instance. My sat may not drop for a few minutes. Is that a sensor error? No, it's accurately depicting my arterial hemoglobin saturation. But my sat simply hasn't dropped yet due to my circulating (mainly venous) and pulmonary oxygen reserves. No doubt it will eventually, and hopefully by that point you've noticed that I've been apneic. But strictly speaking, that reserve is a good thing. You just have to remember that oxygen saturation is an end-point, and your "early warnings" should never be the end-points, but rather the compensatory mechanisms (respiratory rate and volume, etc.) that maintain them.



A good example of this is watching OR intubations on relatively healthy patients. They might be apnic for a decent amount of time before there is any change in Spo2 and by that time they are tubed. When I was shadowing a CRNA he really took his time and went slow to explain everything and even then there was never a dramatic desaturation.


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## Brandon O (Oct 19, 2012)

ChaseZ33 said:


> A good example of this is watching OR intubations on relatively healthy patients. They might be apnic for a decent amount of time before there is any change in Spo2 and by that time they are tubed. When I was shadowing a CRNA he really took his time and went slow to explain everything and even then there was never a dramatic desaturation.



Although for what it's worth, I think you'd notice a sudden acceleration in the desaturation rate if you let them drop below 90%-ish. That's about when you crest the first "Wile E. Coyote" shoulder of the oxyhemoglobin dissociation curve, and each small drop in PaO2 will yield a greater drop in SpO2.

I suppose letting people desat just to observe this would be a bit inappropriate though...


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## VFlutter (Oct 19, 2012)

Brandon Oto said:


> Although for what it's worth, I think you'd notice a sudden acceleration in the desaturation rate if you let them drop below 90%-ish. That's about when you crest the first "Wile E. Coyote" shoulder of the oxyhemoglobin dissociation curve, and each small drop in PaO2 will yield a greater drop in SpO2.
> 
> I suppose letting people desat just to observe this would be a bit inappropriate though...



Agreed, desaturation is exponential not linear and does increase once below 90. 

Another annoying group of people to watch on monitor are patients with sleep apnea. My Spo2 alarms are constantly going off. 

We occasionally have terminal patients who die while on monitor and you are able to observe the whole process. The spo2 gradually then abruptly drop as they pass. It may not be the best data since many have multiple system dysfunction or severe cardiopulmonary problems. We had a patient terminally weened off CPAP a few weeks ago. It was interesting to watch from the monitor side of things. 


I have also caught a few patients going into PEA because of the Spo2 pleth


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## MSDeltaFlt (Oct 20, 2012)

zzyzx said:


> Can anyone provide an explanation for why the reading on a pulse ox may be a minute or too behind the true value? In other words, the pulse ox shows 90% but the patient may already be profoundly hypoxic?



Simple. Interstitial difusion.  The light shines not just through the artery of the finger, but also through tissue/bone/nerves/etc.  Depending on the pt, the probe, software, clinical condition it might take a bit for the values to match up


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## jwk (Oct 20, 2012)

NYMedic828 said:


> The device does not monitor every single passing red blood cell independently. It measures them as sample groups passing through the probe and then reports back its findings on how saturated the sample it measures was. So instead of giving you exact beat for beat measurements on the blood passing the probe at that exact moment it is giving you an averaged measurement that it acquired over a few seconds based on whatever programmed algorithm your device follows. The pleth wave on the other hand should theoretically match up with the radial pulse of the measured limb as that is a visual of what the probe is seeing rather than the number it is giving to you.



Great answer.  Depending on the model/manufacturer, all SaO2 monitors generally take 6-12 seconds to start getting a reading.  Some may even allow the user to configure the averaging time, the "rolling window" the machine is using to compute that average.  If you're using a monitor that shows the pleth waveform, make sure you're getting a good waveform before you believe whatever number might show up initially.  A lot of my PACU nurses want to write down the first number they see, even if there is no baseline waveform on the monitor.


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## sir.shocksalot (Oct 20, 2012)

Brandon Oto said:


> Another big one is simply physiology. You mention the sat may read normal while someone is "hypoxic," but I think that's not actually what you mean. What you mean is, there's been a change in the patient's pulmonary/respiratory status, and you want to know about it, but the sat doesn't reveal it yet.
> 
> Suppose I stop breathing, for instance. My sat may not drop for a few minutes. Is that a sensor error? No, it's accurately depicting my arterial hemoglobin saturation. But my sat simply hasn't dropped yet due to my circulating (mainly venous) and pulmonary oxygen reserves. No doubt it will eventually, and hopefully by that point you've noticed that I've been apneic.


Great explanation. This stuff is why preoxygenation is so important before intubation. If your total lung capacity including your residual volume is filled with oxygen you can remain apneic for a longer time period while still having an adequate ratio of oxygen to facilitate alveolar oxygen diffusion.

I'll try not to reiterate what everyone else has said. Keep in mind that the monitor/pulse ox is a computer and the sensor is just providing data, it takes time to turn that data into something the computer can understand as an oxygen saturation and then display it to you. Also a computer doesn't know if it's getting artifact or a genuine reading, it gets bits of data and interprets it the best it can (sometimes why the monitor starts telling you your patient is in V-Fib when really it's just a lot of artifact). I usually recommend watching the pleth to evaluate if you are getting artifact or a good reading.


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## BLS Systems Limited (Oct 22, 2012)

Another factor could be related to the hemoglobin level.  When Hb is low, the number could be all over the map.  Your question is actually dealt with by the manufacturers when they create the algorithm that goes into their devices.  A lot of that stuff is explained when they do the sales material and presentations-things we tune out while we are looking at the display, pretty lights and battery life.  Different devices can give you different readings depending on the patient and environmental conditions, based on the above responses.


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## Brandon O (Oct 22, 2012)

BLS Systems Limited said:


> Another factor could be related to the hemoglobin level.  When Hb is low, the number could be all over the map.  Your question is actually dealt with by the manufacturers when they create the algorithm that goes into their devices.  A lot of that stuff is explained when they do the sales material and presentations-things we tune out while we are looking at the display, pretty lights and battery life.  Different devices can give you different readings depending on the patient and environmental conditions, based on the above responses.



Are you saying that low hemoglobin count can lead to a poor signal? That makes some sense to me (it seems like it would narrow the transmittance differential between arterial and venous blood), but I haven't seen any research that supports it. Surely a low hematocrit, if anything, would have a larger effect? How different really is the "color" of a Hb-rich vs Hb-poor red cell?


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## BLS Systems Limited (Oct 22, 2012)

Brandon Oto said:


> Are you saying that low hemoglobin count can lead to a poor signal? That makes some sense to me (it seems like it would narrow the transmittance differential between arterial and venous blood), but I haven't seen any research that supports it. Surely a low hematocrit, if anything, would have a larger effect? How different really is the "color" of a Hb-rich vs Hb-poor red cell?



I am basing this on personal experience on several ICU patients (it was a few years ago so the manufacturer may have improved the sensor and software).  On several occasions I experienced a vacillating waveform that increased and decreased with remarkable consistency (very high highs and very low lows with each cycle lasting a minute or two).  The only common denominator that jumped out was low Hb counts (significantly lower).  There may have been other factors, but the Hb stood out as the patients were down a few quarts.


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## Melclin (Oct 22, 2012)

Brandon Oto said:


> Are you saying that low hemoglobin count can lead to a poor signal? That makes some sense to me (it seems like it would narrow the transmittance differential between arterial and venous blood), but I haven't seen any research that supports it. Surely a low hematocrit, if anything, would have a larger effect? How different really is the "color" of a Hb-rich vs Hb-poor red cell?




A few papers (all rubbish) from the late 80's and early 90's suggest that pulse oximetry "may" be inaccurate with Hct <10 in very low saturations. Other literature disputes this. That number is quoted in several books as well but all based as far as I know on the same "evidence". If it even exists at all, it seems a small error at pretty extreme levels of hypoxia and anaemia. I'd argue its pretty irrelevant. You know they're all kinds of sick either way. 

http://www.ncbi.nlm.nih.gov/pubmed/8010546
http://www.ncbi.nlm.nih.gov/pubmed/2352007
http://www.ncbi.nlm.nih.gov/pubmed/2064035

I've never seen newer papers on the topic.


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## Brandon O (Oct 22, 2012)

Interesting, thanks. And of course we should remember that oxygen delivery to tissues is really a factor of both hemoglobin saturation and hemoglobin quantity, so if the latter is inadequate, hypoxia will develop whether or not the lonely remaining hemoglobin are well-saturated. So accurate or inaccurate, we'd need labs to really assess this, which will probably mean both a blood gas and a CBC, thus the pulse ox will be moot.

Or for prehospital purposes without labs, as you say -- they're jes plain sick either way.


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## VFlutter (Oct 22, 2012)

*Rant*

On a side note.... If your patient has Reynaud's syndrome please do not put the Spo2 sensor on their finger. And if you do, please do not call an RRT because it is reading 70% even though your patient is in no distress.

Also, if the spo2 says the HR is 160 you should probably palpate the pulse yourself before calling an RRT. 


Nurses


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## Melclin (Oct 22, 2012)

Brandon Oto said:


> And of course we should remember that oxygen delivery to tissues is really a factor of both hemoglobin saturation and hemoglobin quantity, so if the latter is inadequate, hypoxia will develop whether or not the lonely remaining hemoglobin are well-saturated. So accurate or inaccurate, we'd need labs to really assess this, which will probably mean both a blood gas and a CBC, thus the pulse ox will be moot.
> 
> Or for prehospital purposes without labs, as you say -- they're jes plain sick either way.



Well yes, and cardiac output...in the presence of a circulatory system under enough pressure to maintain adequate perfusion pressures.  

Pulse ox never promised to be the lone ranger, then answer to the question of oxygen delivery, but it clearly has its uses. Its not completely useless just because you dont have a gas and a Hb/Hct.


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## abckidsmom (Oct 22, 2012)

Melclin said:


> Well yes, and cardiac output...in the presence of a circulatory system under enough pressure to maintain adequate perfusion pressures.
> 
> Pulse ox never promised to be the lone ranger, then answer to the question of oxygen delivery, but it clearly has its uses. Its not completely useless just because you dont have a gas and a Hb/Hct.



Cause really, a gray, sweaty guy with a pressure of 70 and sats of 100 is not looking any better because of his excellent saturation.


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## Melclin (Oct 22, 2012)

abckidsmom said:


> Cause really, a gray, sweaty guy with a pressure of 70 and sats of 100 is not looking any better because of his excellent saturation.




*Peers through tired eyes*...I can tell if you're agreeing with me, disagreeing or saying something different altogether :wacko:


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## abckidsmom (Oct 22, 2012)

Melclin said:


> *Peers through tired eyes*...I can tell if you're agreeing with me, disagreeing or saying something different altogether :wacko:



Sometimes I really don't make any sense.  I agree with you.  I think.


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## Brandon O (Oct 22, 2012)

Melclin said:


> Pulse ox never promised to be the lone ranger, then answer to the question of oxygen delivery, but it clearly has its uses. Its not completely useless just because you dont have a gas and a Hb/Hct.



No, course not. Just trying to point out that regardless of questions of accuracy, these variables are going to interact anyway...


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## Melclin (Oct 23, 2012)

Man I must have been in some kind of snippy mood last night. I even got involved in an argument in the comments section of a news web site. How embarassmentingly. 

Drinks all round.

My shout.


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## Brandon O (Oct 23, 2012)

Melclin said:


> Man I must have been in some kind of snippy mood last night. I even got involved in an argument in the comments section of a news web site. How embarassmentingly.
> 
> Drinks all round.
> 
> My shout.



Your punishment will be drinking Foster's.


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## Melclin (Oct 23, 2012)

Brandon Oto said:


> Your punishment will be drinking Foster's.



Jeeze man. Its not like I murdered your mum. Harsh. 

You know I've never actually seen fosters on tap anywhere but in countries that aren't Australia. Not a single soul here drinks it on account of it being, you know, horrible.


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## hogwiley (Oct 23, 2012)

The thing that confuses me when I hear about this lag time is that when I get a reading on someone thats a little low, sometimes If I have them cough and deep breath a bit, or take some drags on an incentive spirometer, Ill usually see their sats increase fairly quickly. I dont really see a lag time. Likewise if I put them on o2 or increase the flow rate.


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## Brandon O (Oct 23, 2012)

hogwiley said:


> The thing that confuses me when I hear about this lag time is that when I get a reading on someone thats a little low, sometimes If I have them cough and deep breath a bit, or take some drags on an incentive spirometer, Ill usually see their sats increase fairly quickly. I dont really see a lag time. Likewise if I put them on o2 or increase the flow rate.



Well, think about the mechanisms we've been discussing. The two main factors in lag are signal averaging and venous/pulmonary oxygen reserves. You can't escape the former, although the exact duration may vary depending on the device, but it's short enough that it won't necessarily be striking. What about the latter? It should make sense that the effect is more profound for creating delay between LOW alveolar oxygen states and a DECREASE in sat. If you suddenly stop breathing or if I decrease your FiO2, you still need to burn through the oxygen in your blood and particularly the oxygen in your lungs before your sat starts to drop. Conversely, if I increase your FiO2 or you increase your minute volume, the effects should be faster; you'll be increasing your blood-borne oxygen content almost immediately. The only delay should be the time for O2 to cross into the circulation in quantity and reach the probe site. In people with poor cardiac output or peripheral perfusion, this can actually be significant, which is why it's a good idea to assume 1-1.5 minutes of lag in all sick people, but in most cases it should be appreciably less.

Make sense? That's why the situations you're describing are on the "increase" side of the fence.


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## hogwiley (Oct 24, 2012)

Brandon Oto said:


> Well, think about the mechanisms we've been discussing. The two main factors in lag are signal averaging and venous/pulmonary oxygen reserves. You can't escape the former, although the exact duration may vary depending on the device, but it's short enough that it won't necessarily be striking. What about the latter? It should make sense that the effect is more profound for creating delay between LOW alveolar oxygen states and a DECREASE in sat. If you suddenly stop breathing or if I decrease your FiO2, you still need to burn through the oxygen in your blood and particularly the oxygen in your lungs before your sat starts to drop. Conversely, if I increase your FiO2 or you increase your minute volume, the effects should be faster; you'll be increasing your blood-borne oxygen content almost immediately. The only delay should be the time for O2 to cross into the circulation in quantity and reach the probe site. In people with poor cardiac output or peripheral perfusion, this can actually be significant, which is why it's a good idea to assume 1-1.5 minutes of lag in all sick people, but in most cases it should be appreciably less.
> 
> Make sense? That's why the situations you're describing are on the "increase" side of the fence.



Yeah makes sense. I kind of had a vague notion of what you said and figured because of that its quicker for them to show a sat increase than a decrease on the pulse ox. I just havent had an opportunity to see this in person, since its not like I take away someones o2 or allow their airway to close to see how long it takes for their sat to drop. Thanks for the good explanation though.


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## hogwiley (Oct 24, 2012)

Melclin said:


> Jeeze man. Its not like I murdered your mum. Harsh.
> 
> You know I've never actually seen fosters on tap anywhere but in countries that aren't Australia. Not a single soul here drinks it on account of it being, you know, horrible.



What would happen if an Australian walked into a pub and ordered a Fosters out loud, would the whole place go silent or everyone laugh assuming your joking?


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## Melclin (Oct 24, 2012)

hogwiley said:


> What would happen if an Australian walked into a pub and ordered a Fosters out loud, would the whole place go silent or everyone laugh assuming your joking?



The later I suspect. Quickly followed by the publican asking what part of the UK you were visiting from.


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## BLS Systems Limited (Oct 24, 2012)

hogwiley said:


> Yeah makes sense. I kind of had a vague notion of what you said and figured because of that its quicker for them to show a sat increase than a decrease on the pulse ox. I just havent had an opportunity to see this in person, since its not like I take away someones o2 or allow their airway to close to see how long it takes for their sat to drop. Thanks for the good explanation though.



For a quick example, try it out on yourself.  Put on the oximeter and hold your breath.  See how long it takes for the sat's to drop and compare it to how fast it takes to return (if you're still awake).  While this isn't the exact same as the CO2 will build as well, it does give an appreciation to those experiencing hypoxia.


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## Brandon O (Oct 24, 2012)

BLS Systems Limited said:


> For a quick example, try it out on yourself.  Put on the oximeter and hold your breath.  See how long it takes for the sat's to drop and compare it to how fast it takes to return (if you're still awake).  While this isn't the exact same as the CO2 will build as well, it does give an appreciation to those experiencing hypoxia.



Don't think I've ever seen a healthy person successfully drop their sat by holding their breath. Will to live and all that...


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## BLS Systems Limited (Oct 24, 2012)

Brandon Oto said:


> Don't think I've ever seen a healthy person successfully drop their sat by holding their breath. Will to live and all that...



I got mine down to 78%.  The secret is to hyperventilate to blow off as much CO2 as possible (drive to breathe and all that).  Don't bear down as the vasovagal stimulation could cause a drop in BP.   As you approach the cusp of the desaturation curve, the SpO2 will start to drop faster and faster.

Fun stuff when its slow on a night shift...


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## Brandon O (Oct 24, 2012)

BLS Systems Limited said:


> I got mine down to 78%.  The secret is to hyperventilate to blow off as much CO2 as possible (drive to breathe and all that).  Don't bear down as the vasovagal stimulation could cause a drop in BP.   As you approach the cusp of the desaturation curve, the SpO2 will start to drop faster and faster.
> 
> Fun stuff when its slow on a night shift...



:lol: Clearly we must all try this.


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## abckidsmom (Oct 24, 2012)

Brandon Oto said:


> :lol: Clearly we must all try this.



Where's the no legal advice warning?  

Kids, don't try this at home and if you do don't say you heard it here. But if you happen to have done it, make sure and post pics of your skin and the monitor so we will believe you.


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## Anjel (Oct 24, 2012)

abckidsmom said:


> Where's the no legal advice warning?
> 
> Kids, don't try this at home and if you do don't say you heard it here. But if you happen to have done it, make sure and post pics of your skin and the monitor so we will believe you.



No legal advice allowed lol


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