Pulse oximiter? (Page 2)

Correct. Mostly useless here to, although if it captures a number during CPR it's nice to say "hey you're doing good CPR" to the person who's doing compressions. Doesn't change treatment though...

Here's the thing. I use pulse oximeters every single day. I have been using them for 20 years. Some are intergrated into ICU monitors, some are handheld, some pocket sized. They all have one common flaw. They are notoriously quick to provide inaccurate numbers. They'll ALWAYS be falsely low, never falsely high.

When I was operating as a military special operations medic, I never once found myself wishing I had one in the field. The benefit of a medical device is that it provides objective data. In the situations you described, objective data can be useful. But it has to be accurate and precise objective data. Even the very best pulse oximeters are persnickity, even under ideal conditions indoors in an ICU. So in the field, if you want objective data to help you make decisions while mountaineering (been there too, both in 1000's of hours of military training and operations and recreationally), or diving, it's important that the data be accurate. It won't be with a pulse oximiter. It won't be precise either, even further compounding the problem if you are using it for serial readings.



Edited by Bill 2009-09-05 2:52 PM

That data looks like it comes from arterial blood gas samples. I'd submit that if you drew and ABG and had a pulse oximter in the field the numbers would not always correlate.

Edited by Bill 2009-09-05 2:48 PM

Why's that? Part of the reason pulse ox is so handy is because it fairly predictably correlates with PaO2, which is the number you really care about anyway.

I'll also add that, having experienced an altitude chamber during my aerospace medicine course in med school, my personal pulse ox dropped to 58% when exposed to the equivalent of 30,000 feet--I needed help putting my oxygen mask back on, and the last thing I remembered was not being able to write my name.

Edited by DrPete 2009-09-05 3:39 PM

It correlates when it works.

I guess my point is the machines themselves aren't terrific. If it reads 100%, I buy that number. When it reads in the 80% range in the absence of a waveform (the field models I've used don't have one) it can't be trusted...

You're absolutely right. Cold fingers/toes/etc. would definitely give you a poor waveform--I guess I was assuming that you threw out the poor waveform...

This thread was interesting.  I'm not sure what the answer truely is.  Just for fun I went over to my office and hooked a Pulse Ox up to my finger and recorded it while running on the treadmill.  I know my heart rate runs about 160 or so while running and 150 or so while cycling.  What I didn't realize is that my pulse ox dropped a few points while running.  I looked up some information on Exercise Induce Hypoxemia.  It sounds from the article that it's fairly common amongst endurance athletes.   I wasn't sure what to make of it.  My neighbor is a cardiologist and he sees a slight drop in saturation with patients during exercise.  The impression I have is that it's probably more complicated physiologically.  How it applies in a training program sounds like it will require further study.

This of course was not a controlled study.

Here's a few of the videos if you're interested.

This was done at 3700 feet, room air, and 75 degrees.

I only take Lisinopril 10mg and am otherwise healthy.

I had ridden my bike fairly rigorously that morning, but considered myself reasonably re hydrated.


Pulse ox while running

I recently had pulmonary emboli.  One of the tests they do to see how you are faring is an "oxy walk."  Walk for 6 minutes hooked up to a pulse oximeter.  THe day I was diagnosed I stayed at 97-98%, which is why they missed it, usually PEs result in a drop.  A week later I was re-admitted to the hospital, and the walk brought me down to 92 or so (may have been 94).  That was enough to cause some concern given the history 

True True and unrelated.  You had a medical condition that the use of pulse oximetry is a useful tool for.

But looking at peripheral hypoxemia whilst exercising tells you nothing about a healthy individual. 

Having a PE by DEFINITION you are not a healthy individual.

Dropping a couple of points isn't hypoxia. You can be in a saturation range from 90-100% without much actual change in your blood oxygenation. There is a steep dropoff only once you hit the high to mid 80's. Look up a "oxygen dissociation curve" for a graphical representation of that.

When you have a PE, you develop something called a V-Q mismatch, or ventilation/perfusion mismatch. What that means is that you can't oxygenate as well because areas of your lung that are ventilated, i.e. getting oxygen, are not getting blood flow so the gas exchange doesn't happen. The reason you didn't drop at rest is because 1. your body wasn't extracting enough oxygen from your blood to make it noticeable, and 2. they only checked an O2 sat--had they looked at your PaO2 (partial pressure of O2 from blood gas analysis) and A-a gradient (basically the difference in O2 between the air you're breathing and your blood) those would've been abnormal.



As you can see in this pic, by the time your SpO2 (% sat) has dropped, your PaO2 has been dropping for a while. That's why O2 sat is so unreliable with small PEs. Of course that launches into the debate about whether those small ones actually need to be treated, etc etc, but that's a different ramble.

Interesting, thanks.   I'd be interested in reading about that debate about treating the small ones (as I am told mine were), but won't hijack this thread  

Most of it is in the critical care arena, where patients have multiple other issues that might make anticoagulation risky. I don't know that there are any large studies out there just yet, but now that we're better at finding PE's it'll take a few years to figure out which patients were the ones who previously had never been diagnosed and would've been fine anyway without treatment.

For a young, healthy triathlete, I would still anticoagulate and go on an aggressive search for an underlying cause unless the reason for the PE was obvious, i.e. recent surgery, a 12-hour plane flight, etc etc.

Aw what the heck, it's a hijack 

I went through the full battery of genetic blood tests with a UCLA hematologist, aPTT ,DRVVT, 20210, beta 2 glycoproteins., Factor V, Factor VIII and more... the works.  All came up negative or insignificant.  I am no doc but probably spent enough time resarching this while home sick (and at work).  I am pretty sure I know where mine came from.  I am also appalled at the level of misdiagnosis in young, healthy people.  I have read some awful horror stories of docs sending people home with anitbiotics only to have them pass on....  You say trips over 12 hours.  Thats one of the problems, I think (with all due respect to docs).  My reading of significant thrombosis research tells me that any trip of over 4 hours - plane, car, bus, etc. presents a risk factor which might be exacerbated by dehydration, the birth control pill, etc.  I think docs (again, with all due respect) *expect* to see a PE patient only if they've been immobilized in a cast, on a super long plane trip, etc.  They don't expect to see PEs in someone like me, so they don't even go there. My doc told me as much.  I am on a few blogs re: PE and DVT. While those cases exist, the number of otherwise healthy people with "unexplained" VTE is staggering... 

I really think increased doctor awareness is required.   On the morning I was admitted to the hospital with severe pain on breathing and multiple bilateral PEs I had been seen by my GP AND my pulmonologist, who both dismissed my shortness of breath and pain on breathing as asthma and a pulled muscle.  4 hours later I was in the telemtry unit.  The pulmonologist said "it was in the back of my mind."  Small comfort if I had been one of the 30% mortality rate.

Thank goodness for sites such as http://www.clotcare.com/clotcare/index.aspx
 and http://www.stoptheclot.org/

OK, rant/hijack/vent over

Chris, did I see you in scuba gear equipment pictures before or am I thinking of someone else?  If so, were you diving any time before the PE? 

Also, I agree with you on doctors misdiagnosed patients and only expecting to see the classic presentations that they have been given through textbooks and not at the bedside (eg: 24 hour trip in a cast), but then again doctors are reacting to clinical presentations, symptoms and manifest signs in the acute care setting.  You were diagnosed finally, although a little later than you expected and pissed off by the comment that it was only in your mind.  But do put aside his bedside manners and know that you were finally diagnosed.  When the bill comes in, put it aside until they call and say that you thought it was only in your mind that you saw it on the counter.  I'm assuming they did finally obtain ABGs and saw the VQ mismatch that DrPete was talking about, unless their blood gas machine wasn't properly calibrated (eg: Correct Barrometric pressure for your area) and if you were previously given O2 before the ABG (again a incorrect reading of the VQ), and the blood gas machine didn't actually compute the VQ, which I have seen many updated hospitals have outdated equipment and not calculate this parameter.  

You said your pretty sure you know where yours came from...where, or rather how?

Since we're hijacking.

Chris, you wouldn't believe the number of people that we see that have the exact same symptoms who DON'T have PE. If anything (in my experience) we tend to be overly suspicious of PE and order far TOO MANY CT scans. For every 20 that I order, I might see one PE, and those are in much higher risk patients than yourself. You have the advantage of hindsight right now. Unfortunately we don't have access to that ahead of time. We're always weight risk/benefit of diagnostic procedures based on our clinical impression of things. Sometimes we're wrong, but we have to make our best call.

Part of the reason medicine is part art and part science.

Yup, good points, Derek.

The "one" sticks in my head, i.e. the patient who had no symptoms but a transiently increased heart rate, but I've ordered a LOT of CT scans for PE, either out of an abundance of caution or out of a strong clinical suspicion for a PE and they've come back negative.

It's tricky stuff. Had you gone through all the scans etc. and found nothing, you'd likely have been mad at doctors for the million dollar workup that diagnosed your cold. It's OK, though, we're used to it.

No diving in the last 6 months, so no relationship.

In my opinion, mine came from racing Wildflower long in warm weather, lots of cramping and dehydration, followed by probably 6 beers that night (dehydration), followed by a 5 hour drive home  with only one quick stop.  First symptoms arose 3 weeks later.  I was diagnosed with a D Dimer followed by angio CT.  I actually wasn't pissed that the doc said it was in my mind, they were looking hard for the cause, but I would think after 5 weeks of finding nothing someone might say "hmmmm, I wonder if....?"  Like I said, I was declared 100% healthy with 95% lung function 4 hours prior to being put in the telemetry unit and given dilaudid for the pain.

To the defensive docs (just kidding ) my rant isn't really about my case, since yes, I was diagnosed correctly.  But (as stated in my original post) it's about the 100 or so personal cases I've read on line (in blogs, not anecdotal case reports) where diagnosis took weeks and months, and sometimes not in time to save the person.  One of the common thread titles is "How long did it take them to diagnose you?"  It's more common than you guys probably like to admit.  This is just my opinion (based on a lot of research, I am an attorney and I had a couple weeks off, I made ti my job) but if you are only looking for cases where someone was flying for 12 hours, I think you are doing your patients a disservice.   You both say you are overly cautious.  Maybe that's where you practice.  The stories I read reflect a serious misunderstanding and ignorance of the possibilites of PEs.  I look at it as a risk /reward.  The risk of having PEs is outweighes by the rewards of discovering them on CT.  Bet those people that are positive are releived that they were correctly diagnosed.

As my GP admitted to me, docs, like everyone else, have a set parameter of experiences to draw upon, and they are often limited by them.  A 42 yo triathlete in relatively good health with unexplained SOB, normal pulse ox and HR, PEs just aren't something you think of.  I don't know how far I had gone down the road by the time the ER doc got around to me, but I was admitted in serious condition.  You guys can tell me what that means.

As for what I had to go through, I went through 5 weeks of continuous testing with my GP, a cardiologist (stress test, echo, 4 or 5 ekgs), a pulmonologist (lung function, etc.), prescribed antibiotics, 2 urgent care visits.  I've gotten bills from these docs for looking for something they could have screened for with a simple D Dimer test.  That part does pizz me off.  Let me also say that had the D Dimer come back positive, and the CT negative, I've got no problem with that.

Yes, I was ultimately diagnosed correctly, and I thank the ER doc for being on the ball.  As I said above, not sure how far down "the road" I was at that point, but I felt like I was in pretty bad shape.  However, a D Dimer test isn't all that expensive, is it?  It's certainly not intrusive or dangerous.  That comes back negative, well they can rule out the PE.  Screens positive, a CT is then warranted.  That part seems like a no brainer to me. 

Anyway, this is a thread for another forum.  I hope the doc types can understand where someone on this side of the stethoscope is coming from.  I've dealt with a a lot of docs recently, some very good and actually listen to what we say, some very bad and dismiss us because we don't have the MD.  I am just trying to raise awareness that it isn't only the 65 year old man who had hip surgery that's going to be considered, and that if you guys happen to see someone that doesn't fit the mold, you think to yourself "hmmmm, I wonder if..." and order a D Dimer, at least.