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Heart rate and altitude


DaveMC

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I am wondering if anyone can answer this for me.  I am very fit 61y/o pilot.  I don't scare easy and I love flying.  I have noticed over the past few years that when I am above 17k my heart rate increases.   on the ground my resting HR is 55-65.  on a average day at work or flying up  to 12K my HR is rarely about 75 maybe 80.  however when I'm above 15k my HR is in the mid to upper 90's consistently.  This is with o2 by mask or by Nasal cannula.  I have a pulse ox and even if I put the o2 flow to a place that puts me at 95-98% sat my heart rate still is elevated.  I am a physician and I have looked and looked and I cant fine any recourses to explain this physiologically. 

 

I am not in distress or having ANY trouble.  My HR is just elevated.   

 

Thoughts?

 

D

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3 minutes ago, DaveMC said:

I am wondering if anyone can answer this for me.  I am very fit 61y/o pilot.  I don't scare easy and I love flying.  I have noticed over the past few years that when I am above 17k my heart rate increases.   on the ground my resting HR is 55-65.  on a average day at work or flying up  to 12K my HR is rarely about 75 maybe 80.  however when I'm above 15k my HR is in the mid to upper 90's consistently.  This is with o2 by mask or by Nasal cannula.  I have a pulse ox and even if I put the o2 flow to a place that puts me at 95-98% sat my heart rate still is elevated.  I am a physician and I have looked and looked and I cant fine any recourses to explain this physiologically. 

 

I am not in distress or having ANY trouble.  My HR is just elevated.   

 

Thoughts?

 

D

I have noticed the same phenomenon of my HR increasing with altitude despite O2 use to keep O2sat nicely high.  I am also a reasonably fit fellow, 54 and sea level HR of 45-55.

I was guessing this must be well known but you are a physician.  I am just a math guy.

We need a flight surgeon I guess - or does cardiac flight surgeon exist as a specialty?  Maybe an exercise physiologist?

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Just now, ArtVandelay said:

Maybe more oxygen is needed, your heart rate is trying to maintain the normal 98% and is compensating.

at sea level the heart responds to low oxygen saturation initially with an increase in rate,  but sats are fine and by itself should not result in elevated rate.  physiologically speaking normal people are o2 driven on heart rate all other thing being the same.

 

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To, my heart rate does not come down if I increase my o2 from say 93 to 97+,

I wonder how much of my higher rate is attributed to anxiety or something else.

Mine has consistently been higher at high altitudes I have no definitive reason why.

 

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1 hour ago, tgardnerh said:

Do you also get a boosted hr when you're satting 95-98 without O2 at more moderate altitude? (eg, thats about where I am baseline in Salt Lake at 4,500 ft).
Conversely, does your HR come back down if you turn up the O2 until your sat is 99-100%?


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No. My hr only really increases at altitudes above 15k. Ans with plenty of o2on board 

 

99-100% sat Is physiologically no different than 93-95%. IMHO 

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1 hour ago, StevenL757 said:

Great question by the OP.  I’d recommend (or ask nicely) @ilovecornfields stop by to offer an expert opinion.

I’m not a cardiologist (although I know there’s at least one here) or a flight surgeon but I’ve noticed the opposite effect - my HR is fine at altitude but the shoots up when I get the weather at my destination and realize I’m going to have to shoot the approach.

That being said, since you ask this is what my “research” revealed. I’ve been on a high fiber diet lately so it was a short “research session”:

The major CV-related effects associated with elevated altitude are:

Immediate decrease in oxygen delivery to the tissues, which requires a cardiac response.

Increase in pulmonary vascular constriction, which increases pulmonary afterload.

Increase in sympathetic nervous outflow, primarily by an increase in epinephrine but also norepinephrine.

https://www.acc.org/latest-in-cardiology/articles/2012/03/30/15/39/altitude-and-the-heart
 

My guess is that even at normal-ish arterial oxygen saturations you’re still getting the hypoxic pulmonary vasoconstriction, decreased end-organ tissue perfusion and sympathetic outflow because even with supplemental oxygen the partial pressure of oxygen in the lungs is lower than at sea level. Maybe you could try breathing from a nonrebreather mask @ 25 LPM at sea level and again at 5000’ and see if you notice the same effect (with someone else flying, of course). I doubt you would. Maybe there’s also an element of peripheral vasodilation from the sun and heat. Is the effect the same at night?

Please let us know when you get the answer.

 

Edited by ilovecornfields
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1 hour ago, ilovecornfields said:

My guess is that even at normal-ish arterial oxygen saturations you’re still getting the hypoxic pulmonary vasoconstriction, decreased end-organ tissue perfusion and sympathetic outflow because even with supplemental oxygen the partial pressure of oxygen in the lungs is lower than at sea level. Maybe you could try breathing from a nonrebreather mask @ 25 LPM at sea level and again at 5000’ and see if you notice the same effect (with someone else flying, of course). I doubt you would. Maybe there’s also an element of peripheral vasodilation from the sun and heat. Is the effect the same at night?

Ooh, boy, I'm DEFINITELY not a cardiologist, and my physiology memory is pretty ancient at this point, but I think the lungs can see the same partial oxygen pressure with enough supplemental O2.  At 15,000' MSL, pressure is reduced by about half, so an FiO2 of 40% should produce exactly the same partial oxygen pressure.  I'm not sure what masks and cannulas are capable of delivering, but I'm guessing they can do that.

On the other hand, I suspect that @ArtVandelay is probably correct and that you're actually not getting the same O2 as sea level, but your pulse ox looks good because your heart is  compensating.   If correct, that would suggest turning your O2 flow further up PAST the point where your SaO2 is normal will eventually result in a slower heart rate.

Edited by jaylw314
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99-100% sat Is physiologically no different than 93-95%. IMHO 


I'll buy that it's clinically no different (and its where I live now, along with a few million other people up here), but I can't see why you wouldn't need to compensate somehow.
To drop off the same oxygen in the tissues, you're gonna need either more blood flow, or a lower venous sat (and a lower partial pressure O2 in the tissues to accomplish that). A higher hemoglobin count will do the job too, but I hope your airplane climbs faster than you can make blood.

A back of the envelope calculation assuming 50% saturation of return blood means the oxygen delivered per blood volume is ~10% lower if nothing else changes, so then you'd expect about 10% higher bloodflow to compensate. HR will have to go up more if venous sat is higher (because the % change in delivery is bigger), and less if stroke volume can also increase, and there are no doubt a half dozen other factors that matter, but the math pencils out to say that a mildly lower saturation should cause a mild compensatory mechanism.

Disclaimer--Not a doctor, just paid a lot of attention when my wife did med school etc.


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12 hours ago, N201MKTurbo said:

Low pressure causes the blood vessels to expand, this requires the heart to pump harder to maintain the same velocity of blood flow.

This is the one that makes the most sense to me.  vasodilation defiantly increases heart rate.  

 

This added with what https://mooneyspace.com/profile/15779-ilovecornfields/ posted also makes a lot of sense.

 

Thank you all

 

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Although the bulk of the physiological effects listed in the literature that I have seen are for situations with no supplemental oxygen. 

Adding O2  particularly by a high quality face mask on free flow would clearly mitigate some of these high altitude effect.  So that brings me back to the vasodilation effect.  

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  • 4 months later...

It's a normal response. I think all previous replies are pretty much correct. Here's what altitude does to the heart.

 

The major cardiovascular related effects associated with elevated altitude are:

  • Immediate decrease in oxygen delivery to the tissues, which requires a cardiac response.
  • Increase in pulmonary vascular constriction, which increases pulmonary afterload.
  • Increase in sympathetic nervous outflow, primarily by an increase in epinephrine but also norepinephrine.

 

Some, but not all of these effects are mitigated by using supplemental oxygen which I personally would recommend at altitudes above 9,000 feet, or lower if your physiology needs it. But even if you do use oxygen, your body will produce chemicals (i.e. epinephrine, norepinephrine as above) that will maintain pressures as your body is exposed to lower atmospheric pressures, but they will increase your heart rate.

There are ways you can reduce this effect by making sure you are well hydrated before each high altitude flight, avoid caffeine (and absolutely avoid alcohol for sure), and physical conditioning / exercise to train your body also is very helpful. 

Edited by JohnB
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  • 2 weeks later...
17 hours ago, Steve Dawson said:

I have very little health care education so it's a guess but what happens to the CO2 you expel when the air pressure is less at altitude? Or is the CO2 expelled as well at altitude as it as on the ground? 

Kind of a complex question for a blood chemist… with a background in transport phenomena…

But, while we wait…  Let’s discuss the transport part…


Stuff gets bound to the hemoglobin…

O2 and CO2 kind of stick and unstick relatively easily….

Compared to CO which binds tightly, and won’t let go for hours….  We would need an extra gallon of blood if we couldn’t dump the CO2 fast enough… and several gallons of blood if CO was an everyday challenge…

Hmmm….

These gasses have to pass across some membrane just to get to the blood cells….

Ahhhhhh…

We don’t ever measure our dissolved CO2 number….   Because it doesn’t become a problem under ordinary flying conditions…

Expect that CO2 is always easily disposed of no matter what altitude we are flying at…

 

PP logic only… I read a biology book somewhere along the way…

Standby for somebody with real knowledge on the subject…

Best regards,

-a-

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19 hours ago, Steve Dawson said:

I have very little health care education so it's a guess but what happens to the CO2 you expel when the air pressure is less at altitude? Or is the CO2 expelled as well at altitude as it as on the ground? 

The lower air pressure itself only has a tiny effect--the speed CO2 comes out of your bloodstream into your lungs is proportional to the amount of CO2 in your bloodstream minus that in the surrounding air.  Since there is such a tiny amount of CO2 in the atmosphere in comparison to your body, reducing air pressure (which cuts down all gases including CO2) barely changes the rate at which CO2 diffuses out of your bloodstream.  

You actually get rid of CO2 a little faster, though, because you're breathing more air volume per minute at altitude as discussed above. 

@carusoam The pCO2 in the body is always around 40mmHg or about 5% of 1 atm :)  pretty easy to remember if you've ever had to grow mammal cell lines in CO2 incubators...

Edited by jaylw314
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