Safe to take a pregnant lady flying?

[flyingcheesehead]

As MAKG said, in fact the air you breathe is changing temperature pretty instantly, and except for really extreme situations, the wet warmth of your nose, mouth, airways, lungs overwhelms the relatively low specific heat of the air entering your lungs.

Now we're starting to make more sense!

During normal nose breathing, air is heated to about 97F and has an 80-90% relative humidity by the time it reaches the carina (that bit where your trachea splits into bronchi to go to your lungs separately). Things go a bit slower than that if you're breathing through your mouth,

Speaking from an engineering mindset, though, when the air reaches the carina there would be a temperature distribution - It'd be 97-98 degrees near the edge of the trachea, but the air in the middle, which hasn't yet had direct contact with warm, most tissues, should still be closer to its original temperature.

but by the time air gets to the alveoli (the air sacs in your lungs that actually do gas exchange with blood), it's safe to assume in all but the weirdest situations (antarctic explorers or mountain climbers caught out) that the air is body-temperature and fully saturated.

How big are alveoli? How big are the tubes by the time they get there?

Exercise is definitely more difficult in heat and humidity. That's because your body relies on the fact that it's dumping heat and water into the air you're breathing in order to cool down. (We may not be dogs, but we still use panting for cooling to some degree.) If the air coming in is already hot and humid, the body isn't pumping that heat and evaporative air into the air for exhalation, and so there is more heat strain on the body overall. Part of the response (though a smallish part) is to try to breathe more (and therefore feel like you can't get enough air) in order to get that cooling going.

A bigger part of the response is that (unlike dogs) we do most of our physiologic cooling through our skin: by sweating and dilating the capillaries in our skin. (If we make our skin warm, that heat is lost by conduction, convection, radiation, and sweat evaporation.) The dilation of capillaries all over the skin drops the resistance to bloodflow in the overall body system, and means your heart needs to pump all the harder to maintain pressure on the system.*

This effect -- and the resultant workload on the heart -- is higher in hot and humid environments as conductive/convective heat loss is less (heat) and evaporative loss from sweat is less effective (humidity). That cardiac load feels very much like air hunger and "can't quite catch my breath" and "can't go any harder, but it's not because of my legs".

Great explanations... Thanks!

* If you're more an electronics person, the heart is a approximately constant-voltage generator. When resistance in the skin drops -- which is in parallel with the rest of the body "load" -- the heart has to ramp up the current in order to maintain the same voltage and thereby keep the other bits of the body running appropriately. Except the problem is even worse, because exercising muscles also become low-resistance loads, and exercise encourages the body to increase voltage (blood pressure) rather than just maintaining it.

Perfect! I'm just a big transistor about to experience thermal runaway.

 

[mcmanigle]

Speaking from an engineering mindset, though, when the air reaches the carina there would be a temperature distribution - It'd be 97-98 degrees near the edge of the trachea, but the air in the middle, which hasn't yet had direct contact with warm, most tissues, should still be closer to its original temperature.

Very true. But (and now I'm admittedly getting into hand-wave-y explanation) flow is pretty turbulent, especially in lower airways. A sponge is probably a better model for the lungs than a tube going into a balloon. On average, from the trachea, the tube will branch about 23 times before it gets to an alveolus. The first 16 divisions (or so) are in the "conducting zone", ie there is no gas exchange there. The final 7-ish divisions are respiratory bronchioles, terminating in the alveoli, all of which perform gas exchange.

So while you can think of pumping air into the trachea and bronchi and upper bronchioles, air is reaching the respiratory zone largely by diffusion. This is also why alveolar air has about 100 mmHg partial pressure of oxygen, compared to the 160 mmHg you're inhaling.

Another (hand-wave-y) way to convince yourself of this is to think of typical lung volumes. An average male is moving about 500mL of air per breath. About 150mL of that remains in the "anatomic dead space" -- non-gas-exchanging airways. The remaining 350mL is mixing with the 2300mL that was sitting in his lungs at the end of normal exhalation (about half of which he could have exhaled if he really tried). So of the total 2650-ish mL in the gas exchanging regions of the lung during a given inhalation, only 13% arrived in "that breath".

I realize these are a lot of tangential arguments rather than hard explanation, but hopefully it's enough to convince you that good mixing has been achieved by the time we're talking about the air actually involved in respiratory gas exchange.

 

[flyingcheesehead]

Very true. But (and now I'm admittedly getting into hand-wave-y explanation) flow is pretty turbulent, especially in lower airways. A sponge is probably a better model for the lungs than a tube going into a balloon. On average, from the trachea, the tube will branch about 23 times before it gets to an alveolus. The first 16 divisions (or so) are in the "conducting zone", ie there is no gas exchange there. The final 7-ish divisions are respiratory bronchioles, terminating in the alveoli, all of which perform gas exchange.

So while you can think of pumping air into the trachea and bronchi and upper bronchioles, air is reaching the respiratory zone largely by diffusion. This is also why alveolar air has about 100 mmHg partial pressure of oxygen, compared to the 160 mmHg you're inhaling.

Another (hand-wave-y) way to convince yourself of this is to think of typical lung volumes. An average male is moving about 500mL of air per breath. About 150mL of that remains in the "anatomic dead space" -- non-gas-exchanging airways. The remaining 350mL is mixing with the 2300mL that was sitting in his lungs at the end of normal exhalation (about half of which he could have exhaled if he really tried). So of the total 2650-ish mL in the gas exchanging regions of the lung during a given inhalation, only 13% arrived in "that breath".

I realize these are a lot of tangential arguments rather than hard explanation, but hopefully it's enough to convince you that good mixing has been achieved by the time we're talking about the air actually involved in respiratory gas exchange.

Wow - That's a really good explanation. Thanks! I had no idea how little air was arriving per breath and how much of a role diffusion played. I remember learning in health class way back when that while we normally inhale air that's about 21% O2, we exhale air that's about 5% less (16%). So if we're only replacing about 13% of the air in our lungs, it clearly takes at least several breaths for the process to occur. It also seems that a large part of the function of our breathing in the outer parts of our lungs (the alveoli end of things) is not only gas exchange, but encouraging that mixing and diffusion process.

In terms of the "conducting zone", is there really NO gas exchange there, or just a much lower proportion of gas exchange? Is there a difference between the tissues in the conducting zone vs. the gas exchange zone, or is it simply that the "mixture" (to use an aviation term) is much richer towards the alveoli (much more surface area for exchange to occur) thus meaning that the most gas exchange occurs there?

 

[mcmanigle]

It also seems that a large part of the function of our breathing in the outer parts of our lungs (the alveoli end of things) is not only gas exchange, but encouraging that mixing and diffusion process.

Yes, you can think of it as three different "pools":

Tidal volume (air exchanged with each breath)
-- in diffusion exchange with --
Residual volume (air retained in lungs)
-- in rapid gas exchange equilibrium with --
Arterial blood

Of course, it's not perfect, not least because some of the tidal volume is in direct contact with the respiratory zone (and therefore in respiratory equilibrium with arterial blood), but it's a relatively small component as we saw in the last post. (To complicate things further, there is also some blood that skips the respiratory areas of the lung entirely on its way around, in a situation called ventilation/perfusion mismatch.)

Also, going back to 16% O2 in exhaled air: That's correct, and when you figure that the alveolar air is only about 13% O2, you see that the "extra" 3% oxygen is coming from the "dead zone" air that was never gas exchanged.

Nice to see that the math works out as well, as the exhaled air is:
150 mL dead zone air at 21% + 350 mL respiratory air at 13% = 500 mL air at 15-16% (to within the sig-figs in my ballpark numbers)

(As bonus points, think about what all this means for mouth-to-mouth CPR.)

Just as important as the oxygen buffering (as far as physiology is concerned) is the CO2 buffering. The equilibrium between blood and air is achieved rapidly in the alveolar capillaries (except in pretty severe pathology), so the gas partial pressures in your arterial blood are the same as that in your capillary air. The bicarbonate buffer system is the major determinant of blood pH, and relies on tight control of blood CO2 concentration. If that were bouncing up and down with every breath you took, you'd be in trouble.

In terms of the "conducting zone", is there really NO gas exchange there, or just a much lower proportion of gas exchange? Is there a difference between the tissues in the conducting zone vs. the gas exchange zone, or is it simply that the "mixture" (to use an aviation term) is much richer towards the alveoli (much more surface area for exchange to occur) thus meaning that the most gas exchange occurs there?

There is a difference in tissues; it's not just a matter of relative surface area contributed by each portion. Of course, nothing in real life is absolute, and there is a small border zone in which the cell type in the epithelium transitions, but unless you have a reason to be digging really, really deep, respiratory zone exchanges and conduction zone does not.

 

[Swampfox201]

I'd be careful. The pressure change on climb out can make the baby pop out.

 

[danhagan]

I'd be careful. The pressure change on climb out can make the baby pop out.

That's because their bellies expand as altitude increases (like a balloon)

 

[acrophile]

I say that based on my engineering knowledge, mainly thermodynamics and heat transfer, not just a hunch

My mistake, then. I got the wrong impression--apologies!

Air density is affected by temperature, thus on a hot, humid day the air's density will be lower than normal. I think what you meant to say in that last sentence is pressure, not density, can be higher or lower than average.

At a given pressure, hot and humid air indeed has lower density than dry and cool air at the same pressure. But that density can still be higher than average, if that pressure is much higher than average. (Admittedly, though, if it's hot enough, then pressure won't feasibly be high enough to result in above-average density.)

Body temperature won't rise much - Only a few degrees F warmer and you're going to be in a world of hurt - so I don't think the body's extra heat would be contributing much to the air density inside your lungs.

Agreed. I was suggesting that the mechanism was other than via the temperature's effect on air density in your lungs.

 

[flyingcheesehead]

My mistake, then. I got the wrong impression--apologies!

No prob. I can't stand science deniers either.

At a given pressure, hot and humid air indeed has lower density than dry and cool air at the same pressure. But that density can still be higher than average, if that pressure is much higher than average. (Admittedly, though, if it's hot enough, then pressure won't feasibly be high enough to result in above-average density.)

My thoughts exactly.

 

[flyingcheesehead]

mcmanigle, fascinating stuff you've posted here, and it's greatly increased my understanding of the breathing process and the reason behind PA vs. DA when it comes to physiology. Thanks!

Now can someone please take us to the obligatory flame war and thread closing? This thread is way too informative.

 

[gkainz]

interesting that we would question flying with a pregnant woman, while nobody I know would think twice about heading west from here while pregnant.

"Hey Honey, let's go have a picnic at Vale."

 

[Jaybird180]

mcmanigle, fascinating stuff you've posted here, and it's greatly increased my understanding of the breathing process and the reason behind PA vs. DA when it comes to physiology. Thanks!

Now can someone please take us to the obligatory flame war and thread closing? This thread is way too informative.

I'd be careful. The pressure change on climb out can make the baby pop out.

Already started

 

[MAKG1]

interesting that we would question flying with a pregnant woman, while nobody I know would think twice about heading west from here while pregnant.

"Hey Honey, let's go have a picnic at Vale."

Or up Pike's Peak / Mt. Evans.

It's a LONG drive to the delivery room from up there.

 

[Zeldman]

After reading many interesting and very informative replies, I just have one question:






Is it safe to take a pregnant lady flying?

 

[mcmanigle]

Is it safe to take a pregnant lady flying?

Guidelines here, page A10 (page 12 of the PDF).

Summary:
- In normal pregnancy, hypoxia is not an issue up to commercial cabin pressures (8000')
- A pregnant person might be more predisposed to discomfort from ear squeeze, bowel gas, and airsickness, but standard precautions should minimize that.
- When turbulence is a possibility, especially third trimester, keep the seatbelt fastened low over the hips / thighs for the whole flight.
- Pregnant people are at increased risk for DVT (leg clot), so commercial flight advice is to walk around every 1-2 hours. Translate that to stops (or at least a good stretch) accordingly.
- Pregnant people who also have low oxygen carrying reserve (eg anemia) should probably get that under control before flying.
- If the fetus has decreased respiratory reserve (eg IUGR, post maturity, preeclampsia, hypertension, placental infarction), think about the oxygen issue again and consider a pulse oximeter and/or preemptive supplemental oxygen.


The following are more about quick access to medical care, so for shortish flights or flights where landing and calling an ambulance wouldn't be the end of the world, it's less a big deal:
- In the first trimester, if you're having bleeding or pain associated with the pregnancy.
- In the last trimester, if beyond 36 weeks or a history of pre-term delivery, cervical incompetence, bleeding, or increased uterine activity ("I'm not sure if these are contractions or not, but let's go!")