MadSci Network: Medicine

Re: Why does a person that is hyperventilating has to breathe into a bag

Date: Mon Mar 28 20:07:03 2005
Posted By: Mark Sullivan, 2nd Year Internal Medicine Resident
Area of science: Medicine
ID: 1111506423.Me

Hi Petmo,
   Good question.  Hyperventilation as medically defined is a respiratory 
rate greater than 30 breaths per minute.  Normal is roughly 8-30.  In 
order to answer your question we have to take a look a blood chemistry and 
acid/base balance.  Because I don't know your background in this area I am 
going to be as basic as possible.

   Red blood cells are responsible for carrying/delivering oxygen (O2) to 
the cells.  Also disolved within the blood is Bicarbonate (H2CO3) and 
Carbon Dioxide (CO2).  Now, the acid/base balance of the blood is such 
that the body attempts to maintain the pH (hydrogen concentration) within 
a narrow range of 7.40 +/- 0.4.  Anything above 7.44 is considered 
alkaline, and anything less than 7.36 is considered acidic.  Truthfully, 
the body doesn't like being out of this range and does multiple things to 
change this according to how long either acidosis or alkalosis has been 
present.  As for what causes these changes in pH, there are numerous, and 
for now we are only going to look at hyperventilation as it pertains to 
your question.

    Now, the part of the blood responsible for keepting this balance is 
really the CO2 and HCO3.  If we look at these chemically, CO2 can be 
changed into HCO3 as follows:

H2O + CO2 <-> H2CO3

   As you can see I do not have an equal sign as the arrows indicate that 
the conversion can go either way.  What makes the difference is how much 
CO2 or H2CO3 is added to either side of the equation.  If we increase the 
CO2, then there is an imbalance of H2CO3, so the equation shifts to the 
right until that balance is achieved, and visa versa.  But this balancing 
act is not quite instantaneous, and there are several mechanisms in play.

  If we have too much CO2 or too little H2CO3, then the blood becomes too 
acidic (pH < 7.36) and if we have too little CO2 or too much H2CO3 then 
the blood is alkalotic.  The thing that changes most rapidly is the CO2 
concentration.  And this is because of the respiratory rate and the 
ability to rapidly change CO2 loss through the lungs.  Hold your breath or 
slow your rate down and you get too much CO2, increase your rate and you 
decrease the amount of CO2.  The kidneys are primarily in charge of the 
H2CO3 by changing how much is filtered or reabsorbed, but this is a much 
slower process.  So the two ways to really change your acid base balance 
are through the kidneys or the lungs.  If you were to have a problem with 
your breathing then your kidneys would try to compensate for that, and 
visa versa (again).
   We already defined hyperventilation as an increased respiratory rate > 
30.  Sometimes this can be very drastic with rates into the 40's and 
50's.  When that happens, you breathe off your CO2 very rapidly and this 
changes the pH of your blood to a more alkalotic range.  Over time the 
kidneys could probably compensate, that is if you could last long enough 
breathing that fast, but most people would tire out.  By breathing into a 
paper bag you are actually rebreathing CO2, and therefore you can prevent 
blowing off some of that CO2.  BUT, not all cases of hyperventilation are 
due to anxiety, so that trick only works when it is truly diagnosed by a 
physician.  Sometimes people hyperventilate because they have increased 
losses of H2CO3 from the kidneys, which means their blood is too acidic.  
If that happens the lungs try to compensate by getting rid of CO2 and 
making the blood more alkalotic.  If you were to breath into a paper bag 
in this case you would not be helping yourself, and in fact could do 
yourself a lot of harm.

   Like I mentioned before, there are a lot of causes of acid/base 
disorders that is much more complex than I am getting into at this time.  
If you would like to look at this more from a chemistry and physiology 
point of view here is a very technical web site review  ht
tp:// that I 
found on line.  believe me it is not very simple.  And as I said above the 
paper bag trick is not a cure, and not even something I would recommend to 
someone until I have excluded all the other causes of hyperventilation.  
Good luck to you.

Mark Sullivan


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