Crash Course on Cellular Respiration R/T ABGs--Where Does Bicarbonate Come From?

So we know that we eat food for energy, and we breathe in oxygen and release carbon dioxide.  We also know that with metabolic and respiratory homeostasis, that the lungs release the carbon dioxide at an appropriate rate to the amount of oxygen that is used, and that the kidneys retain bicarbonate and release protons in the urine.  But how do these two facts relate to each other?  

The energy we eat is always broken down into glucose, which has a chemical formula of C6H12O6, and that floats around in our blood stream until it's shunted into a cell (skeletal muscle, brain, whatever...) by insulin.  At the same time, oxygen (O2) is taken in, crosses the very thin membrane of the alveoli into the capillaries, onto a hemoglobin, and is carried by the red blood cell into a cell (skeletal muscle, brain, whatever...).  Inside of the cell the Citric Acid, Krebs Cycle, and e- Transport chain (to make 38 ATP) takes place (see how you really do need to remember your Intro to Bio?), producing carbon dioxide, water (which is why we have insensitive water loss of 600 mL/day, right?) and 38 ATP (theoretically).  

Now, remember your Intro to Chem, where all molecules dissolved in a fluid are in constant homeostatic flux?  That's what's happening here:

CO2 + H2O <--> H2CO3 <--> HCO3- + H+

Once the CO2 and H2O dissolve into the blood, it exists as CO2, H2O, H2CO3, HCO3-, and H+ all a the same time (actually, very little of it stays as H2CO3--it's kind of that in between, super fast, invisible step).  When that blood travels to the kidneys, the kidneys just pick out the H+ and that goes into the urine, making urine acidic.  Some of the water is excreted in the urine also.  Then the blood flows up to the lungs, where the CO2 crosses over into the alveoli and gets exhaled with some H2O as well.  

Magnesium sulfate and flushing

How I felt today:

Today was the OB ATI test.  70 questions in 70 minutes.  This was my question 68.  I was stumped, and tired, and frankly, by this point, didn't give two craps.  I just needed to get out of that room.  Now that I've finally got a few hours of sleep and had some time to think about it, I'm pretty sure I got it wrong, so I decided to do a post-mortem on it.

I'd like your input.

Here's the question:

A nurse is caring for a patient with preeclampsia with magnesium sulfate.  Which of these symptoms should you report to the provider?  Select all that apply.

a) Respiratory rate of 20 per minute
b) A urinary output of 400 mL/4hr
c) Epigastric pain
d) Facial flushing
e) Lack of deep tendon reflexes

I'm pretty confident I got this answer wrong, so what did you answer?  Why?  I'll give you a minute to think, and I'll explain my answer choices and reasonings below.

By the way, this is what a vial label for magnesium sulfate IM might look like:

http://dailymed.nlm.nih.gov/dailymed/image.cfm?id=59573&name=274e5d29-figure-02%2Ejpg

Ok, here's the question again:

A nurse is caring for a patient with preeclampsia with magnesium sulfate.  Which of these symptoms should you report to the provider?  Select all that apply.

a) Respiratory rate of 20 per minute
b) A urinary output of 400 mL/4hr
c) Epigastric pain
d) Facial flushing
e) Lack of deep tendon reflexes

I chose c) and e).  I think I should have chosen c), d), and e).

I had some confusion over this question for 2 reasons.  With the phrase "which of these symptoms" it was unclear to me whether the question was asking for the symptoms for preeclampsia progressing to eclampsia (essentially, the treatment was failing), or asking for the symptoms of magnesium sulfate overdose, or both.  In the moment my brain just thought: "Well, I'm going to report any potentially life threatening symptoms," because, you know, real life.  A good test taking tactic, if you don't know the answer to a question is to strike out the obviously WNL (within normal limits) answers.

a) Incorrect.  A respiratory rate of 20 breaths per minute is within normal range.
b) Incorrect.  An output of 30 mL/hour is "normal" on NCLEX questions.  30mL/hr x 4hrs = 120mL, so 400 mL/4hr is more than sufficient.
c) Epigastric pain.  Here's where it got a little confusing.  Was this question strictly about calling for signs of magnesium toxicity?  Or was it asking for what I should alert the doctor about in general?  Because epigastric pain, especially RUQ (right upper quadrant pain) is classic s/s of liver swelling, which happens with preeclampsia because the elevated blood pressure causes the blood to back up from the heart into the systemic circulation.  (The first organs to be affected by this systemic increase in blood pressure are your brain, heart, kidneys, and liver.)  This is why with preeclampsia and eclampsia, you'll see elevated liver enzymes.
d) Facial flushing.  I couldn't, for the life of me, remember with 100% certainty that facial flushing was a side effect of magnesium sulfate.  My gut said yes, but my brain was being a non-committing coward.  More on this later.
e) Magnesium sulfate classified as an anticonvulsant because it is a CNS depressant.  It blocks the transmission of Ach (acetylcholine), which decreases motor nerve impulse.  It would make sense, then, that if someone was given too much mag sulfate, that it would cause a cessation of motor nerve impulses, leading to a lack of deep tendon reflexes.  For a video explanation on how to perform deep tendon reflexes, click here.

So I got home and researched magnesium sulfate, and this is why I think I should have picked d):

http://www.healthline.com/health/pregnancy/preterm-labor-magnesium-sulfate#Overview1

Basically, it would have similar results to a calcium channel blocker.  It does not have the same mechanism of action, but if you think about it, calcium channel blockers block the calcium channels in smooth vascular muscles, prevent an influx of calcium, which causes vasodilation (hence, why a side effect of nifedipine is orthostatic hypotension).  Similarly, if you decrease the calcium blood levels by administering magnesium sulfate, there won't be an influx of calcium through channels on the smooth vascular muscles, so your body will be unable to vasoconstrict.  The vessels will remain vasodilated.  What else happens with vasodilation?  Flushing!  You get red.  When it's hot outside, or when you work out, or when you are embarrassed, your blood vessels dilate, causing flushing.

I'm not 100% sure, but it makes sense, right?  And, in the words of Ms. Soberano, "If you can defend your reasoning, then it's a good answer."

Now, if I could figure out why we use calcium gluconate as the antidote...

References:
http://www.drugs.com/sfx/magnesium-sulfate-side-effects.html
http://www.rxlist.com/magnesium-sulfate-side-effects-drug-center.htm
http://www.surgeryencyclopedia.com/La-Pa/Laxatives.html
http://en.wikipedia.org/wiki/Magnesium_sulfate
http://www.healthline.com/health/pregnancy/preterm-labor-magnesium-sulfate#Overview1
http://www.ncbi.nlm.nih.gov/pubmed/1331782
Mosby's 2013 Nursing Drug Reference
https://www.youtube.com/watch?v=0sqCIzuotWo
http://en.wikipedia.org/wiki/Calcium_channel_blocker
http://www.drugs.com/nifedipine.html