QUESTIONS

AND NOW WHAT YOU’VE ALL BEEN WAITING FOR!

THE ACID BASE REVIEW
FOR THE MENTALLY CHALLENGED STUDENT.

I will begin assuming you know absolutely nothing & work up from there.  Hopefully by the end, you will be as good as anyone in working out these problems.  Here we go:

1. Ok, I’ll figure you know a little. I’ll figure you know what neutral is in a generic sense & that pH 7 is neutral.  I’ll also be so bold as to figure you know that an acid is a proton donor & a base is a proton acceptor.  So let’s go back to pH. Define pH please.
    
2. What is normal blood pH? What is normal p(02)?
    
3. Explain the difference between acidosis & acidemia.
    
4. When total CO2 is reported on a blood panel, what is included in this measurement?
    
5. What’s the difference between a volatile acid & a non-volatile acid?
    
6. Who remembers the Henderson-Hasselbach equation?  No one?  I’ll give you a hint.  Here it is for determining blood pH:
   
                                   [bicarb]
    pH = pK + LOG   -------------
                              [carbonic acid]
   
   So, I’ll ask an easy question: What the hell is a pK?
    
7. So everyone take out their blood gas machines.  You have a respiratory acidosis if pC02 is > _______. You have a respiratory alkalosis if pC02 is < ________.
    
8. Sounds straight forward, then. If pC02 is high then you have resp. acidosis. If pC02 is low you have resp. alkalosis.  Is metabolic acidosis & alkalosis going to be easy too? Can we just see if bicarb is high or low? Noooo.  To determine if you have a metabolic acidosis or alkalosis, you have to look at the base excess.
   
   What are the rules for the base excess in detemining if there is alkalosis or acidosis?
    
9. Base excess?!  We’re really reaching into your memories now. Wasn’t there a formula about the base excess?  Yes, there was.  Here it is:
   
   HC03 amount that you need to give (in meq) = 0.3 x kg x base excess
   
   So, where did the 0.3 come from?

We’ll stop there & continue with part two when you’re ready.

ANSWERS

1. pH is the negative logarithm of the hydrogen ion concentration.
   
   Huh? i.e. if  [H+] = 1 x 10^-7  then pH=7
   (Everyone get that [] is the symbol for concentration & ^ is the exponent sign)
   
   
   
   
    
2. Normal blood pH is 7.4. Normal partial pressure of oxygen is 100 torr (same as mm of Hg).  This is, of course, for the patient breathing room air.  The general rule for patientÕs receiving supplemental oxygen is that p(02) should be at least 4x the concentration of 02 being breathed.  A really crappy p(o2) would be 60.  Realize though that in a normal venous sample p(02) is about 40.
   
   
   
   
    
3. Acidosis is a metabolic state that increases acidity. Acidosis & alkalosis can (& frequently do) go on inside the same patient.  Acidemia is the state where the blood pH is < 7.4.
   
   
   
   
    
4. Total C02 = C02 + Carbonic acid (H2C03) + bicarb (HC03)
   95% of this value is bicarb so you CAN consider total C02 to be a measurement of bicarb & not be too far off.
   
   
   
   
    
5. A volatile acid can be turned into a gas & excreted from the lung (like carbonic acid - it is easily turned into C02 & exhaled). A non-volatile acid must be removed by the kidney, or metabolized (lactic acid would be a good example.)
   
   
   
   
    
6. Didn’t we do this as undergrads?  pK is the pH at which 50% of an acid is ionized. For carbonic acid, that pH is 6.1
   (Memorize the Henderson-Hasselback equation as shown. Consider the bicarb component to be the metabolic component & the carbonic acid component to be the respiratory component.  The normal ratio of bicarb to carb. acid is 20:1)
   
   
   
   
    
7. resp. acidosis if pC02 > 46 torr
   resp alkalosis if pC02 < 36 torr.
   
   
   
   
    
8. Normal base excess is 0±4 meq/L
   <-4 is metabolic acidosis
   >+4 is metabolic alkalosis
   
   
   
   
    
9. 0.3 represents the “treatable space.”  The extracellular fluid represents about 20% of the body weight but because of exchange between cells & extracellular fluid, the figure is brought up to 30%.