fluid and electrolytes

jesa · Nov 7, 2008

I know how you feel, when I started it was like learning a foreign language. just keep at it and it will eventually make sense.

as far as acidosis/alkalosis, here's how I remembered it:

acidosis pH is low

alkalosis pH is high

for respiratory look at the CO2 level

for metabolic look at the HCO3 level

if it's respiratory then the CO2 level goes in the same direction as the pH (both CO2 and pH are high for alkalosis, both low for acidosis)

if it's metabolic then the HCO3 level and pH go in opposite directions (HCO3 high, pH low then metabolic acidosis, HCO3 low, pH high then metabolic alkalosis)

this is a nice one to make a simple arrow diagram for

Daytonite, BSN, RN · Nov 7, 2008

Thank you for all the great links.
Do not know how to reply to PM. I'm new.
Just picked you because I needed some encouraging words. That is all!

I thought it was rather bizarre. Had no idea what you were talking about.

CO2emission · Nov 7, 2008

Hi,

My advice is you will get confused if you use this forum to build your knowledge because everyone has their own understanding and although people mean well, there needs to be some authenticity, references and peer consensus so you don't head off in the wrong direction.

If you have some knowledge about basic chemistry the rest will come easy.

I think this is something that you either do yourself or, you find a mentor/tutor. Take heart though many people have difficulty with this area of physiology.

You can try this site for some fast track tutorials with concept maps but its not free.

http://www.rapidlearningcenter.com/biology/anatomy-physiology/22-Fluid-Electrolyte-and-Acid-Base-Balance.html

Good luck.

Nurse2Doc2008 · Nov 8, 2008

Here's what I do.

#1. Look at the pH. If it's below 7.4, you have an acidosis. If it's above 7.4, you have an alkalosis.

#2. Look at the pCO2. Realize that pCO2 is determined by the patient's ventilation....so, if the patient is hypoventilating (like someone who has a drug overdose, for example), they won't blow off CO2, so their pCO2 will rise. On the other hand, if they're breathing too fast (like someone hyperventilating, for example), they blow off too much CO2, and their pCO2 will fall. CO2 is an acid, so too much causes acidosis, while too little causes alkalosis. Thus, a patient who has overdosed on drugs will develop a respiratory acidosis, essentially b/c they don't exhale any CO2, and a patient who is hyperventilating will develop a respiratory alkalosis, since they are blowing off too much CO2. So, to generalize, in a primary respiratory acidosis, pH is low and pCO2 is high. In a primary respiratory alkalosis, pH is high and pCO2 is low.

Now, what happens if the pH is high and the pCO2 is also high? Or, what if the pH is low and the pCO2 is also low? Well, recall that the pCO2 can change to compensate (ie bring the pH back towards a more normal range) for a primary metabolic acidosis/alkalosis as well. Which brings us to #3...

#3. Look at the Bicarb. For the sake of this discussion, consider bicarb a base. When the amount of bicarb in the blood falls, you have a metabolic acidosis, whereas when the amount of bicarb in the blood rises,you have a metabolic alkalosis. So, to generalize, in a primary metabolic acidosis, the pH is low and the bicarb is low. In a primary metabolic alkalosis, the pH is high and the bicarb is high.

As I mentioned above, the pCO2 can change to compensate (ie bring the ph back towards normal) for a metabolic acidosis/alkalosis. You could think about this in a sort of telological way...as in, if pH is low because bicarb is low (ie primary metabolic acidosis), you'd "want" to get rid of some acid. What's a good way to get rid of acid? Blow off some CO2 (hyperventilate!), and generate a respiratory alkalosis. So in this situation, you'd have a low pH, low bicarb, and low pCO2 (you describe this as a primary metabolic acidosis with respiratory compensation). What do you think would happen if you had the opposite situation, as in a primary metabolic alkalosis? Your body would "want" to bring the pH back towards normal, so how do we generate some acid to bring it back to normal? You gut it....retain some CO2 and underventilate. In this situation, you'd have a high pH, high bicarb, and high pCO2 (you describe this as a primary metabolic acidosis with respiratory compensation).

So, can you "compensate" for a primary respiratory acid-base disturbance? Of course! Just like you compensated with respiration for a metabolic disturbance, your kidneys are able to compensate for a respiratory disturbance by adjusting the amount of bicarb present in the blood. We can work through this similarly....say you have a primary respiratory acidosis, meaning the pH is low and the pCO2 is high. What do your kidneys "want" to do? Put more bicarb back into the blood to bring the pH back to normal. So in this case, you'd have a low pH, high pCO2, and high bicarb (you call this a primary respiratory acidosis with metabolic compensation). Last but not least, if you have a primary respiratory alkalosis, meaning pH is high and pCO2 is low, your kidney would want to get rid of more bicarb, thus bringing the pH back towards normal. In this case, you'd have a high pH, low pCO2, and low bicarb.

Whew. That's it! It sounds confusing, but I tihnk if you think about what each thing means (pH, bicarb, and pCO2, it makes it easier to reason through it rather than just memorize something). If you really understand how this works, it'll really help you understand vent settings and tons of other clinical topics.

A few things I got confused about in the beginning with all of this was how to determine which was the primary disturbance and which was the compensation. That's why I always look at pH and pCO2 first and figure that out before I look at bicarb. If pH is low and pCO2 is also low, you know there's no way you could have a respiratory acidosis, since CO2 would have to be high to cause acidosis. You know that the CO2 has changed to compensate for something, namely a change in the bicarb. That's when you check the bicarb, and make sure it's low. Similarly, if the pH is high and pCO2 is also high, you know you can't have a primary respiratory alkalosis, since high pCO2 would cause acidosis, not alkalsos. A check of the bicarb will reveal that it is high.

Note that some times you will see uncompensated respiratory or metabolic acidosis or alkalosis...that's basically just the first few scenerios I described, where only bicarb OR pCO2 changes with pH changes.

One other thing to remember is that although you can somewhat "compensate" for a primary acid-base disturbance, you won't compensate enough to return pH to normal, and certainly won't overcompensate and push the pH beyond, so don't worry about this as a possible point of confusion. (This goes for testing purposes...in real life, all bets are off.)

Lastly, remember what I described above is sort of the short story....a lot of patients, especially in the ICU, will have mixed acid-base disorders, like both a primary respiratory acidosis and a primary metabolic alkalosis. There are fancy formulas to figure this stuff out, as well as to figure out the degree of compensation, but I'm betting that's beyond the scope of your test.

So, to summarize:

Respiratory Acidosis= low pH with high pCO2. If there is metabolic compensation, bicarb will be high.

Respiratory Alkalosis=high pH with low pCO2. If there is metabolic compensation, bicarb will be low.

Metabolic Acidosis=low pH with low bicarb. If there is respiratory compensation, pCO2 will be low.

Metabolic Alkalosis=high pH with high bicarb. If there is respiratory compensation, pCO2 will be high.

There's more to discuss....etiologies of various disturbances, formulas to calc if compensation is appropriate, mixed acid-base d/o, etc.

Sorry this post is so uber long. Hopefully it is helpful. Tell me if you have any questions--I love acid-base/electrolytes!