9 - Acid-Base Concepts Flashcards
What is the equation for acid base homeostasis in the body? What equation defines this?
CO2 + H2O <-> H2CO3 <-> HCO3- + H+
Henderson Hasselbach: [H+] = 24 (pCO2)/(HCO3)
It’s just important to know that pCO2 and bicarb move in a similar way. If you’re retaining CO2 to keep pH stable you have to increase bicarb as well. If that doesn’t happen ,there’s a secondary problem.
What must our bodies make in order to maintain a constant pH?
We make a lot of acid ~22,000 mmol a day to keep our pH at 7.4 or close.
Where in the nephron plays a role in acid-base interactions? What enzyme is involved in this?
In the proximal tubule, HCO3- is reabsorbed.
H+ is secreted in the intercalated cells in the collecting duct.
Carbonic anhydrase, aids in the conversion of carbon dioxide to carbonic acid and bicarbonate ions.
What two things are needed to identify acid-base imbalances?
Arterial blood gas: pCO2 and pH
Basic chemistry: HCO3-, Na+, Cl-
Define primary respiratory (acute or chronic) acidosis and alklalosis?
Primary respiratory acidosis: pCO2 >40 mmHg - hypoventilating and retaining CO2
Primary respiratory alkalosis: pCO2 <40 mmHg - hyperventilating and exhaling CO2
Define primary metabolic (acute or chronic) acidosis and alklalosis?
Can be +/- anion gap.
Primary metabolic acidosis: HCO3- <24 mmol/L
Primary metabolic alkalosis: HCO3- >24 mmol/L
Which combination of acid-base disturbances cannot occur?
Repiratory acidosis and alkalosis because you can’t breathe slowly and quickly at the same time.
A 68 yo male presents with shortness of breath. He has a long history of tobacco use. His labs are as follows:
pH: 7.23
pCO2: 68 mmHg
HCO3-: 30 mmHg
What is the primary acid-base disturbance?
Respiratory acidosis.
Acidosis because the pH is lower than 7.4 and respiratory because his pCO2 is above 40.
How does compensation for acid-base disturbances work?
Metabolic compensation for respiratory disorders and respiratory compensation for metabolic disorders.
It’s the body’s way of bringing pH back to 7.4
What is the compensation equation for metabolic acidosis in order to predict pCO2?
-pCO2 = 1.5 x [HCO3-] + 8
OR
- pCO2 = last 2 digits of pH
- Remember: pCO2 compensates in the same direction as bicarb!*
How would you calculate pCO2 compensation for metabolic alkalosis?
-PCO2 = 15 + [HCO3-]
But the maximum pCO2 is 55 mmHg because humans can only hypoventilate to a certain extent.
Describe acute and chronic compensation for respiratory acidosis (PaCO2 > 40)?
Acute: bicarb increases 1 mmol/L per 10 mmHg increase in PaCO2
Chronic: bicarb increases 4 mmol/L per 10 mmHg increase in PaCO2
Bicarbonate increases more in chronic respiratory acidosis because the kidneys change in order to absorb bicarb.
Describe acute and chronic compensation for respiratory alkalosis (PaCO2 < 40)?
Acute: bicarb decreases 2 mmol/L per 10 mmHg derease in PaCO2
Chronic: bicarb decreases 4 mmol/L per 10 mmHg decrease in PaCO2.
Remember: bicarbonate compensates in the same direction as the chance in pCO2.
What is the purpose of calculating compensation?
To determine if a secondary disorder is present.
If you’re not compensating appropriately, then a secondary problem is present.
If you have a primary metabolic disturbance and your bicarbonate is 15 mmol/L, then what should your pCO2 be to compensate? What if your pCO2 is below or above this value?
pCO2 = (1.5 x 15) + 8 = 30 mmHg
If your pCO2 is below 30 mmHg you have secondary respiratory alkalosis. If your pCO2 is above 30 mmHg, then you have secondary respiratory acidosis.
If you have a primary respiratory disturbance and your pCO2 is 60 mmHg, when you do you expect your HCO3 to be to compensate?
- In acute, your bicarb should go up 1 mmol/L for every 10 mmHg.
- In chronic, your bicarb should go up 4 mmol/L for every 10 mmHg of pCOa.
So in acute you should compensate by increaseing HCO3 by 2 mmol/L and in chronic you should compensate by increaseing HCO3 by 8 mmol/L
If bicarb is higher than expected you have secondary metabolic alkalosis. If bicarb is lower than expected to compensate then you have secondary metabolic acidosis.
What are some causes of an elevated anion gap?
MUD PILES
M: methanol intoxication
U: uremia
D: diabetic ketoacidosis
P: propylene glycol
I: isoniazid
L: lactic acid
E: ethylene glycol intoxication
S: salicylate intoxication
What are the major cations and anions in our blood? How do we calculate the anion gap?
Cations: Na+ (normally 140)
Anions: HCO3- (normally 24) and Cl- (normally 106)
Anion gap = Na+ - (Cl- + HCO3-)
Normally the anion gap is 12
