Metabolic Acidosis

Question 0

What’s the anion gap?

Answer 0

Anion gap = Na - (Cl + HCO3)

It’s the mEq of anions that aren’t Cl- and HCO3-, which are often the anions of acids.

Question 1

4 types of cause of metabolic acidosis?

Answer 1

Loss of bicarb.
Insufficient bicarb production.
Increased endogenous organic acid generation.
Ingestion of exogenous acid (or substance that becomes acid).

Question 2

What’s a normal anion gap?

Answer 2

About 10.

Question 3

2 organs from which bicarb can be lost, causing a metabolic acidosis?

Answer 3

Renal loss.
GI loss (diarrhea, usually).

Question 4

If an added or retained organic acid is causing metabolic acidosis, what will the ion gap be?

Answer 4

The anion gap will be >10, because the acid’s anion is present.

Question 5

What kind of anion gap does diabetic ketoacidosis produce?

How about acidosis caused by alcohol ingestion?

Answer 5

Both produce high anion gap metabolic acidosis.

Question 6

What kind of anion gap will acidosis from reduced GFR produce? Why?

Answer 6

The anion gap will be high.

There is failure of bicarb absorption/production, but there is also retention of phosphoric and sulfuric acid.

Question 7

3 types of renal tubular acidosis (RTA)?

Answer 7

Proximal (Type II) - failure or bicarb reabsorption.
Distal (Type I) - defect in acid excretion (bicarb generation).
Distal hyperkalemic (Type IV) - defect in acid excretion (ammoniagenesis).

Question 8

How does proximal RTA affect the amount of bicarb that appears in the urine?

Answer 8

As bicarb isn’t being absorbed as well in proximal RTA, bicarb will start to appear in the urine at lower serum bicarb concentrations.

Question 9

Review: How is bicarb absorbed in the proximal tubule? Name 4-5 required proteins.

Answer 9

Na/K ATPase generates Na+ gradient.
Na+/H+ exchanger moves H+ into lumen. (and H+ pump)
HCO3- + H+ –> H2O + CO2 via carbonic anhydrase.
CO2 + H2O diffuse into cell, carbonic anhydrase converts to HCO3- + H+.
Na/HCO3 cotransporter moves HCO3 to blood.
(note that an H+ keeps cycling from lumen to cytosol, but bicarb is reabsorbed)

Question 10

Is HCO3 reabsorption normal in distal RTA?

Answer 10

Yes.

Question 11

What can’t be done to the urine in distal RTA?

Answer 11

The urine can’t be acidified.

Question 12

There are 5 listed mechanism for distal RTA. 3 involved impaired proton secretion. What are those 3 precise mechanisms?

Answer 12

H+ ATPase defect. (primary or autoimmune)
H+/K+ defect. (rare, toxins)
H+ backleak. (amphotericin toxicity)

Question 13

2 mechanisms for distal RTA that don’t directly involve H+ secretion?

Answer 13

Defect in Carbonic Anhydrase II (usually also causes proximal RTA).
Defect in HCO3/Cl exchanger (brings HCO3 to blood).

These are rare.

Question 14

How is serum K+ in both proximal and distal RTA?

Answer 14

Typically a little low.

(several reasons… including increased aldosterone, increased distal Na+ delivery,

Question 15

What broader syndrome can proximal RTA be a part of?

Answer 15

Fanconi Syndrome: Impaired glucose, amino acid, bicarb, etc. reabsorption.

Question 16

What underlies hyperkalemic RTA?

Answer 16

Aldosterone insufficiency / lack of response to aldosterone.

Question 17

2 main endogenous acids that cause metabolic acidosis?

Answer 17

Lactic acid

Ketoacids

Question 18

5 common toxins that cause an exogenous acidosis?

Answer 18

Methanol.
Ethylene glycol.
Diethylene glycol.
Toluene.
Aspirin.

Question 19

Difference between Type A and Type B lactic acidemia?

Answer 19

Type A: Caused by relative hypoxia (shock, severe anemia, etc.)
Type B: Caused by impaired metabolism of lactate.

Question 20

What’s the osmolar gap?

Answer 20

The difference between observed and calculated osmolality.

Question 21

What’s a common osmolyte implicated in high osmotic-gap acidosis?

Answer 21

Ethanol is the most common osmolyte found in high osmotic gap acidosis.

Question 22

What’s the old treatment for methanol poisoning? Why?

Answer 22

Ethanol, as it would compete for alcohol dehydrogenase, decreasing conversion of methanol to formic acid.

Question 23

What’s the significance of delta anion gap / delta HCO3?

Answer 23

If there is just one process happening, the magnitude of change of the anion gap and the HCO3 gap should be about the same.
If there’s mismatch, another acid-base disorder may be co-occurring.

Question 24

4 clinical features of acidosis?

Answer 24

Hyperventilation.
Hemodynamic compromise.
Hyperkalemia.
Musculoskeletal problems (when chronic).

Question 25

How do the lungs compensate in acidemia?

Answer 25

Increased ventilation -> reduced pCO2

Question 26

How can you judge if the respiratory compensation to acidemia is appropriate?

Answer 26

Winter’s formula.

Question 27

What is Winter’s formula?

Answer 27

Expected pCO2 = 1.5*bicarb + 8 +/- 2.

in metabolic acidosis

Question 28

Affect of metabolic acidosis on NH4+ synthesis / excretion?

Answer 28

Increased synthesis and excretion.

Question 29

Treatment for severe acidosis?

Answer 29

Give bicarb (or equivalent such as citrate).

Question 30

Risks of giving NaHCO3?

Answer 30

Hypernatremia /volume overload.
Hyperkalemia.
CO2 generation (bad if there’s too much).
Overshoot alkalosis.