Acid/Base Disorders

Question 1

How do you determine if an acid base disturbance is simple or mixed?

Answer 1

You assess the compensation. If the compensation is not as expected, then an additional acid base disorder is present and the acid base picture is mixed. An acid base disturbance is “mixed” when 2 or more acid base disturbances are present.

Question 2

What causes respiratory alkalosis?

Answer 2

Hyperventilation

• anxiety, fever, pain
• lung disease
• liver disease
• sepsis
• brain disease
• pregnancy

Question 3

How can you tell if respiratory alkalosis is acute or chronic?

Answer 3

Acute is before renal compensation and chronic is after renal compensation (3-5 days)

Question 4

What are the compensation rules for acute respiratory alkalosis and chronic respiratory alkalosis?

Answer 4

Acute compensation

Bicarbonate goes down 2 for every 10 change in PCO₂

Chronic compensation

Bicarbonate goes down 4 for every 10 change in PCO₂

Question 5

pH 7.5

P_CO2 20

HCO₃^- 20

P_aO2 80

What’s going on?

Answer 5

Acute Respiratory Alkalosis

For acute compensation, the bicarbonate drops 2 for every 10 P_CO2 dropped. So, a P_CO2 of 20 is a 20 P_CO2 drop (from a normal value of 40). So, you would expect a bicarbonate drop of 4. The bicarbonate is 20 (normal value is 24) so this matches what is expected.

Question 6

What are the compensation rules for acute respiratory acidosis and chronic respiratory acidosis?

Answer 6

For acute respiratory acidosis, bicarbonate goes up by 1 for every 10 that P_CO2 goes up.

For chronic respiratory acidosis, the bicarbonate goes up by 4 for every 10 that P_CO2 goes up.

Question 7

pH 7.3

P_CO2 50

HCO₃^- 25

P_aO2 58

What’s going on?

Answer 7

Acute Respiratory Acidosis

Remember…

Acute compensation is 1:10

Chronic compensation is 4:10

Question 8

What is contraction alkalosis?

Answer 8

It is a form of metabolic alkalosis where you lose chloride-rich fluid but not bicarbonate. After fluid loss, the amount of bicarbonate doesn’t increase but the concentration does.

Examples of this are vomiting, NG suctioning, and diuretics (loop and HCTZ)

Question 9

Metabolic alkalosis can happen from increased renal proton excretion. In the kidneys, whenever a H⁺ is excreted a bicarbonate is reabsorbed. What are 2 settings that can cause this?

Answer 9

• Mineralocorticoid excess
  
  • Primary hyperaldosteronism
  • Cushing’s syndrome
  • Congenital adrenal hyperplasia
  • Hyperreninism
  • Renal artery stenosis
• Diuretics
  
  • Reabsorbing Na causes a negative lumen which favors proton movement out into the lumen. This results in reabsorption of bicarbonate.

Question 10

How can hypovolemia cause metabolic alkalosis?

Answer 10

Hypovolemia causes Na resorption. Na resorption created a negative lumen which favors H⁺ movement into the lumen causing bicarbonate resorption.

Question 11

How can you tell if metabolic alkalosis is being caused by hypovolemia? If it’s caused by hypovolemia, how do you treat?

Answer 11

Check urine chloride.

In hypovolemia, chloride is also reabsorbed. So, if urine chloride is low (less than 20), you should consider that the metabolic alkalosis may be caused by hypovolemia.

This is treated with normal saline with sodium and chloride

Question 12

What is the compensation rule for metabolic alkalosis?

Answer 12

PCO₂ increases between 0.25 to 1 per increase of 1 HCO₃^-

Question 13

July 4th picnic, vomiting 24 hrs, BP 90/40, HR 125

pH 7.5

P_CO2 50

HCO₃^- 34

Urine Na 25

Urine Cl 9

What’s going on?

Answer 13

Chloride responsive metabolic alkalosis due to loss of H⁺ from vomiting with appropriate respiratory compensation.

0.25 to 1 P_CO2 per 1 bicarbonate.

The bicarbonate went from 24 to 34 which is a change of 10. So, the P_CO2 should increase between 2.5 and 10. The P_CO2 increased by 10 which is within what is expected from the rule.

The urine Cl is less than 10 which suggests that the patient is chloride is deficient.

Question 14

Failure of kidney to handle the daily acid load results in metabolic acidosis (aka renal tubular acidosis). What are the 2 general categories of RTA?

Answer 14

Proximal RTA (failure to resorb bicarbonate)

Distal RTA (failure to secrete H+ and generate bicarbonate – resulting in urine anion gap that is positive)

Question 15

Is urine anion gap positive in Renal loss of bicarbonate (e.g. RTA) or GI (e.g. diarrhea, surgical drains, fistulas)?

Answer 15

Renal

Question 16

Explain how serum and urine anion gap works for metabolic acidosis

Answer 16

When bicarbonate is lost in either GI or kidneys, Cl goes up to compensate.

So, in renal and GI loss of bicarbonate, serum anion gap is normal.

Serum anion gap calculation:

Na - ( HCO₃^- + Cl^- ) = 6-12

Urine anion gap is positive for renal loss and negative for GI loss.

Determining the cause of a metabolic acidosis that lacks a serum anion gap often depends on determining whether the kidney is appropriately excreting acid. The urine anion gap is an ‘artificial’ and calculated measure that is representative of the unmeasured ions in urine. Usually the most important unmeasured ion in urine is NH4+ since it is the most important form of acid excretion by the kidney. Urine NH4+ is difficult to measure directly, but its excretion is usually accompanied by the anion chloride. A negative urine anion gap can be used as evidence of increased NH4+ excretion. In a metabolic acidosis without a serum anion gap:

A positive urine anion gap suggests a low urinary NH4+ (e.g. renal tubular acidosis causing renal loss of bicarbonate).

A negative urine anion gap suggests a high urinary NH4+ (e.g. diarrhea causing GI loss of bicarbonate).

Question 17

What is the compensation rule for metabolic acidosis?

Answer 17

1 to 1.5 decrease of P_CO2 per 1 decrease of bicarbonate

Question 18

4th of july picnic, diarrhea for 3 days

pH 7.2

PCO₂ 26

HCO₃^- 10

Serum Na 140, Cl 120, HCO₃^- 10

Urine Na 10, K 10, Cl 40

Answer 18

Non Anion Gap Metabolic Acidosis from GI bicarbonate loss with appropriate respiratory compensation

Compensation rule: 1 to 1.5 decrease of PCO2 per 1 decrease of bicarbonate.

Bicarbonate went from 24 to 10 which is a change of 14. The compensation should be between 14 and 21. The PCO₂ is 26 which is a change of 14 which matches the rule.

Serum anion gap = 140 - 130 = 10. normal anion gap means GI or Renal bicarbonate loss.

Urine anion gap = 10 + 10 - 40 = -20. Negative anion gap means GI bicarbonate loss.

Remember that urine anion gap is used to indirectly measure ammonium elimination. Ammonium isn’t easily measured in urine so you use the anion gap to estimate it. When ammonium excretion is increased, Cl is increased too. So, with a negative anion gap, that means Cl is high which means ammonium excretion is high too. In GI bicarbonate loss, the kidneys try to compensate for the acidic state by increasing ammoniagenesis to bind to the H⁺s to eliminate as ammonium. This excess ammonium in urine shows up as a negative anion gap.

Question 19

Metabolic acidosis can happen from intake of acid. Explain why this causes an anion gap metabolic acidosis.

Answer 19

When you ingest an acid, the H⁺ from the acid will consume HCO₃^- causing metabolic acidosis. The conjugate base anions are still sticking around after the proton left so there is an anion gap. Remember that in non anion gap metabolic acidosis, when bicarbonate is lost, Cl is increased to compensate. This doesn’t happen in this case because the conjugate base anions are present. However, these anions can’t be measured in the serum so you get an anion gap due to the loss of bicarbonate and a Cl that doesn’t increase.

Question 20

What can cause anion gap metabolic acidosis?

Answer 20

KARL

• Ketones (metabolism)
  
  • Starvation (ketones)
  • DKA (ketones)
• Aspirin and other toxins/ingestions
  
  • Ethylene glycol ( → glycolic acid → oxalic acid)
  • Methanol (formaldehyde → formic acid)
  • Ethanol (ketones)
  • Aspirin (acetylsalicylic acid → salicylic acid)
• Renal failure (sulfate, urate, phosphate hippurate)
• Lactic acid (hypoxia)