4 Acid/Base concepts

Question 1

What is the acid/base homeostasis equation of the body?

Answer 1

CO₂ + H₂O <-> H₂CO₃ <-> HCO₃^- + H⁺

Question 2

What is the normal pH of the blood?

Answer 2

7.4

Question 3

Where are HCO₃^- and H⁺ handled in the renal tubules?

Answer 3

HCO₃^- = proximal tubule (carbonic anhydrase)

H⁺= collecting duct

Question 4

What is a normal pCO₂? What is it called when the body deviates from this normal?

Answer 4

Normal= 40 mmHg

**changes from normal usually due to frequency of breathing:

>40= acidosis (hypoventilating)

<40= alkalosis (hyperventilating)

Question 5

What is a normal HCO₃^- ? What is it called when the body deviates from this normal?

Answer 5

Normal= 24 mmol/L

**changes from normal usually due to changes in metabolism:

<24 = acidosis

>24 = alkalosis

Question 6

What is the only combination of acid-base disturbances that cannot happen simultaneously?`

Answer 6

A respiratory acidosis and respiratory alkalosis (cannot be both hyper and hypoventilating)

Question 7

For metabolic acidosis, how do you calculate the estimated compensation?

Answer 7

• in metabolic acidosis, you have low HCO₃^-
• to calculate your expected repiratory compensation (aka change in pCO₂)…
  
  • pCO₂= 1.5 x [HCO₃^-] + 8
  • pCO₂= last 2 digits of pH (e.g. pH=7.21, expect pCO₂= 21)

Question 8

For metabolic alkalosis, how do you calculate the estimated compensation?

Answer 8

• in metabolic alkalosis, you have high HCO₃^-
• to calculate your expected repiratory compensation (aka change in pCO₂)…
  
  • pCO₂= 15 + [HCO₃^-]
  • HOWEVER, you have a maximum pCO₂ of 55 mmHg (because you can only decrease your breathing so low before you need to breath)

Question 9

For respiratory acidosis, how do you calculate the estimated compensation?

Answer 9

• in respiratory acidosis, you have high pCO₂
• to calculate your expected metabolic compensation (aka change in HCO₃^-)…
  
  • acute: bicarb increases 1 mmol/L per 10 mmHg increase in pCO₂
  • chronic: bicarb increases 4 mmol/L per 10 mmHg increase in pCO₂

Question 10

For respiratory alkalosis, how do you calculate the estimated compensation?

Answer 10

• in respiratory alkalosis, you have low pCO₂
• to calculate your expected metabolic compensation (aka change in HCO₃^-)…
  
  • acute: bicarb decreases 2 mmol/L per 10 mmHg decrease in pCO₂
  • chronic: bicarb decreases 4 mmol/L per 10 mmHg decrease in pCO₂

Question 11

What are some causes for an elevated anion gap?

Answer 11

• acid has been added exogenously or increased endogenously in the body
• MUDPILES:
  
  • Methanol intoxication
  • Uremia (renal failure)
  • Diabetic ketoacidosis
  • Propylene glycol (drug vehicle)
  • Isoniazid
  • Lactic acid
  • Ethylene glycol intoxication
  • Salicylate intoxication

Question 12

Describe a normal anion gap

Answer 12

Normally 10-12

Question 13

How do you calculate the anion gap?

Answer 13

Gap = Na⁺ - (Cl^- + HCO₃^-)

e.g. normally…. 140 - (104 + 24) = 12

Question 14

How is the anion gap used clinically?

Answer 14

• Helps physicians find how much of a substance a patient may have ingested (mudpiles)
• e.g. you get a metabolic panel and see the anion gap is 27. If you have a previous panel (or assume normal is 10-12), you can subtract 27-12 and find the patient may have ingested 15 mEq of acid

Question 15

What is a non-anion gap metabolic acidosis?

Answer 15

• If a patient has a normal calculated anion gap, but still low bicarbonate (you would expect an elevated gap in acidosis)
• possible causes= ingestion of HCl or renal tubular acidosis
  
  • Cl is buffered with Na, decreasing NaHCO₃ levels but you need to add the NaCl to the level there before the acidosis