Approach to Metabolic Acidosis and Alkalosis

Question 1

Metabolic Acidosis

Answer 1

Metabolic acidosis is defined as a primary decrease in the bicarbonate concentration of the plasma.

Question 2

What is the respiratory adaptation for metabolic acidosis?

Answer 2

Hyperventilation to reduce PCO2

Question 3

What is [H+] at pH = 7.4?

Answer 3

[H+] = 80

Question 4

How is the anion gap calculated?

Answer 4

AG = (Na+) - (HCO3‐ + Cl‐)

Question 5

What are the top 3 most represented ions in the body?

Answer 5

1. Na+
2. Cl-
3. HCO3-

Question 6

What is the normal range of the anion gap?

Answer 6

9-12 mEq

Question 7

What are the causes of Increased Anion Gap Acidosis?

Answer 7

• Increased organic acid production

- Failure to excrete inorganic anions

Question 8

How does generation of organic acid increase the anion gap?

Answer 8

Generation of organic acid will lead to the titration of the HCO3- which will decrease its levels by a proportionate
amount

Question 9

What are the causes of Normal Anion Gap (Hyperchloremic) Acidosis?

Answer 9

• Gastrointestinal loss of bicarbonate
• Renal loss of bicarbonate
• Failure to excrete acid
• Administration of acid
• Administration of large amounts of saline (dilution)

Question 10

How does Type II renal tubule acidosis cause acidosis?

Answer 10

Disease of the PT that leads to decreased filtration
of HCO3- but with complete reabsorption - HCO3- still
decreased because of decreased filtration

Question 11

How does Type I renal tubule acidosis cause acidosis?

Answer 11

Disease of the DT that is due to several mechanisms:

1. Inability to excrete H+ ion - problem with H+ ATPase
2. Reuptake of H++ ions

Question 12

Metabolic Alkalosis

Answer 12

Metabolic alkalosis is defined as a primary increase in the bicarbonate concentration of the plasma.

Question 13

What is the respiratory adaptation for metabolic alkalosis?

Answer 13

Hypoventilation to increase PCO2

Question 14

What are some causes of metabolic alkalosis?

Answer 14

• Net loss of H+ from ECF
• Net addition of HCO3‐ to ECF
• Chloride depletion (contraction)

Question 15

What happens with HCl loss from the stomach?

Answer 15

If HCl is lost from the body because of vomiting or gastric drainage then there is a net gain of bicarbonate

Question 16

What does aldosterone excess cause and how?

Answer 16

Metabolic alkalosis.

It causes hypokalemia which will shift K+ from inside the cells out of the cells to try and maintain equilibrium. H+ will be taken into the cells causing increased plasma pH

Question 17

What can cause HCO3- gain?

Answer 17

• Exogenous alkali administration 
– bicarbonate
– lactate 
– citrate 
– acetate

Question 18

How does the GI have excess Cl- loss?

Answer 18

• Secretion of chloride into stool (villous adenoma)

* Failure of gut reabsorption of chloride (congenital chloridorrhea)

Question 19

Bartter’s syndrome

Answer 19

Genetic defect of the loop diuretic‐sensitive Na‐ K‐2Cl cotransporter in the loop of Henle (“endogenous loop diuretic”)

Question 20

Gitelman’s syndrome

Answer 20

Genetic defect of the thiazide‐sensitive Na‐Cl cotransporter in the distal tubule (“endogenous thiazide”)

Question 21

What causes skin loss of Cl-?

Answer 21

Cystic fibrosis ‐‐ loss of chloride in excess of bicarbonate in sweat

Question 22

What happens to ionized Ca2+ with increased pH?

Answer 22

Increased pH will increase the amount of Ca2+ bound to albumin - decreased ionized fraction

Question 23

Metabolic Alkalosis Lab Findings

Answer 23

Elevated [HCO3‐], decreased [Cl‐], usually low [K+]

Question 24

Chloride‐responsive type Metabolic alkalosis

Answer 24

Kidney is avidly reabsorbing chloride because of persistent volume (and chloride) depletion that developed during the generation of metabolic alkalosis.

Question 25

Chloride‐resistant type (urine Cl‐ > 20 mmol/L) Metabolic alkalosis

Answer 25

Mineralocorticoid excess states ‐‐ can be either primary (low renin) of secondary (high renin) - hyperaldosteronism

Question 26

What can be used for treatment of metabolic alkalosis?

Answer 26

• Potassium administration
• Acetazolamide
• Volume repletion
• Intravenous HCl or NH4Cl

Question 27

A 70‐year‐old man presents with weakness.

• Blood chemistries reveal (inmmol/L): Na+145, K+3.0, Cl‐ 120, CO2 16; (in mg/dL): Urea nitrogen 30, Creatinine 1.2.
• Arterial blood gases (ABG) reveal: pH 7.35, PCO2 30 mmHg, HCO3‐ 16 mEq/L.

Q: What is the acid base disturbance?
A. Metabolic acidosis
B. Metabolic alkalosis
C. Mixed metabolic acidosis/metabolic alkalosis 
D. No metabolic disturbance

Answer 27

A. Metabolic acidosis

Question 28

A 70‐year‐old man presents with weakness.

• Blood chemistries reveal (inmmol/L): Na+145, K+3.0, Cl‐ 120, CO2 16; (in mg/dL): Urea nitrogen 30, Creatinine 1.2.
• Arterial blood gases (ABG) reveal: pH 7.35, PCO2 30 mmHg, HCO3‐ 16 mEq/L.

What is the nature of the metabolic acidosis?

A. High‐anion gap acidosis
B. Normal anion gap (hyperchloremic) acidosis

Answer 28

B. Normal anion gap (hyperchloremic) acidosis

145 - (120+16) = 9

Question 29

A 30‐year‐old female presents with fatigue and weakness. She denies vomiting or diarrhea and takes no medications. Her physical examination is normal except for mild orthostatic hypotension. Blood chemistries reveal (in mmol/L): Na+ 138, K+ 2.9, Cl‐ 90, CO2 40. Arterial blood gases (ABG) reveal: pH 7.50; PCO2 53 mmHg; HCO3‐ 40 mEq/L. The urine pH is 7.5. Urine chemistries reveal (in mEq/L): Na+ 30 K+ 30 Cl‐ 5.

Q: What is the acid base disorder?

A. Metabolic acidosis
B. Metabolic alkalosis
C. Mixed metabolic acidosis/metabolic alkalosis
D. No metabolic disturbance

Answer 29

B. Metabolic alkalosis

Question 30

A 30‐year‐old female presents with fatigue and weakness. She denies vomiting or diarrhea and takes no medications. Her physical examination is normal except for mild orthostatic hypotension. Blood chemistries reveal (in mmol/L): Na+ 138, K+ 2.9, Cl‐ 90, CO2 40. Arterial blood gases (ABG) reveal: pH 7.50; PCO2 53 mmHg; HCO3‐ 40 mEq/L. The urine pH is 7.5. Urine chemistries reveal (in mEq/L): Na+ 30 K+ 30 Cl‐ 5.

What is the likely diagnosis?

A. Diuretic intake
B. Bartter’s syndrome
C. Gitelman’s syndrome
D. Surreptitious vomiting
E. Hypokalemic periodic paralysis

Answer 30

D. Surreptitious vomiting