Acid/Base

Question 1

What is the effect of alkalosis on K status?

Answer 1

High pH causes a shift of K+ into cells from the ECF, creating a greater driving force for K+ secretion; high pH also de-inhibits apical K+ channels, allowing greater flux from cells to the tubule lumen

Alkalosis produces hypokalemia

Question 2

Acetazolamide

Answer 2

Blocks carbonic anhydrase

Weak diuretic action, as distal parts of the nephron compensate for decreased proximal reabsorption of Na+

Causes wasting of bicarbonate in the urine; may cause metabolic acidosis, or be used to treat metabolic alkalosis

Question 3

How much nonvolatile acid is produced each day?

Answer 3

60 mEq / day (sulfuric acid, phosphoric acid)

Question 4

Effect of hypokalemia on plasma pH

Answer 4

Low ECF K+ causes a shift of H+ into cells; in the tubular cells, this means that more H+ is available for secretion, causing an alkalosis

Question 5

Effect of hyperkalemia on plasma pH

Answer 5

Increased ECF K+ causes a shift of H+ out of cells; in the tubular cells, this means that less H+ is availabe for secretion and so H+ is inappropriately retained in the ECF, producing an acidosis

Question 6

Respiratory Alkalosis - Compensation

Answer 6

Decrease in HCO3-

Acutely, HCO3- drops by 2 mEq/L for every 10 mmHg decrease in PCO2

Chronically, HCO3- drops by 4 mEq for every 10 mmHg decrease in PCO2

Question 7

Respiratory acidosis - Compensation

Answer 7

Increased synthesis of bicarbonate

Acutely, bicarbonate increases by 1 mEq/L for every 10 mmHg increase in PCO2

Chronically, HCO3- increases by 4 mEq/L for every 10 mmHg increase in PCO2

Question 8

Post-hypercapnea

Answer 8

Development of metabolic alkalosis in a patient with chronic respiratory acidosis who has received mechanical ventilation; pCO2 is rapidly lowered but compensatory HCO3- remains high

Question 9

Effect of mineralocorticoids on acid/base status

Answer 9

Mineralocorticoids act on the H+ ATPase on the apical membrane of the intercalated cell in the distal tubule, stimulating it to secrete H+ into the tubule lumen, which is accompanied by bicarbonate resorption

Question 10

2 types of metabolic alkalosis

Answer 10

Chloride (saline) responsive

Chlorine (saline) unresponsive

Question 11

Chloride responsive metabolic alkalosis

Answer 11

Urine Cl < 20mEq/L

Low urine chloride reflects Cl- depletion as a major maintenance factor for metabolic alkalosis

Usually associated with intravascular volume depletion

Question 12

Choride unresponsive metabolic alkalosis

Answer 12

Urine Cl > 20 mEq/L

Caused by hyperaldosteronism and Cushing’s syndrome; metabolic alkalosis is generated and maintained by renal H+ loss rather than Cl- depletion

Question 13

What is the effect of Chloride on metabolic alkalosis?

Answer 13

Chloride depletion results in the resorption of bicarbonate by the kidney, maintaining metabolic alkalosis

Question 14

Metabolic alkalosis - Compensation

Answer 14

Decreased ventilation with increased retention of CO2

PCO2 increases from 40 by 0.25-1x the increase in HCO3- over 24

Question 15

Treatment of metabolic alkalosis

Answer 15

Mechanical hypoventilation with maintenance of oxygenation

Chloride responsive - infusion of NaCl (saline)

Chloride resistant - spironolactone to block mineralocorticoid effect

Question 16

Normal serum anion gap

Answer 16

Na - Cl - HCO3- = 9 +/- 3

Question 17

Causes of metabolic acidosis - 2 categories

Answer 17

1. Loss of bicarbonate - Renal vs. extra-renal

2. Addition of acid

Question 18

Renal Tubular Acidosis - 2 types

Answer 18

Proximal renal tubular acidosis - failure of proximal tubule to reabsorb filtered HCO3-

Distal renal tubular acidosis - decreased free H+ excretion

Question 19

Loss of bicarbonate - 2 mechanisms

Answer 19

Renal - urine pH > 5.3 suggests failure of kidney to reabsorb filtered HCO3- or to excrete free H+

Extra-renal - urine pH < 5.3 suggests that the kidney is functioning normally; HCO3- loss is likely GI in origin (diarrhea)

Question 20

Urine anion gap

Answer 20

Urine Na + K - Cl

Indirectly assesses whether NH4+ is increased in response to metabolic acidosis; if NH4+ production is increased the Cl- should also increase to maintain electroneutrality; as Cl- increases, urine AG becomes negative

Negative AG suggests that NH4+ production by the kidney is occurring and that the non-anion gap metabolic acidosis is due to GI loss of HCO3-

Positive AG suggests that NH4+ production is impaired and that an RTA is present

Question 21

Causes of anion gap metabolic acidosis

Answer 21

Methanol
Uremia
DKA
Propylene glycol
Isoniazid
Lactate
Ethylene glycol
Salicylate