Section 4: Metabolic Acidosis and Alkalosis Flashcards
List the causes of metabolic acidosis with an increased anion gap
- Methanol
- Uremia
- Diabetic ketoacidosis
- Paraldehyde
- INH; Iron toxicity
- Lactic acidosis
- Ethylene glycol
- Rhabdomyolysis
- Salicylates
Use the mnemonic “MUDPILERS”
List the causes of normal anion gap metabolic acidosis
- Diarrhea
- Renal tubular acidosis
What is the pathophysiology of lactic acidosis?
Rx?
This is caused by any form of hypoperfusion, such as hypotension, resulting in anaerobic metabolism. Anaerobic metabolism leads to glycolysis, which results in the accumulation of lactic acid.
Treat the underlying cause of hypoperfusion.
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8048-8072). . Kindle Edition.
What is the pathophysiology of aspirin overdose?
Rx?
Aspirin overdose originally gives respiratory alkalosis from hyperventilation. Over a short period, metabolic acidosis develops from poisoning of mitochondria and the loss of aerobic metabolism. This gives lactic acidosis.
Treat with bicarbonate, which corrects the acidosis and increases urinary excretion of aspirin
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8048-8072). . Kindle Edition.
What is the pathophysiology of methanol toxicity?
Rx?
This toxic alcohol leads to formic acid and formaldehyde production
Look for an intoxicated patient with visual disturbance. After getting a methanol level, order fomepizole or ethanol administration. These substances block the production of formic acid and allow time for dialysis to remove the methanol
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8048-8072). . Kindle Edition.
How does renal failure lead to anion gap metabolic acidosis?
Renal failure prevents the excretion of the 1 mEq/ kg of organic acid (uremic acid) that is formed each day. This is an indication for dialysis
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8048-8072). . Kindle Edition.
What acids are produced in diabetic ketoacidosis that leads to the increased anion metabolic acidosis?
- Acetone
- Acetoacetate
- Betahydroxybutyric acid
Rx of metabolic acidosis in DKA
A low serum bicarbonate is the fastest single test to tell if a patient’s hyperglycemia is life threatening.
Treat with normal saline hydration and insulin. Place the patient in the ICU.
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8070-8082). . Kindle Edition.
Rx of ethylene glycol toxicity
Treat the same as methanol intoxication with fomepizole or ethanol, which blocks the production of oxalic acid and allows time for dialysis to remove the ethylene glycol.
Look for an intoxicated patient with a renal abnormality, such as oxalate crystals in the urine. There is also renal failure and hypocalcemia, because the oxalate binds with calcium to form crystals. Suicide attempt with ethylene glycol is key.
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8070-8082). . Kindle Edition.
Explain the pathophysiology of normal anion gap metabolic acidosis in diarrhea
Diarrhea causes metabolic acidosis via increased bicarbonate loss from the colon. The colon secretes both bicarbonate and potassium, so the potassium level will be low (hypokalemia) as well. Because there is increased chloride reabsorption, there is hyperchloremia, and that is why there is a normal anion gap.
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8086-8090). . Kindle Edition.
List the clinically significant types of renal tubular acidosis (RTA)
RTA Type I (Distal RTA)
RTA Type II (Proximal RTA)
RTA Type IV (hyporeninemic hypoaldosteronism)
Explain the pathophysiology of distal RTA (Type I)
There is an inability to excrete hydrogen ions (acids) in the distal tubule. This results in the accumulation of acid in the body. The urine pH rises because the body cannot excrete acid. In an alkaline environment, stones will form. Serum potassium is low (body excretes + ions in the form of K + since it can’t excrete H +) and serum bicarbonate is low
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8091-8112). . Kindle Edition.
How to diagnostically test for distal RTA?
Rx of RTA Type I
- Test by intravenously administering acid (ammonium chloride— which should lower urine pH secondary to increased H + formation). In distal RTA the person cannot excrete the acid, and the urine pH stays abnormally basic
- Treat by administering bicarbonate. The proximal tubule is still working; therefore the patient will still absorb the bicarbonate
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8091-8112). . Kindle Edition.
Explain the pathophysiology of proximal RTA (Type II)
There is an inability to reabsorb bicarbonate in the proximal renal tubule. Initially there is an elevated urine pH, but when the body loses substantial amounts of bicarbonate, the urine pH drops. Because urine pH is often low, kidney stones do not develop. A low serum bicarbonate leaches calcium out of the bones, and there is also osteomalacia
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8091-8112). . Kindle Edition.
Diagnostic testing for proximal RTA
Rx?
Test by giving bicarbonate. A normal person with metabolic acidosis will absorb all of the bicarbonate, and there should still be a low urine pH in a normal patient. In proximal RTA, the patient cannot absorb the bicarbonate and the urine pH rises from the bicarbonate malabsorption
Treat by giving a thiazide diuretic, which results in a volume contraction. The contracted blood volume raises the concentration of serum bicarbonate. Large quantities of serum bicarbonate are also given (bicarbonate is generally ineffective and that is why they must be used in such high amounts).
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8091-8112). . Kindle Edition.
CF of hyporeninemic hypoaldosteronism (Type IV)
There is decreased aldosterone production or effect. Look for a diabetic patient with a normal anion gap metabolic acidosis. This is the only RTA with an elevated potassium level.
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8113-8115). . Kindle Edition.
Rx of RTA type IV
Treat with aldosterone administration in the form of fludrocortisone, which is the steroid with the highest mineralocorticoid content
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8117-8158). . Kindle Edition.
Compare the types of RTA
- Types I has high urine pH while types II, and IV have low urine pH
- Both types I and II have low serum potassium while type IV has high serum potassium
- Type I has stone but not types II and IV
- Test for type I by giving acid, for type II by giving bicarbonate. There is urine sodium loss in type IV.
- Treatment for type I is with bicarbonate; for type IIwith thiazide diuretic and high dose bicarbonate; type IV with fludrocortisone
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8117-8158). . Kindle Edition.
How do you distinguish between diarrhea and RTA as the cause of the normal anion gap metabolic acidosis?
- ** Urine Anion Gap (UAG)**
UAG = Urine Na+ – Urine Cl-
When acid is excreted from the kidney, it goes out as NH4Cl. Acid excretion from the kidney goes out with chloride
If you can excrete acid from the kidney, the urine chloride goes up. If the urine chloride is up, then the number (UAG) is negative. Diarrhea causes a negative UAG, because the kidney can excrete acid and the net UAG is negative. In metabolic acidosis, a negative UAG means the kidney works.
If you cannot excrete acid from the kidney, the urine chloride goes down. This gives a positive number (UAG). In RTA, you cannot excrete acid from the kidney. The urine chloride will be low, and the UAG will be positive.
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8158-8161). . Kindle Edition.
List the causes of metabolic alkalosis
- Volume contraction
- Conn’s syndrome or Cushing’s syndrome
- Hypokalemia
- Milk-alkali syndrome
- Vomiting
How does volume contraction lead to metabolic alkalosis?
Rx?
Volume contraction leads to metabolic alkalosis because there is a secondary hyperaldosteronism, which causes increased urinary loss of acid
Treat the underlying cause
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8162-8179). . Kindle Edition.
How does Conn’s syndrome or Cushing’s syndrome lead to metabolic alkalosis?
Rx?
Hyperaldosteronism resulting from primary hyperaldosteronism (Conn syndrome) or Cushing syndrome causes urinary acid loss
Surgically remove the adenoma. Also look for hypokalemia, which often accompanies the increased urinary acid loss
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8162-8179). . Kindle Edition.
How does hypokalemia lead to metabolic alkalosis?
Hypokalemia causes metabolic alkalosis, because potassium ions shift out of the cell to correct the hypokalemia. This shifts hydrogen ions into the cell in exchange for the potassium ions leaving
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8162-8179). . Kindle Edition.
How does Milk-Alkali syndrome result in metabolic alkalosis?
Metabolic alkalosis occurs from the administration of too much liquid antacid
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8162-8179). . Kindle Edition.
How does vomiting result in metabolic alkalosis?
Vomiting causes a loss of acid from the stomach. In addition, the loss of fluids leads to volume contraction and secondary hyperaldosteronism.
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8162-8179). . Kindle Edition.
