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.
