2025 – PAGE 369 – ACID-BASE DISORDERS

RENAL TUBULAR ACIDOSIS TYPE IV (RTA IV)

Renal tubular acidosis type IV (RTA IV) is LOW-YIELD. But if you must know, look for a NON-GAP acidosis and HYPERKALEMIA. Both RTA I and II have HYPOkalemia. NH4Cl secretion is intact, so urine pH is < 5.5. This is low-yield because it’s almost never seen in children. It can be seen in hypOaldosteronism and diabetes mellitus.

MNEMONIC: If you take the IV in RTA IV and turn the V on its side, you kind of get I<, which looks like a K and should remind you of ↑<⁺, hyperkalemia for RTA IV.

MNEMONIC PEARL: RTAs and diarrhea both cause a non-gap metabolic acidosis. If given the urine chloride value, it can help. Urine chloride is LOW in RTAs (< 15) and HIGH in diarrhea (> 15). If you understand the RTA section, you will know this is not correct, but the mnemonic to follow could help you remember this subtle point. RTA is at the level of the kidney. As one would imagine, the kidney can’t get rid of HCl (or NH4Cl), so urine Cl is LOW (<15). In diarrhea, the kidneys are just fine, so they can get rid of HCl, so urine Cl is HIGH. In all likelihood, you will only come across RTA I on the exam, so you should be fine.

METABOLIC ALKALOSIS

PEARL: If you see a HYPOKALEMIA in a patient with a metabolic alkalosis, FIX THE POTASSIUM FIRST! Otherwise the alkalosis probably will not correct.

The best way to categorize a metabolic alkalosis is by whether it is chloride-responsive or chloride-unresponsive.

• CHLORIDE RESPONSIVE: Look for a urine chloride of < 15. If you see that, then assume that the body is STARVING for chloride and holding onto as much as possible. You need to give normal saline!
  • VOMITING: Due to loss of HCl
  • DEHYDRATION: Causes a contraction alkalosis through multiple mechanisms in the kidney in an effort to compensate. It’s especially bad when you are already vomiting out HCl!
  • DIURETICS: Occurs with loop diuretics and thiazides, but only if there is insufficient dietary chloride (like in a patient on NaCl/salt restriction). If a metabolic alkalosis is given in a patient on a diuretic, the patient will also likely have a concurrent hypokalemia.
    • PEARLS: Loop diuretics can block reabsorption of Na+, K+, and Cl- and cause hyponatremia, hypokalemia, and hypochloremia. So, they usually cause a hypochloremic hypokalemic metabolic alkalosis. BUT furosemide can also cause hypernatremic dehydration (we won’t go into why), hypercalciuria without hypocalcemia (which can result in stones/nephrocalcinosis), and hypomagnesemia.
    • THIAZIDES (like hydrochlorothiazide or chlorthalidone): Cause sodium and chloride losses. They can also cause hypokalemia. Bicarb is not lost.
  • PEARL: Chronic laxative use, diuretics, and excessive vomiting all cause hypOkalemic hypOchloremic metabolic ALKALOSIS. (KNOW THIS!)
• CHLORIDE UNRESPONSIVE: Look for a urine chloride of > 15. If you see that, then assume that the body has plenty of chloride and has no interest in getting more (therefore, saline will not help).
  • CUSHING’S SYNDROME: More to come
  • HYPERALDOSTERONISM: More to come
  • BARTTER, GITELMAN, and LIDDLE SYNDROMES: If a metabolic alkalosis is presented with a high urine chloride and hypokalemia, keep these syndromes in mind. If any of these are in your answer choices, keep the pearls/rules below in mind.
    • BARTTER SYNDROME(AKA BARTTER’S SYNDROME): LOW to normal blood pressure. Findings identical to those on loop diuretics: hypokalemic, hypochloremic metabolic alkalosis, with high urinary K, Cl.
    • GITELMAN SYNDROME(AKA GITELMAN’S SYNDROME): NORMAL blood pressure.
    • LIDDLE SYNDROME (AKA LIDDLE’S SYNDROME): HIGH blood pressure with low levels of renin and aldosterone.
    • MNEMONIC: The blood pressure follows the alphabet for these! BGL, B – G – L = LOW – NORMAL – HIGH.