#51 – Renal Tubular Acidosis



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Definition

  • Group of disorders that cause a metabolic acidosis due to defects in the renal tubules
    • Net retention of HCl
    • Net loss of NaHCO3

Pathophysiology

The kidney regulates acid-base balance two main ways:

  • Reabsorption of filtered HCO3
    • >80% of the bicarbonate filtered by the glomerulus is reabsorbed in the proximal renal tubules via Na-H exchange
  • Acid excretion
    • Collecting ducts of the nephron excrete hydrogen ions buffered by NH3 and PO3 (so the pH of the urine doesn’t destroy the nephron)
      • Extra production of NH3 is stimulated by intracellular acidosis.
  • 3 step process
    • Reabsorption of sodium to create a negative gradient in the tubular lumen
    • Excretion of hydrogen by H-K-ATPase and reabsorption of potassium
    • Prevention of hydrogen ions from diffusing back out of the tubular lumen

Initial Presentation

  • Patients diagnosed with an RTA must first be diagnosed with a metabolic acidosis
    • Decreased pH with decreased HCO3
  • After this is determined, the anion gap must be calculated and found to be normal
    • AG = Na – (Cl + HCO3) = 8-12

Differential for NAGMA

  • Ureteric diversion
  • Small bowel fistulae
  • Excessive saline
  • Diarrhea
  • Carbonic anhydrase inhibitors
  • Renal tubular acidosis
  • Adrenal insufficiency
  • Pancreatic fistulae

Type I (distal) RTA

  • Cause
    • Defect in the distal hydrogen ion excretion
  • Pathophysiology
    • Failure of the H-ATPase proton pump (most common cause)
      • Inability to acidify urine < 5.5
      • Hypokalemia
    • Increased hydrogen ion permeability of the luminal membrane

Type II (proximal) RTA

  • Cause
    • Defect in proximal bicarbonate reabsorption
  • Pathophysiology
    • Damage to the proximal tubule that leads to progressive bicarbonate wasting in the urine

Type IV (hypoaldosteronism)

  • Cause
    • Reductions in aldosterone secretion and responsiveness
  • Pathophysiology
    • Decreased rate of proton secretion rather than an intrinsic defect in the tubule’s capacity to generate normal pH gradient
    • Hyperkalemia causes reduced urine NH4, which in turns leads to more acidic urine
      • Hydrogen ions have nothing to bind to

Diagnostic Work-Up

  • RTAs should be considered in any patient with a normal anion gap metabolic acidosis
    • Need ABG and BMP
  • Once this determination is made:
    • Urine pH
      • > 5.5 in type I (distal)
      • < 5.5 in type II (proximal) and type IV
    • Urine ammonium
      • Elevated in type II (proximal)
      • Decreased in type I (distal) and type IV
      • Most labs can’t measure urine ammonium directly:
        • Urine Anion Gap (urine Na+K+Cl)
          • (+) UAG = > 20
            • Type I (distal) and type IV
          • (-) UAG = < – 20
            • Type II (proximal)
    • Serum potassium
      • Elevated in type IV
      • Decreased in type I and II

Treatment

  • Type I (distal)
    • Urinarary Alkali Therapy
      • Sodium bicarbonate
        • Increased risk of nephrolithiasis due to bicarbonaturia
          • Use potassium citrate instead
  • Type II (proximal)
    • Much more difficult to treat due to the INCREASED bicarbonate diuresis during bicarbonate therapy
    • Alkali therapy (10x the dose for type I) AND potassium salt repletion as bicarbonaturia INCREASES urinary potassium losses
    • Thiazide diuretics if large alkali doses ineffective or not tolerated
      • Diuresis reduces urinary bicarbonate loss by increasing proximal sodium reabsorption
        • Which secondarily increased bicarbonate reabsorption
  • Type IV
    • Stop any medication causes or treat underlying condition (hypoaldosteronism)
      • Mineralcorticoid (fludrocortisone) and glucocorticoid (hydrocortisone)
    • Potassium repletion
Up-To-Date. 2019



References

  1. Rodríguez Soriano J. Renal tubular acidosis: the clinical entity. Journal of the American Society of Nephrology : JASN. 2002; 13(8):2160-70. [pubmed]
  2. Skelton LA, Boron WF, Zhou Y. Acid-base transport by the renal proximal tubule. Journal of nephrology. ; 23 Suppl 16:S4-18. [pubmed]
  3. Hamm LL, Nakhoul N, Hering-Smith KS. Acid-Base Homeostasis. Clinical journal of the American Society of Nephrology : CJASN. 2015; 10(12):2232-42. [pubmed]
  4. The Curbsiders.  Episode 104. https://thecurbsiders.com/internal-medicine-podcast/104-renal-tubular-acidosis-kidney-boy-joel-topf-md
  5. DB’s Medical Rants.  http://www.medrants.com/archives/8897
  6. Oh M, Carroll HJ. Value and determinants of urine anion gap. Nephron. 2002; 90(3):252-5. [pubmed]
  7. Rodríguez Soriano J. Renal tubular acidosis: the clinical entity. Journal of the American Society of Nephrology : JASN. 2002; 13(8):2160-70. [pubmed]
  8. Karet FE. Mechanisms in hyperkalemic renal tubular acidosis. Journal of the American Society of Nephrology : JASN. 2009; 20(2):251-4. [pubmed]

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