Acidosis, Metabolic, Normal Anion Gap

KEY FEATURES

Essentials of Diagnosis

• Decreased HCO₃^– and hyperchloremia (usually) with acidemia

• Normal anion gap acidosis is mainly caused by loss of bicarbonate in the gastrointestinal (GI) track or renal tubular acidosis (RTA)

General Considerations0.

• The hallmark of this disorder is that the low HCO₃^– of metabolic acidosis is associated with hyperchloremia, so that the anion gap remains normal

• Decreased HCO₃^– is seen also in respiratory alkalosis, but the pH distinguishes between the two disorders

• Two major causes

  • Bicarbonate loss from the GI tract

  • Defects in renal acidification (RTA)

• The compensatory increase in serum chloride (hyperchloremia) maintains electroneutrality and a normal anion gap

• The urinary anion gap can differentiate between these two causes

• Table 23–12 outlines anion gap in metabolic acidosis

Gastrointestinal HCO₃^– Loss

• The GI tract secretes bicarbonate at multiple sites

• The most common cause of a non-anion gap metabolic acidosis from the GI tract is diarrhea (loss of bicarbonate rich stool fluid)

• An infrequent cause is a ureterosigmoidostomy, where ureters are implanted into the sigmoid colon for urinary diversion

  • Unlike the bladder, colonic mucosa secretes bicarbonate in exchange for chloride, resulting in metabolic acidosis

  • This procedure is now rarely performed in the United States, though remains popular in other countries

• More commonly, a neobladder is created using a loop of bowel (generally ileum or colon)

  • This procedure has significantly decreased the incidence of metabolic acidosis

  • Non-anion gap metabolic acidosis, however, can still occur when contact time between urine and mucosa is increased, typically as a result of an anastomotic stricture

Renal Tubular Acidosis

• Defined as hyperchloremic acidosis with a normal anion gap and normal or near-normal glomerular filtration rate, in the absence of diarrhea

• Three major types of RTA can be differentiated by ...