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Key Features

Essentials of Diagnosis

  • 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

General Considerations

  • Most common causes

    • Gastrointestinal (GI) HCO3 loss

    • Defects in renal acidification (renal tubular acidoses)

  • The urinary anion gap can differentiate between these two causes

  • Table 21–13 outlines anion gap in metabolic acidosis

Table 21–13.Anion gap in metabolic acidosis.1
Renal tubular acidosis (RTA)

  • 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 the clinical setting: urinary pH, urinary anion gap (see below), serum K+ level

  • Type I (distal H+ secretion defect)

    • Due to selective deficiency in H+ secretion in the distal nephron

    • Low serum K+

    • Despite acidosis, urinary pH cannot be acidified (urine pH > 5.5)

    • Associated with autoimmune disease, hypercalcemia

  • Type II (proximal HCO3 reabsorption defect)

    • Due to a selective defect in the proximal tubule's ability to adequately reabsorb filtered HCO3

    • Low serum K+

    • Urine pH < 5.5

    • Associated with plasma cell myeloma and nephrotoxic drugs

  • Type IV (hyporeninemic hypoaldosteronism)

    • Only RTA characterized by hyperkalemic, hyperchloremic acidosis

    • Defect is aldosterone deficiency or antagonism, which impairs distal nephron Na+ reabsorption and K+ and H+ excretion

    • Urine pH < 5.5

    • Renal salt wasting is frequently present

    • Most common in diabetic nephropathy, tubulointerstitial renal diseases, AIDS, and hypertensive nephrosclerosis

Clinical Findings

Symptoms and Signs

  • Symptoms are mainly those of the underlying disorder

  • Compensatory hyperventilation may be misinterpreted as a primary respiratory disorder

  • When acidosis is severe, Kussmaul breathing (deep, regular, sighing respirations) occur and are indicative of intense stimulation of the respiratory center

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