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ESSENTIALS OF DIAGNOSIS

ESSENTIALS OF DIAGNOSIS

  • High HCO3 with alkalemia (high pH).

  • Evaluate effective circulating volume by physical examination.

  • Urinary chloride concentration differentiates saline-responsive alkalosis from saline-unresponsive alkalosis.

GENERAL CONSIDERATIONS

Metabolic alkalosis is characterized by high serum HCO3 levels. The development of metabolic alkalosis requires its “generation” from loss of acid or gain of alkali, and its “maintenance” from the kidney’s inability to excrete excess bicarbonate.

The causes of metabolic alkalosis are classified into two groups based on chloride responsiveness and are generally distinguished using urine chloride values (Table 21–15). The compensatory increase in PCO2 rarely exceeds 55 mm Hg; higher PCO2 values imply a superimposed primary respiratory acidosis.

Table 21–15.Metabolic alkalosis.

A. Chloride-Responsive Metabolic Alkalosis (UCl less than 20 mEq/L)

Chloride-responsive metabolic alkalosis involves the loss of chloride and extracellular volume. In vomiting and nasogastric suction, loss of acid (HCl) generates the alkalosis and volume contraction from chloride loss maintains the alkalosis. Distally acting diuretics that cause chloride loss, eg, loop and thiazide diuretics, are a common cause of metabolic alkalosis. UCl levels can be unreliable in these settings since these diuretics increase UCl. These disorders cause concurrent hypokalemia, which can exacerbate metabolic alkalosis by stimulating H+ secretion and ammoniagenesis. In respiratory acidosis, the kidneys compensate by increasing renal HCO3 retention, which causes volume expansion and hence NaCl excretion. If the hypercapnia is corrected rapidly, the kidneys will attempt to correct the alkalosis by excreting HCO3; if sufficient Cl is unavailable, bicarbonaturia will halt and metabolic alkalosis will persist. This process has been termed posthypercapneic metabolic alkalosis.

In alkalosis, bicarbonaturia causes obligatory sodium excretion as the accompanying cation and UNa levels are unreliable markers of extracellular volume.

B. Chloride-Unresponsive Alkalosis (UCl more than 20 mEq/L)

1. Excess mineralocorticoid ...

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