Alkalosis, Metabolic

Essentials of Diagnosis

• High HCO₃^– with alkalemia

• Evaluate effective circulating volume by physical examination

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

General Considerations

• Etiology can be classified into saline responsive or saline unresponsive (Table 21–15)

• Saline responsive

  • A sign of extracellular volume contraction

  • Much more commonly encountered than saline-unresponsive alkalosis, which implies excessive total body bicarbonate with either euvolemia or hypervolemia

  • Characterized by normotensive extracellular volume contraction and hypokalemia

  • Hypotension or orthostatic hypotension may be seen

  • Generally associated with hypokalemia, due partly to the direct effect of alkalosis on renal potassium excretion and partly to secondary hyperaldosteronism from volume contraction

• Saline unresponsive

  • Implies a volume-expanded state as from hyperaldosteronism with accompanying hypokalemia from the renal mineralocorticoid effect

Symptoms and Signs

• No characteristic symptoms or signs

• Hypotension and orthostasis may occur

• Weakness and hyporeflexia occur if serum K⁺ is markedly low

• Tetany and neuromuscular irritability occur rarely

• Arterial blood pH and bicarbonate are elevated

• With respiratory compensation, arterial PCO₂ is increased

• Serum potassium and chloride are decreased

• There may be an increased anion gap

• The urine chloride can differentiate between saline-response (< 25 mEq/L) and unresponsive (> 40 mEq/L) causes

Medications

Saline-responsive

• Correct the extracellular volume deficit with isotonic saline

• Discontinue diuretics

• H₂-blockers or proton pump inhibitors may be helpful in patients with alkalosis from nasogastric suctioning

• If pulmonary or cardiovascular disease prohibits adequate resuscitation, acetazolamide will increase renal bicarbonate excretion

• Hypokalemia may develop because bicarbonate excretion may induce kaliuresis

• Dialysis with low-bicarbonate dialysate may be required in severe cases, especially when reduced kidney function is present

Saline-unresponsive

• Blockage of aldosterone effect with a mineralocorticoid antagonist

Therapeutic Procedures

• Mild alkalosis is generally well tolerated

• Severe or symptomatic alkalosis (pH > 7.60) requires urgent treatment

Surgery

• Therapy for saline-unresponsive metabolic alkalosis includes surgical removal of a mineralocorticoid-producing tumor

When to Refer

• If expertise is needed for the work-up

• For treatment of saline-unresponsive metabolic alkalosis

When to Admit

• For persistent metabolic alkalosis in the absence of hypovolemia or associated with severe hypokalemia

• If administration of acid is needed for emergency therapy