How does excessive vomiting cause metabolic alkalosis?

How Excessive Vomiting Causes Metabolic Alkalosis

Excessive vomiting causes metabolic alkalosis primarily through the loss of hydrochloric acid from the stomach, leading to a decrease in hydrogen ions and chloride, which results in increased serum bicarbonate levels and blood pH. 1

Pathophysiological Mechanism

The development of metabolic alkalosis from vomiting involves two key phases:

1. Generation Phase

• Loss of Gastric Acid: Vomiting expels gastric contents containing hydrochloric acid (HCl), resulting in:
  • Depletion of hydrogen ions (H+)
  • Loss of chloride ions (Cl-)
  • Relative increase in bicarbonate concentration in the blood

2. Maintenance Phase

Several factors contribute to sustaining the alkalotic state:

• Volume Depletion: Leads to:

  • Increased proximal tubular reabsorption of sodium and bicarbonate
  • Reduced glomerular filtration rate (GFR)
  • Activation of the renin-angiotensin-aldosterone system

• Chloride Depletion: Critical factor that:

  • Impairs the kidney's ability to excrete excess bicarbonate
  • Creates "chloride-depletion alkalosis" 2

• Hypokalemia: Commonly occurs with vomiting and:

  • Promotes intracellular shift of H+ ions
  • Stimulates renal ammoniagenesis
  • Enhances bicarbonate reabsorption in the kidney

• Secondary Hyperaldosteronism: Develops due to volume depletion and:

  • Increases H+ secretion in distal tubule
  • Enhances sodium reabsorption and potassium excretion
  • Further worsens hypokalemia

Laboratory Findings

Typical laboratory abnormalities in vomiting-induced metabolic alkalosis include:

• Elevated serum bicarbonate (>26 mmol/L)
• Elevated arterial pH (>7.43)
• Hypochloremia (decreased serum chloride)
• Hypokalemia
• Increased blood urea nitrogen (BUN) due to volume depletion
• Compensatory increase in PaCO₂ due to hypoventilation
• Paradoxical aciduria (despite systemic alkalosis)

Clinical Manifestations

Metabolic alkalosis from excessive vomiting may present with:

• Lethargy and confusion
• Muscle weakness and cramps
• Cardiac arrhythmias (especially with hypokalemia)
• Hypotension (due to volume depletion)
• Tetany (in severe cases)
• Respiratory depression (compensatory hypoventilation)

Treatment Approach

The cornerstone of treatment for vomiting-induced metabolic alkalosis is:

1. Correction of Volume Depletion:

   • Isotonic saline (0.9% NaCl) administration to restore intravascular volume
   • Addresses both hypovolemia and chloride deficiency

2. Electrolyte Replacement:

   • Potassium chloride supplementation to correct hypokalemia
   • Chloride replacement to enable renal bicarbonate excretion 1

3. Treatment of Underlying Cause:

   • Address the cause of vomiting (e.g., antiemetics for nausea, treatment of pyloric stenosis)

4. Monitoring:

   • Serial assessment of acid-base status and electrolytes
   • Careful attention to fluid status

Special Considerations

• In cases of severe alkalosis (pH >7.55), more aggressive correction may be required due to associated increased mortality 3
• High cation-gap amino acid solutions may be beneficial in treating chloride-depletion alkalosis with hyponatremia 2
• Paradoxical metabolic acidosis can rarely occur with severe vomiting due to other concurrent processes 4

Understanding this pathophysiological sequence is crucial for appropriate management of patients with vomiting-induced metabolic alkalosis, as addressing both the generation and maintenance factors is essential for successful treatment.