How Vomiting Causes Metabolic Alkalosis
Vomiting causes metabolic alkalosis primarily through the loss of gastric acid (HCl), which leads to a decrease in hydrogen ions and chloride in the body, resulting in an increase in serum bicarbonate and pH.
Pathophysiological Mechanism
Initial Phase: Direct Loss of Acid
- When vomiting occurs, hydrochloric acid (HCl) from the stomach is lost
- This directly reduces the body's hydrogen ion content
- Loss of chloride ions creates hypochloremia
Volume Depletion and Renal Compensation
Volume contraction effects:
- Vomiting leads to dehydration and decreased effective circulating volume
- This activates the renin-angiotensin-aldosterone system (RAAS)
- Increased aldosterone promotes sodium reabsorption and potassium/hydrogen ion excretion in the distal tubule
Paradoxical aciduria:
- Despite systemic alkalosis, the urine becomes paradoxically acidic
- This occurs because the kidneys prioritize volume restoration over acid-base balance
- Sodium is reabsorbed with bicarbonate in the proximal tubule, while hydrogen ions are secreted
Hypokalemia development:
- Potassium is lost in vomitus
- Volume depletion and aldosterone increase promote further potassium excretion
- Hypokalemia worsens metabolic alkalosis through intracellular-extracellular hydrogen ion shifts
Maintenance Phase: Chloride Depletion
- Chloride depletion is critical in maintaining metabolic alkalosis
- Low chloride availability limits the kidney's ability to excrete bicarbonate
- This creates "chloride-responsive" metabolic alkalosis, where chloride replacement is necessary for correction
Laboratory Findings
- Blood gases: Elevated pH, increased bicarbonate
- Electrolytes: Hypochloremia, hypokalemia
- Urine electrolytes: Low urinary chloride (<10 mEq/L), high urinary sodium and potassium
- Other findings: Possible hypocalcemia, increased neutrophil count
Clinical Manifestations
- Neurological: Lethargy, confusion, seizures (in severe cases)
- Cardiovascular: Arrhythmias (especially with hypokalemia)
- Musculoskeletal: Muscle weakness, cramps
- Respiratory: Compensatory hypoventilation
Treatment Approach
- Fluid resuscitation: Chloride-rich solutions (normal saline) to correct volume depletion and provide chloride
- Electrolyte replacement: Potassium supplementation for hypokalemia
- Addressing underlying cause: Treating the cause of vomiting
Special Considerations
- In patients with renal insufficiency, metabolic alkalosis may be more persistent due to impaired ability to excrete bicarbonate
- Chronic vomiting (as in eating disorders) can lead to more severe and treatment-resistant metabolic alkalosis
- Paradoxically, some conditions with vomiting may present with metabolic acidosis instead, particularly if there is concurrent starvation ketosis or renal dysfunction
Understanding this pathophysiology is essential for proper diagnosis and management of patients presenting with vomiting and acid-base disturbances.