In an infant with congenital hypertrophic pyloric stenosis, what is the typical electrolyte disturbance and how should it be corrected before definitive surgical pyloromyotomy?

Electrolyte Disturbances in Congenital Hypertrophic Pyloric Stenosis

Typical Electrolyte Pattern

Infants with congenital hypertrophic pyloric stenosis classically present with hypochloremic, hypokalemic metabolic alkalosis due to persistent non-bilious vomiting and loss of gastric hydrochloric acid. 1, 2

The characteristic biochemical derangements include:

• Hypochloremia (low serum chloride) occurs in approximately 28% of patients at presentation 1
• Hypokalemia (low serum potassium) develops in approximately 9% of cases 1
• Metabolic alkalosis (elevated serum bicarbonate) is the most common finding, present in approximately 71% of infants at presentation 1
• Paradoxical aciduria may occur despite systemic alkalosis, as the kidneys attempt to conserve hydrogen ions while excreting potassium 3

Preoperative Correction Protocol

Surgery must be delayed until complete correction of fluid and electrolyte abnormalities is achieved, as pyloric stenosis is a metabolic emergency requiring biochemical stabilization, not a surgical emergency. 2, 4

Fluid Resuscitation Strategy

• Administer isotonic saline (0.9% NaCl) with added potassium chloride as the primary resuscitation fluid to correct both volume depletion and chloride deficit 1
• Calculate chloride replacement needs based on the severity of metabolic alkalosis: approximately 10 mmol/kg of chloride is required to reduce plasma bicarbonate by 3 mmol/L on average 1
• Avoid chloride-free solutions during initial resuscitation, as chloride replacement is essential for correcting the metabolic alkalosis 1

Potassium Replacement

• Do not administer potassium until adequate urine output is established (>1 mL/kg/hour) to avoid hyperkalemia in oliguric patients 5
• Add potassium chloride to maintenance fluids once urine output is confirmed, typically at 20-40 mEq/L in IV fluids 1
• Potassium repletion will fail if concurrent hypomagnesemia is not corrected, so assess and replace magnesium as needed 6

Monitoring Requirements

Monitor the following parameters every 6-12 hours during initial resuscitation: 5, 1

• Serum sodium, potassium, chloride, and bicarbonate
• Blood urea nitrogen and hematocrit (to assess volume status)
• Urine output and specific gravity
• Body weight (to track fluid balance)
• Acid-base status (venous or arterial blood gas)

Surgical Readiness Criteria

Proceed to pyloromyotomy only when ALL of the following are achieved: 1, 2, 4

• Serum chloride >100 mmol/L
• Serum potassium >3.5 mmol/L
• Serum bicarbonate <30 mmol/L (ideally <28 mmol/L)
• Adequate urine output (>1 mL/kg/hour)
• Clinical signs of adequate hydration (normal skin turgor, moist mucous membranes)

Common Pitfalls to Avoid

• Rushing to surgery before complete metabolic correction increases perioperative complications and anesthetic risk 2, 4
• Using balanced or hypotonic solutions instead of normal saline with KCl will not adequately correct the chloride deficit driving the metabolic alkalosis 1
• Failing to establish urine output before potassium administration risks life-threatening hyperkalemia 5
• Inadequate chloride dosing based solely on serum chloride rather than the severity of alkalosis will result in incomplete correction 1
• Assuming normal electrolytes rule out significant dehydration: the severity of metabolic alkalosis is a more reliable indicator of fluid deficit than physical examination alone in vomiting infants 1