A 4-week-old infant is admitted with a diagnosis of hypertrophic pyloric stenosis. The infant has been vomiting for the past week and shows signs of dehydration. Which nursing intervention should be the priority immediately after admission?

The priority nursing intervention is to establish IV access and monitor electrolyte balance to correct fluid and electrolyte imbalances before surgery. Other options are contraindicated or less immediate.

Understanding Hypertrophic Pyloric Stenosis (HPS)

Infantile hypertrophic pyloric stenosis (IHPS) is a condition where the circular muscle of the pylorus becomes markedly thickened, leading to progressive gastric outlet obstruction. In a 4-week-old infant, this manifests as projectile, non-bilious vomiting. The vomitus is non-bilious because the obstruction is proximal to the duodenum, where bile enters the gastrointestinal tract. Over a week of persistent vomiting, the infant loses large amounts of gastric secretions, which are rich in hydrochloric acid (HCl).

The Priority: Physiological Correction Before Surgical Correction

The immediate priority is not to rush to surgery, but to stabilize the infant’s fluid and electrolyte status. The correct intervention is to establish intravenous access and monitor electrolyte balance. While a pyloromyotomy is the definitive surgical treatment for HPS, it is an urgent, not an emergent, procedure. The infant has been vomiting for a week, which means a significant metabolic derangement has developed that must be corrected preoperatively to ensure safe anesthesia and a positive surgical outcome.

The Pathophysiology of the Metabolic Derangement

The persistent vomiting of gastric contents leads to a specific and predictable set of electrolyte imbalances:
- Loss of Hydrochloric Acid (HCl): The stomach’s parietal cells secrete HCl. Vomiting depletes hydrogen ions (H⁺) and chloride ions (Cl⁻) from the body.
- Hypochloremia: Serum chloride levels drop significantly because Cl⁻ is lost directly in the vomitus.
- Metabolic Alkalosis: The loss of H⁺ causes the blood pH to rise, resulting in a hypochloremic metabolic alkalosis. This is a hallmark finding in HPS.
- Hypokalemia: Initially, the kidneys attempt to compensate for the alkalosis by excreting potassium (K⁺) in exchange for retaining H⁺. Over time, this renal compensation, combined with poor intake, leads to total body potassium depletion and hypokalemia.
- Paradoxical Aciduria: In the face of systemic alkalosis, the kidney’s priority becomes sodium and volume conservation due to dehydration. The distal tubules will excrete H⁺ in exchange for sodium, even if the body is already alkalotic. This results in acidic urine despite the blood being alkaline, a classic finding in advanced HPS.

Analyzing the Interventions Based on the Provided Evidence

The study by Saleem et al. (2026) directly supports the principle of prioritizing fluid and electrolyte correction. The research highlights that infants with IHPS present with "significant metabolic derangement requiring preoperative fluid resuscitation" ^[1]. The study compared conventional fluid therapy with a chloride-guided bolus approach, with the entire premise being that correcting the biochemical imbalance is the essential first step before any surgical intervention ^[1]. The goal of therapy is to guide the infant safely to the operating room, not to rush them there while physiologically unstable.

- Intervention 1: Begin oral rehydration therapy with small, frequent feedings.
This is incorrect and dangerous. The infant has a mechanical gastric outlet obstruction. Any oral intake, even small amounts, will likely be vomited, worsening fluid and electrolyte losses and increasing the risk of aspiration. The gastrointestinal tract is not a functional route for rehydration in this scenario.

- Intervention 2: Position the infant in a prone position to prevent aspiration.
This is incorrect. The standard position to reduce the risk of aspiration in a vomiting infant is upright or with the head of the bed elevated, not prone. Placing an infant prone also increases the risk of sudden infant death syndrome (SIDS). The priority is to manage the airway, but the correct positioning is not prone.

- Intervention 3: Prepare the infant for immediate surgical intervention.
This is incorrect and reflects a failure to recognize the urgency of medical stabilization before surgery. Taking an infant with a hypochloremic, hypokalemic metabolic alkalosis to the operating room creates a high risk for life-threatening cardiac arrhythmias and respiratory depression under anesthesia. The preoperative optimization with intravenous fluids, as the focus of the cited study, is the standard of care ^[1].

- Intervention 4: Establish intravenous access and monitor electrolyte balance.
This is the correct priority. The immediate nursing intervention is to secure IV access to begin fluid resuscitation, typically with an isotonic solution like 0.9% sodium chloride, which provides the chloride needed to correct the alkalosis. Simultaneously, blood is drawn for a baseline metabolic panel to assess the severity of the hypochloremia, hypokalemia, and metabolic alkalosis. The study’s focus on a "chloride-guided" protocol underscores that monitoring serum chloride is a key indicator of resuscitation progress ^[1]. Potassium replacement is added to IV fluids only after adequate urine output is confirmed, as administering potassium to a patient with poor renal function can cause dangerous hyperkalemia. The infant is ready for surgery only when the metabolic alkalosis is corrected, typically evidenced by a serum chloride level above 100 mEq/L and a bicarbonate level below 30 mEq/L. This process of medical stabilization turns a surgical emergency into a safe, planned procedure.