Situation: A nurse on the pediatric surgical ward cares for infants with congenital anomalies of the bowel and nervous system. An infant is awaiting bowel surgery, and the surgeon orders rectal irrigations with warm normal saline. The mother asks why the nurse uses warm normal saline and never tap water. Which explanation is correct?

Tap water is hypotonic; large volumes in a dilated colon are absorbed and can cause water intoxication and hyponatremia in a small infant. Isotonic warm normal saline decompresses the bowel without shifting fluid and electrolytes. Hypertonic phosphate enemas are also avoided in infants.

A rectal irrigation in an infant awaiting bowel surgery is not the same as a routine cleansing enema in an older child or adult. The choice of fluid matters because the infant’s colon is already dilated and its mucosa is vulnerable. The safest fluid is warm isotonic normal saline, and the reason is directly tied to tonicity and fluid shifts.

Tap water is hypotonic relative to plasma. When a hypotonic solution is instilled into a dilated, poorly motile colon, the water does not simply pass through. It moves across the bowel wall along the osmotic gradient into the intravascular space. In a small infant, even a modest volume of absorbed free water can overwhelm renal excretory capacity. The result is water intoxication with dilutional hyponatremia, which can progress to cerebral edema, seizures, and cardiovascular instability. This is why tap water is never used for large-volume rectal irrigations in infants.

Normal saline 0.9% is isotonic. Because its osmolality matches that of plasma, it stays largely within the bowel lumen. It mechanically softens and evacuates stool, decompresses the dilated colon, and does so without creating a gradient that pulls water into the circulation or draws electrolytes out of the body. Warming the saline to body temperature also reduces cramping and prevents hypothermia in a small infant.

Fluid	Tonicity	Effect on fluid/electrolytes	Safety in infant rectal irrigation
Normal saline 0.9%	Isotonic	Minimal net fluid shift; no electrolyte imbalance	Safe, preferred
Tap water	Hypotonic	Absorbed into circulation; dilutional hyponatremia, water intoxication	Avoid
Hypertonic phosphate enema	Hypertonic	Draws water into lumen; risk of hyperphosphatemia, tetany, dehydration	Avoid in infants

Key point! The danger of tap water is not infection or poor stool softening. The primary concern is systemic absorption of free water leading to hyponatremia. Sterility is a separate issue, and enterocolitis is not the expected complication of tap water irrigation in this context.

Watch out! Hypertonic phosphate enemas are also avoided in infants because they can cause severe electrolyte disturbances, including hyperphosphatemia and hypocalcemic tetany. The safest approach for bowel decompression before surgery is warm isotonic saline irrigation.