Early experiences with the stable isotope method in children.

A new technique to estimate water uptake by the gastrointestinal tract was tested in infants undergoing elective surgery. Patients were randomly allocated to receive either sterile water or a glucose-electrolyte solution labeled with the stable isotope H2O18. After absorption, the isotopic O18 comes to rapid equilibrium with plasma bicarbonate and subsequently appears in the breath as C18O2. Breath and blood samples were collected at five-minute intervals after administration of the test solution. Mass spectrometric analysis enabled detection of isotopic O18 enrichment at parts per million levels in blood and in breath CO2. Enrichment of blood and breath C18O2 above baseline levels is detectable within five minutes of administration of the labeled test solution and rises to a plateau between 30 and 90 minutes. It is hypothesized that the rate of increase of enrichment is proportional to the unidirectional flux of water from lumen to gut.