A 68 year old woman presents with an obviously incarcerated umbilical hernia. She has gross abdominal distention, is clinically dehydrated, and repos persistent fecaloid vomiting for the past 3 days. Although tired, weak, and thirsty, she is awake and ale and her sensorium is not paicularly affected. Laboratory analysis reveals a serum sodium concentration of 118 mEq/L. Which of the following is the most likely physiologic explanation for the serum sodium?
Gastrointestinal tract fluids have a sodium concentration very close to that of plasma; as they are lost (internally or externally), they should be replaced with isotonic, sodium-containing fluids. But that is not what patients typically do at home. Thirsty and unable to eat solid (sodium-containing) foods, they drink water, Coke, and tea, fluids without significant amounts of sodium, which the body avidly retains because of the severe volume depletion. Endogenous water from catabolic activity is also retained. Dilutional hyponatremia eventually develops. She does not have "water intoxication" . This term denotes abnormal water retention due to excessive water infusion at a time when there is a high level of ADH in the blood. This patient is retaining water because she is desperately volume-depleted, not because high volumes of water are being forced into her. The hyponatremia is not due to the loss of hypeonic fluid . There are no hypeonic fluids in the gut, or anywhere else for that matter. The only hypeonic fluid that we can lose is highly concentrated urine, but we usually do so as a physiologic response to save water. Yes, we often see comatose and convulsing patients when they have this much hyponatremia , but that happens when water retention is massive and fast. Slow water retention allows the brain to adapt. One can see even lower serum sodium concentrations in patients with a clear sensorium. Ref: Molina P.E. (2013). Chapter 10. Endocrine Integration of Energy and Electrolyte Balance. In P.E. Molina (Ed), Endocrine Physiology, 4e.
She has acute water intoxication
She has been vomiting and trapping hypeonic fluids in the bowel lumen
She has vomited and sequestered sodium-containing fluids, and has retained endogenous and ingested water
There must be a laboratory error, because such a serum sodium level would have produced coma