2025 – PAGE 375 – 376 – FLUIDS & ELECTROLYTES

2025 – Core Study Guide 20 – Fluids and Electrolytes 2025 – PAGE 375 – 376 – FLUIDS & ELECTROLYTES

HYPONATREMIA

Hyponatremia is MUCH more common than hypernatremia. Look for a sodium level less than 130 mEq/L. If the level is < 120, watch out for seizures! Seizures can occur as free water starts moving into CNS cells at about 125 mEq/L. For babies, remember NOT to give free water until at least 6 months of age. That’s when their kidneys can handle the job of getting rid of excess water. Regarding diagnosing WHY there is hyponatremia, always consider:

    • volume status (especially the effective circulating volume)
    • serum osmolality
    • urine osmolality
    • urine sodium

Regarding correction of sodium, remember that rapid correction (faster than 12 mEq/day) can result in central pontine myelinolysis.

  • PEARL: It’s possible that you will only be given a urine sodium level. For the exam, use a urinary sodium of 20 as your cutoff for the LOWER LIMIT of normal. So, if a urine sodium level is < 20, the kidneys are either holding on to sodium tightly, or they are not holding on to water tightly enough. If you get confused, it’s probably in your best interest to assume that a low urine sodium reflects low/decreased sodium excretion.
  • PEARL: Formula to correct = (desired Na – measured Na) x Weight in kg x 0.6. Then ADD 3 mEq/kg as the daily maintenance amount of sodium needed. This will give you the total amount of sodium needed for the next 24 hours to reach the desired sodium level you input into the equation, which should never be greater than 12 above the measured sodium. So, if measured is 115, desired should be no more than 127.
    • SHORTCUT: If the desired change in sodium is 12 mEq over 24 hours, then 12 x 0.6 = 7.2, and a simpler formula for the correction of sodium = (7 x Weight in kg) + (3 mEq x Weight in kg). Using basic algebra, this comes out to 10 x Weight in kg. So, 10 mEq of sodium is needed per kg of weight to raise the sodium by 12 mEq/l over the next 24 hours. This formula includes the sodium needed for maintenance. It does NOT calculate what kind of sodium-containing solution is needed, or how much water should be given. Those can vary depending on the clinical scenario, for example, the degree of estimated dehydration.
  • DEHYDRATION:
    • Children with hyponatremic dehydration have hypotonic body fluids with serum osmolarity less than 270 mOsm/kg (270 mmol/kg). This can lead to fluid shifts from the extracellular to the intracellular space.
    • The urine will be concentrated, so expect a HIGH urine osmolality (> 400 mOsm/kg).
    • The kidneys are working to retain sodium/water, so expect a LOW urine sodium. This is a KEY POINT. Do not assume that a concentrated urine means an elevated urine sodium.
  • DIURETICS: May vary depending on the diuretic, but in general, if a diuretic is at fault for hyponatremia, that means it’s forcing sodium out. Expect a HIGH urine sodium and probably a LOW urine osmolality since free water is being forced out as well.
  • ACUTE TUBULAR NECROSIS & RENAL FAILURE: Expect a HIGH urine sodium because the kidneys are not able to do their job of holding onto sodium.
  • GASTROENTERITIS and DIARRHEA: This usually does NOT cause a hyponatremia until the patient is only given (or is only tolerating) hypotonic liquids (like free water). The patient is volume depleted due to GI losses, so the kidneys hold onto as much sodium as they can. Therefore, the urine Na+ is LOW (very low, often < 10). If there happens to be a metabolic alkalosis from vomiting, then the urine sodium could be normal or high, but the chloride level will still be low (low yield).
  • PSYCHOGENIC POLYDIPSIA: Refers to patients who just tend to drink a lot of water on a fairly chronic basis. Serum becomes dilute, and the kidneys are excreting as much water (using sodium to do it) as possi­ble. So look for the serum osmolality and urine osmolality to be low.
  • ACUTE WATER INTOXICATION: This refers to drinking lots of water over a very short time. Look for a healthy, afebrile child having change in mentation or a seizure after being around a body of water (tub/pool). Think of it as a VERY acute issue, so total body sodium is normal. This is a hypervolemic hyponatremia, though the child will not be edematous and will look euvolemic. Again, the serum osmolality and urine osmolality will be low.
  • NEPHROTIC SYNDROME: Patients are edematous due to 3rd spacing from hypoalbuminemia. The total body water (TBW) is normal, BUT the intravascular department is depleted. Patients can get hyponatremia. Assume it’s the same as being dehydrated. Urine sodium is LOW (very low, often < 10).   
  • SYNDROME OF INAPPROPRIATE ADH SECRETION (SIADH): Excess ADH results in excess water retention and an INCREASED total body water (and sometimes BP). Urine output may be decreased to < 1 cc/kg/day. There is serum hyposmolality (<300 mOsm) and hyponatremia because of retained water. Urine is concentrated/hyperosmolar (>100 mOsm). Urine sodium is > 25 (normal to high). Possible etiologies of SIADH include brain tumor/bleed, pneumonia or other lung disease, cancer, infection, recent surgery, and Guillain Barre. It’s a diagnosis of exclusion, so other etiologies of hyponatremia should be ruled out first. Treat with fluid RESTRICTION. Use Demeclocycline (diuretic) or Fludrocortisone if fluid restriction is not working and you note that the sodium level is persistently < 120.
    • MNEMONIC: Assume the urine sodium being high in SIADH because there is no drive to hold on to sodium given the hypervolemic state. Part of the kidney must hold on to water due to the ADH, but there is no problem with sodium excretion, so that part of the kidney is trying to EXCRETE sodium in an effort to excrete water.
  • (DOUBLE TAKE) PSEUDOHYPONATREMIA: The short story: PSEUDOHYPONATREMIA results when glucose levels exceed the upper limit of normal by 100. This results in a decrease in sodium by 1.6 for every increase in glucose by 100.
    The long story: PSEUDOHYPONATREMIA is a phenomenon that typically refers to the hyperosmolar state caused by hyperglycemia in which the increased oncotic pressure in the extracellular space pulls water from the intracellular space. This dilutes the sodium in the extracellular fluid, resulting in a decreased sodium concentration but possibly normal total body sodium. Pseudohyponatremia can ALSO refer to a situation in which elevated triglycerides or proteins (both are big) cause the measured sodium per overall volume (not sodium per volume of water) to be low. This is likely a low-yield fact, but it could be seen in a nephrotic case with hyperlipidemia.
  • CEREBRAL SALT WASTING: This is low-yield and complicated; it is controversial whether it even exists. The blood and urine electrolyte picture is the same as in SIADH with low serum sodium and osmolality and inappropriately high urine sodium and osmolality. The important difference between CSW and SIADH is that in CSW there is volume depletion. Look for hyponatremia, high urine sodium and osmolality, and clinical evidence of hyponatremia such as hypotension, high hematocrit, and poor skin turgor.
Previous Topic
Back to Lesson
Next Topic