Disorders of Water Balance: Hyponatremia Flashcards
What is hyponatremia defined as?
Serum or plasma [Na^+] < 135 mEq/L
What is the normal range for serum [Na^+]?
138 to 142 mEq/L
What factors determine serum [Na^+]?
- Total body Na^+ * Total body K^+ * Total body water (TBW)
What are the potential causes of hyponatremia?
- Increase in total body water * Decrease in Na^+ * Decrease in K^+ * Combination of both
What prerequisite is necessary for the development of hyponatremia?
A defect in kidney water excretion in the presence of normal water intake
What hormone’s high levels contribute to the retention of water in hyponatremic patients?
Antidiuretic hormone (ADH)
What urine osmolality value is indicative of an inability to lower urine osmolality in hyponatremic patients?
<100 mOsm/kg H2O
What is the first step in the approach to a patient with hyponatremia?
Measure Serum Osmolality
What does hypoosmolality rule out in the context of hyponatremia?
Pseudo (factitious)- and hypertonic hyponatremia
What type of hyponatremia is referred to as true hyponatremia?
Hypotonic hyponatremia
What does urine osmolality help differentiate in the context of hypovolemic, hypervolemic, and euvolemic hyponatremia?
Low osmolality (<100 mOsm/kg H2O) from high osmolality (>100 mOsm/kg H2O)
What are the three classifications of hypotonic hyponatremia based on volume status?
- Hypovolemic hyponatremia * Hypervolemic hyponatremia * Normovolemic hyponatremia
What factors should be assessed in the patient’s history when evaluating hyponatremia?
- Fluid loss (diarrhea, vomiting) * Medications * Medical conditions * Intravenous (i.v.) fluid use
What laboratory tests are pertinent in assessing hyponatremia?
- Serum chemistry * Uric acid * Lipid panel * Complete blood count * Fractional excretion of Na^+, uric acid, phosphate
What is pseudohyponatremia?
Artifactually low serum [Na^+] in patients with severe hyperlipidemia or hyperproteinemia
What method is used to determine serum [Na^+]?
Ion-selective electrode (potentiometric) method
What is the correction factor for serum [Na^+] decrease due to hyperglycemia?
1.6 mEq/L for each 100 mg/dL glucose above normal (up to 400 mg/dL)
What is the condition called when serum osmolality is high due to hyperglycemia?
Hypertonic hyponatremia
What defines the osmola gap?
The difference between measured and calculated serum osmolality
What substances can also increase plasma osmolality aside from glucose?
- Urea * Methanol * Ethanol * Ethylene glycol
Fill in the blank: Hyponatremia can develop when water intake is ______ kidney excretion.
greater than
True or False: Patients with pseudohyponatremia are usually symptomatic.
False
