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Hyponatremia
Conceptually similar to Hypernatremia, Hyponatremia is typically (an excess of) WATER to sodium. See the diagnostic algorithm, approach to management, and calculator below.
Hyponatremia is the most common electrolyte abnormality in hospitalized pediatric patients. This can present in hypo-, eu- and hypervolemic states. Disorders can be thought of those losing salt in the urine or losing salt elsewhere, thereby causing their urine to reabsorb as much salt as possible. One may also think of ADH dependent (intact renal function) vs ADH independent hyponatremia (the "system" is chronically reset, such as beer potomania or renal failure).
The first thing we want to confirm is whether this is true hyponatremia. If you recall your osmotic equation:
2Na + Glucose/18 + BUN/2.8 (we convert glucose and BUN to mmol/L). Normal serum osmolality is 275-295 mOsm/kg H2O.
The reason we care about sodium is not for sodium sake per se (though Na+ has important functions as well), but for the osmolality it provides, i.e. loss or gain of sodium changes serum osmolality affects the vascular fluid status as well as other organs, such as the brain.
So as long as there is enough serum osmolality, the sodium level is less important, but we need to confirm sodium/osmlolaity is normal as sodium provides most of the serum osmolality.
Sodium on a chemistry is estimated after dilution, not directly measured and it is based on a standard plasma volume. (VBG or ABG directly measure sodium, so we consider these more reliable in dysnatremias).
Therefore, conditions such as hyperlipidemia and hyperproteinemia which change the underlying plasma volume assumptions by reducing the water content, can cause the sodium on the analyzer to read low after dilution.
Hyperglycemia is an osmotic draw, thereby reducing the estimated serum sodium as well. When you correct the glucose, the sodium levels return to normal.
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For every 100mg of glucose above 100mg/dL, sodium decrease by 1.6 mEq/L.
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E.g. serum glucose is 750mg/L, calculated Na 125mEq/L, corrected Na+= ~135mEq/L
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Once we determine low serum osmolality, we determine volume status where the algorithm is fairly similar (but opposite) that of hypernatremia.
SIADH gets a special mention due to the frequency we see it. SIADH can be a result of nausea, pain, pulmonary diseases (pneumonia) , can be diagnosed in a patient with:
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Hyponatremia with hypo-osmolality
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Euvolemia
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Urine sodium >20, often >40mEq/L (patient is euvolemic, so the kidneys excrete sodium to avoid fluid overload/hypertension)
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Urine Osm >100mOsm/kg (inappropriately since we want the urine to be maximally dilute to get rid of the excess water of hyponatremia).
The hourly rate of fluid replacement + maintenance for a patient with sodium derangements based on calculations can be extremely high. We therefore often max out our fluid rates at around 1.5 Maintenance (calculated fluids vs suggested fluids). If your patient can tolerate PO as well, that may be another option to get additional fluid
SIADH urine osmolality >100mOsm/kg)
Calculator
Max correction rate - 8 meq/24hrs in acute hyponatremia
Max correction rate - 6 meq/24hrs in chronic hyponatremia
(slow rate to avoid ODS)
Sodium deficit return in the first 24 hrs:
If time to return <=24 hrs - will return full Na deficit
If time to return >24 but <=36 - will return 2/3 Na deficit
If time to return >36 but <=48- will return 1/2 Na deficit
If time to return >48 - will return 1/3 Na deficit
Maintenance fluid (mL)
Maintenance fluid (mL)
Na deficit
Na deficit
Fluid Deficit
Fluid Deficit
Remaining Balance (mL)
Remaining Balance (mL)
If symptomatic or Na < 120, use 3%NS bolus 3-5 ml/kg. It will increase Na to 2.5-4 meq/L
Time to 140 mEq/L
Description | H₂O | Na |
|---|---|---|
Calculations:
- Maintenance: Holliday-Segar (See "Fluids and Electrolytes")
- Isotonic deficit: 1gm=1mL
- additional sodium deficit: 0.6x Baseline wt (135-current Na)