Hyponatremia

Most common electrolyte disorder in clinical practice.
Disturbance in sodium-water balance.
Skilled management is essential
– Hyponatremia can be life threatening
– Inappropriate therapy can be deleterious.
Hyponatremia can be a contributing factor to morbidity and death in hospitalized patients.

Serum Na = Total body sodium / Total body water

Hyponatremia could be due to
1. Increase in body water
2. Decrease in body sodium
3. Both

Remember the basics of the body’s fluid compartments.

  • TBW = WEIGHT x 0.5 (women) or 0.6 (men)
  • TBW x 1/3 = ECF
  • TBW x 2/3 = ICF
  • ECF x 2/3 = Interstitial compartment
  • ECF x 1/4 = Intravascular compartment
  • Water will move freely to balance osmolalities: Therefore all compartments have equal osmolalities.
  • Sodium is the major extracellular cation.

Serum osmolality = 2 X (Na)  + Glucose/18   +  BUN/2.8

  • Sodium is the primary determinant of Plasma osmolality.
  • Relative osmolality determines the osmotic shift of water across membranes.

Normal control of sodium

Normal sodium level is maintained despite changes in intake of salt and water.

  • Renal mechanisms – Flexibility of renal water excretion.
  • Thirst
  • Vasopressin

The Axis of Sodium

  • The kidney controls the size of the ECFV by controlling Na excretion.
  • Kidney is programmed to keep the ECFV within an acceptable range.

IMPORTANT TO KNOW : That when ECFV increases mechanisms are triggered to excrete Na as the response. When ECFV decreases mechanisms are triggered to retain Na as the response.

Approach to hyponatremia

Confirm that it is true hyponatremia

Pseudohyponatremia :Hyperproteinemia,Hyperlipidemia }   Non aqueous phase

Plasma —>         Aqueous phase / Non-aqueous phase

Sodium exists in aqueous phase.

Selective sodium electrodes solve this problem.

Translocationalhyponatremia
Na falls by 1.4meq/L for each 100mg% increase in glucose.

Eg: If Na = 140meq/L, when glucose = 100mg%. If glucose increases to 400mg%, then sodium will be 140 – (1.4X 3) = 135.8meq/L

True Hyponatremia

1.  Hypovolemic

  • Body water  ↓
  • Body sodium ↓↓

2.  Euvolemic

  • Body water ↑
  • Body sodium ↔

3.  Hypervolemic

  • Body water ↑↑
  • Body sodium ↑

 

I . Hypovolemic hyponatremia.

1) When Urine Na > 20

Renal losses

  • Diuretic use
  • Salt losing nephropathies
  • Ketonuria
  • Cerebral salt wasting
  • Osmotic diuresis
  • Mineralocorticoid def
  • Bicarbonturia : RTA , Met alkalosis(Vomiting)

2) When Urine Na < 10

Extra renal losses

  • Vomiting
  • Diarrhoea
  • Third space losses : Burns , Pancreatitis , Traumatised muscles

II. Hypervolemic Hyponatremia

1) Whene Urine Na > 20

  • ARF
  • CRF

2) When Urine Na < 10

  • Nephrotic syndrome
  • Cardiac failure
  • Cirrhosis

III. Euvolemic Hyponatremia

When urine Na > 20

  • Glucocorticoid deficiency
  • Hypothyroidism
  • Physical or emotional stress
  • Drugs
  • SIADH

All have high ADH.

 

 

 

 

 

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