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

Pseudohyponatremia :Hyperproteinemia,Hyperlipidemia }   Non aqueous phase

Plasma —>         Aqueous phase / Non-aqueous phase

Sodium exists in aqueous phase.

Selective sodium electrodes solve this problem.

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.