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.
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