Hyponatraemia

Question 1

Classify severity of hyponatraemia

Answer 1

Mild: 130 - 135
Mod: 120 - 130
Sev: < 120

Question 2

Why is serum osmolarity relevant to the management of hyponatraemia

Answer 2

Hypotonic hyponatraemia leads to cerebral oedema (large fluid shifts)

Prior to treatment, identification of tonicity of fluid is crucial as this aids in the rapid diagnosis and treatment of hyponatraemia

Question 3

Classify hyponatraemia according to tonicity

Answer 3

Hypotonic hypoNa (sOsm < 280)

• Hypovolaemic
• Euvolaemic
• Hypervolaemic

Isotonic hypoNa (sOsm 280 - 295)

• Pseudohyponatraemia
• e.g. hyperTG / paraproteinemia (multiple myeloma)

Hypertonic hypoNa (sOsm > 295)

• Mannitol
• Glucose

Question 4

What is the formula for correction of Na for high glucose

Answer 4

Corrected Na = Measured Na + Glucose/4

all in mmol/L

Question 5

Why is acute hyponatraemia treated differently from chronic hyponatraemia

Answer 5

Astrocytes in the brain transport osmotically active cytoplasmic solutes into the extracellular space to counteract the hypotonic environment. This process takes 24 - 48 hours and is the reason why acute and chronic hyponatraemia are treated differently

Question 6

What regulates the release of ADH and which of these has an overriding effect

Answer 6

1. Increased plasma osmolarity –> increases ADH
2. Decreased plasma osmolarity –> decreases ADH
3. Low effective circulating volume –> increases ADH
4. High effective circulating volume –> decreases ADH

So if low osmolarity (should decrease ADH) and low effective circulating volume (should increase ADH) –> the overriding effect is an increase in ADH due to the low effective circulating volume.

Question 7

Describe your approach to finding the cause for hyponatraemia

Answer 7

1. Serum osmolarity < or > 280
   If isotonic/hypertonic: mannitol/glucose/paraprotein/TGs
   If hypotonic then:
2. Urine osmolarity < 100 or > 100
   If < 100 then normal water excretion: Primary polydipsia
   If > 100 then impaired water excretion then:
3. Volume status:
   Hypovolaemic
   - Urine Na < 20: GI loss/Haemorrhage/3rd spacing/burns
   - Urine Na > 20: Diuretic / Adrenal insufficiency
   Euvolaemic
   - SIADH
   Hypervolaemia
   - Urine Na > 20 CHF/Cirrhosis/Nephrotic
   - Urine Na > 20 Renal failure

Question 8

E.g.
80 kg female
Measured Na 118
Desired Na 130

What is the:

1. Na deficit
2. Volume of 0.9% NaCl required to correct this deficit
3. Over how many hours should the saline be infused to ensure a correction rate of 0.5 mmol/L per hour

Answer 8

E.g.
80 kg female
Measured Na 118
Desired Na 130

1. Na deficit = TBW (Na desired - Na measured)
2. TBW = 0.5 x weight

So
Na deficit = 40L (130mmol/L - 118mmol/L)
Na deficit = 480 mmol

And
1 L of saline contains 154 mmol of Na
So volume required:

480 mmol / 154 mmol/L = 3.117 L or 3117 mL

How fast to give it: Need correction rate
- usually 0.5 mmol/L per hour

This means you want to increase the Na concentration by 0.5 mmol/L per hour

Our desired Na is 130 mmol/L from 118 mmol/L
= 12 mmol/L increase at a rate of 0.5 mmol/L per hour

12 mmol/L

0. 5 mmol/L.hr

= 24 hours

So

Volume = 3117 mL over 24 hours
= 130 mL/hour.

Question 9

List 4 indications for A-line insertion

Answer 9

1. Continuous intra-arterial BP monitoring
   - Co-morbidities + Major surgery
   - Critical ill + require titrated vasoactive medications
2. To predict fluid responsiveness
   - Automated calculation: SPV / PPV / SVV
3. For frequent blood sampling
4. Identification of abnormal arterial pressure waveforms