Electrolytes

Question 1

Classify the causes of hypernatraemia

Answer 1

Hypovolaemic
1. Extrarenal loss
- GI (vomiting and diarrhoea)
- Skin (fever, burns, excessive sweating)
2. Renal loss
- Intrinsic renal disease
- Loop diuretics
- Osmotic diuretics glucose, urea, mannitol)

Euvolaemia
1. Extrarenal loss
- Skin (fever, excessive sweating)
2. Renal loss
- Diabetes Insipidus (central and nephrogenic)
3. Reduced intake
- Inability to access water
- Primary hypodipsia

Hypervolaemic
1. Hypertonic fluid administration (5% NaCL. NaHCO3. TPN)
2. Mineralocorticoid excess.
3. Adrenal tumours

Question 2

Discuss the treatment of hypernatraemia

Answer 2

1. Review fluid regime
2. Replace free water
   - Oral = effective if GIT ok
   - IV/NGT also ok

Amount of water necessary to replace existing deficits
Water deficit = TBW x [(Na/140)-1]

Where TBW males is 50% body weight (40% in females)

3. Replace this deficit slowly. Plasma osmolarity should be lowered at a rate of no more than 2mOsm/L/hour to avoid cerebral oedema

Question 3

How quickly should hypernatraemia be corrected and why

Answer 3

1. HyperNa < 24 hour can be corrected within 24 hour
2. If unknown duration hyperNa or longer than 48 hours then plasma osmolality should not be lowered by more than 2 mOsmol/L/hour

Question 4

How do you evaluate hyponatraemia in the ICU

Answer 4

Step 1. History and Physical examination
Step 2. Calculate serum osmolarity

[serum] = 2 x [Na] + [urea] + [glucose]

Normal [serum] = pseudohyponatraemia
- Severe hypertriglycerideamia
- Hyperproteinaemia

High [serum] = Severe hyperglycaemia

Low [serum] –> go to step 3

Step 3
Hypovolaemic hyponatraemia
- Drugs (e.g. thiazides)

Euvolaemic hyponatraemia
- SIADH
- Psychogenic polydipsia
- adrenal insuifficiency
- hypothyroidism

Hypervolaemic hyponatraemia
- CCF
- Liver cirrhosis
- Nephrotic syndrome

Question 5

How is hyponatraemia managed

Answer 5

1. Is ? Rx required: symptoms/severity
2. Timescale ? developed over 48 hours?
3. Avoid overzealous correction –> CPM

Asymptomatic or Na > 120
- fluid restirction (50 - 60% maintenance)

Symptomatic or Na 110 - 115
- Rapid correction: 2 mmol/L for first 4 hours or until asymptomatic.
- MAX 8 mmol/day

Question 6

What is the maximum rate of correction of hyponatraemia if it developed over 48 hours

Answer 6

Not more than 8 mmol/L/day

Question 7

How can you calculate how much the Na will increase with your infused fluid?

Answer 7

Increase P[Na] = Infusate [Na] - P[Na] / (TBW + 1)

Question 8

We want to increase the plasma sodium concentration of a 60 kg woman from 110mmol/L to 120 mmol/L over 24 hours. How is this done?

Answer 8

1. Determine Sodium deficit

Na deficit = (Target Na - Plasma Na) x TBW

TBW is total body water and is equal to lean body mass x 0.6 in males and 0.5 in females.

So,
Na deficit = (120 - 110) x 0.5 x 60
Na deficit = 10 x 30
Na deficit = 300 mmol

2. Then determine fluid type and rate of infusion. Consider patient volume status here.

5% NaCl has ± 1 mmol Na per 2 mls
0.9% NaCl has ± 1 mmol Na per 6.5 mls

So if we use 5% NaCl a volume of 600 mls will be required to be infused over 24 hours i.e. 25 ml/hour. The next day the sodium should be 120.