Lab Case 22 – Interpretation

Elderly male from nursing home presents critically unwell with severe hypernatraemia following a seizure.

Issues:

1. Seizure and altered mental state – secondary to high sodium, intracranial haemorrhage, other.

2. Tachycardia and low blood pressure for age – shock, large fluid deficit

3.  Life threatening hypernatraemia

4. Possible end of life situation 

Severe hypernatraemia and hyperchloridaemia

aim to correct sodium at rate not exceeding 0.5-1 mmol/hr

HCO3 normal

anion gap = 21  –  elevated, causes in this patient include :

a. Lactate – shock, sepsis, other

b. Toxins – metformin, paracetamol, iron, salicylates

c. Unlikely here – DKA (glucose 6.9 and no ketones given), renal failure ( normal creatinine)

Urea 23.5 with normal Creatinine – severe volume depletion and pre renal failure

Fluid deficit = assume average weight of 70 kg = 70X0.5X(183/140-1) = 10.75 litres

Total fluid replacement – add ongoing insensible losses and replace over 24 hours

Initial fluid bolus to correct shock ( can use normal saline )

Mildly elevated glucose – stress response, unlikely to cause osmotic diuresis

osmolality = 396 – severe hyperosmolar state, with large fluid deficit (as above)

Interpretation – Elderly male with severe hypernatraemia and large fluid deficit presents following a seizure, in shock. He requires airway support, fluid bolus to correct shock, slow sodium correction, assessment of underlying cause and discussion of possible end of life issues ( severe hypernatraemia associated with more than 50% mortality )

 

HYPERNATRAEMIA

In general

1. Deranged thirst response or altered behavioural response, usually elderly and institutionalised.

2.  Diabetes insipidus – nephrogenic, neurogenic

3. Free water loss

Assessment – aims

1. Fluid/ volume status

2. Acute/chronic

3. Thorough neuro examination

Mechanism – cellular dehydration causes cell shrinkage. The response is altered resting potentials of electrically active membranes. After 1 hour of hypernatraemia there is generation of intracellular organic solutes to restore volume and avoid structural damage. ( too rapid fluid rehydration can cause cerebral oedema). Cell dehydration in the CNS can cause altered mental state, seizures or intracranial haemorrhage ( rupture of bridging veins).

CAUSES

1. Hypovolaemia – renal (osmotic dieresis, post obstructive, kidney disease), extra renal (diarrhoea, vomiting, fistula, burns)

2. Hypervolaemia – sodium gain ( hypertonic saline, NaHCO3, accidental ie. formula problems in infants)

3. Euvolaemia – renal (diabetes insipidus- nephrogenic, neurogenic), extra renal (insensible losses)

DDX – HHS

Additional investigations – urine sodium and osmolality, BSL, CT head

Treatment aims –

1. Restore normal tonicity

2. Investigate and treat underlying cause

FLUID DEFICIT CALCULATION

weight (kg) X %TBW X ( Na/140-1) in litres

TBW

young men 0.6

young women and elderly men 0.5

elderly women 0.4

 

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