Emergency Medicine

Acid–Base, Electrolyte and Renal Emergencies 137

ELECTROLYTE DISORDERS

(iii) Normovolaemic hyponatraemia

(a) urine osmolality > serum osmolality:

• syndrome of inappropriate antidiuretic hormone secretion
  (SIADH) due to head injury, meningoencephalitis, CVA,
  pneumonia, COPD, neoplasia, human immunodeficiency
  virus (HIV) infection, drugs such as carbamazepine,
  NSAIDs and antidepressants


• positive-pressure ventilation, porphyria
  (b) urine osmolality < serum osmolality:


• hypotonic post-operative fluids such as 5% dextrose or 4%
  dextrose 1/5 normal saline, transurethral resection of the
  prostate (TURP) irrigation fluid, psychogenic polydipsia,
  ‘tea and toast’ diet, beer potomania.
  (iv) Hypervolaemic hyponatraemia
  (a) urinary sodium <20 mmol/L: congestive cardiac failure,
  cirrhosis, nephrotic syndrome, hypoalbuminaemia, hepatorenal
  syndrome
  (b) urinary sodium >20 mmol/L: steroids, cerebral salt wasting,
  chronic renal failure, hypothyroidism.

3 Clinical features progress as the serum sodium level drops, but depend also
on the rate of fall, i.e. the more rapid the fall the greater the symptoms:
(i) Na >125 mmol/L: usually asymptomatic.
(ii) Na 115–125 mmol/L: lethargy, weakness, ataxia, and vomiting.
(iii) Na <115 mmol/L: confusion, headache, convulsions, and coma.

4 Assess the underlying volume status:
(i) Look at the skin turgor, jugular venous pressure (JVP), measure
lying and sitting blood pressure (BP), listen for basal crackles.

5 Send blood for FBC, U&Es, LFTs, thyroid function and serum osmolality.
Send urine for sodium and osmolality.

6 Perform an ECG and request a CXR.

MANAGEMENT

1 Commence high-f low oxygen by face mask.

2 Asymptomatic patients:
(i) Discontinue implicated drug therapy and treat the underlying
medical condition, e.g. antibiotics for sepsis.
(ii) Restrict fluid intake to 50% of estimated maintenance fluid
requirements in SIADH, i.e. around 750 mL/day.
(iii) Aim to increase the serum sodium gradually by 0.5 mmol/L
per h, to a maximum rate of 12 mmol/L per 24 h.

3 Get senior ED doctor help if the patient has neurological symptoms.
(i) Administer 3% hypertonic saline to raise serum sodium levels by
1 mmol/h.