Background
Water loss in excess of salt deficit
(each litre dehydration = ↑5mEq Na +)
- Patients can't drink or xs loss e.g. sweat/heat stroke
- GI loss without replacement (vomit, diarrhoea etc.)
- Renal disease (can't concentrate urine)
- Diabetes insipidus
- Osmotic diuresis (↑Glu, DKA, HONC,HypoKalaemia)
Xs salt (either ingestion or iatrogenic IV fluids)
- ↑ mineralocorticoids (Conn's)
- ↑Glucocorticoids (Cushing's)
Clinical
- Dehydration++
- Often asymptomatic if gradual onset
- Later weakness, ataxia, tremor, rigidity and hyperreflexia
- May have confusion/ lethargy / coma (brain dehydration)
- Beware - overall may have HYPOnatraemia in total body tissues
Investigations
Calculate Water deficit
- Total body water x (serum Na-140)/ (140)
- TBW is 50% in males and 40% in females
So male with Na of 160 has free water deficit of (0.5 x 70) x 20/140 = 5 Litres - Aim to replace deficit plus maintenance and ongoing losses
- Investigate for underlying cause
Management
- Stop ongoing losses
- Correct water deficit (SLOWLY [days] - avoid cerebral oedema)
- Oral water or 5% dextrose IV (beware too rapid correction)
- Aim for 1mmol Na+ drop / hr
- Beware - correct Na+ deficit if dehydrated
- Investigate for and treat underlying cause