Sodium and Fluid Balance Flashcards
What is the commonest electrolyte abnormality in hospitalised patients
Hyponatraemia
What is hyponatraemia
Serum sodium < 135mmol/L
What is the underlying pathogenesis of hyponatraemia
Increased extracellular water
What hormone controls water levels in the body
ADH (vasopressin)
Retains water through the action on water channels (aquaporin 2)
What is the MOA of ADH
Acts on V2 receptors in the collecting duct
Acts via inserting aquaporin 2 channels
Acts on V1 recptors on vascular smooth muscle as a vasoconstrictor at higher concentrations
Where are V1 receptors found
Vascular smooth muscle
What are the two main stimuli for ADH secretion
Serum osmolality (mediated by hypothalamic osmoreceptors)
Blood volume/pressure (mediated by baroreceptors in carotids, atria, aorta)
What is the effect of increased ADH secretion on serum sodium
Hyponataraemia
Increased water reabsorption leads to dilution of serum sodium
What is the first step in the clinical assessment of a patient with hyponatraemia
Clinical assessment of volume status
What are the three outcomes of a volume assessment
Hypovolaemic
Euvolaemic
Hypervolaemic
What are the clinical signs of hyponatraemic hypovolvaemia
Tachycardia Postural hypotension Dry mucous membranes Reduced skin turgor Confusion/drowsiness Reduced urine output Low urine Na (<20)
What are the clinical signs of hyponatraemic hypervolaemia
Raised JVP
Bibasal crackles on chest auscultation
Peripheral oedema
Causes of hyponatraemic hypovolaemia
Extra-renal: Diarrhoea, Vomiting
Renal: Diuretics, Salt losing nephropathy
Causes of hyponatraemic euvolmaenia
Hypothyroidism
Adrenal insufficiency
SIADH
Causes of hyponatraemic hypervolaemia
Cardiac failure
Nephrotic syndrome, renal failure
Cirrhosis
What are the causes of SIADH
CNS pathology Lung pathology Drugs (SSRI, TCA, opiates, PPIs, carbamazepine) Tumours Surgery
Tests in a hyponatraemia hypovolaemic patient
Clinical assessment
Tests in a hyponataemia euvolaemic patient
TFTs
Short synacthen test
Plasma and urine osmolality (low plasma and high urine osmolality)
Tests in a hyponatraemic hypervolaemic patient
Fluid overload?
Diagnosis of SIADH
No hypovolaemia
No hypothyroidism
No adrenal insufficiency
Reduced plasma osmolality AND increased urine osmolality (>100)
Management of a hypovolaemic patient with hyponatraemia
Volume replacement with 0.9% saline
Management of a hypervolaemic patient with hyponatraemia
Fluid restriction
Treat the underlying cause
Management of a euvolaemic patient with hyponatraemia
Fluid restriction
Treat the underlying cause
Signs of severe hyponatraemia
Reduced GCS
Seizures
Seek expert help! (treat with hypertonic 3% saline)
What is the most important point to remember while correcting hyponatraemia
Serum sodium NOT be corrected >8-10mmol/L in the first 24 hours as there is risk of osmotic demyelination (central pontine myelinolysis)
Signs of osmotic demyelination due to rapid correction of hyponatraemia
QUadriplegia Dysarthria Dysphagia Seizures Coma Death
Drugs used to treat SIADH
If water restriction is insufficient
Demeclocycline - reduces responsiveness of collecting tubule cells to ADH, monitor U&Es (risk of nephrotoxicity)
Tolvaptan - V2 receptor antagonist
Hypernatraemia
Serum sodium >145mmol/L
Main causes of hypernatraemia
Unreplaced water loss: GI losses, sweat losses, renal losses (osmotic diuresis, reduced ADH release/action (Diabetes insipidus)
Patient cannot control water intake (e.g. very young, very old)
Investigations in a patient with suspected diabetes insipidus
Serum glucose (exclude diabetes mellitus) Serum potassium (exclude hypokalaemia) Serum calcium (exclude hypercalcaemia) Plasma and urine osmolality Water deprivation test
Treatment of hypernatraemia
Fluid replacement
Treat the underlying cause
A 70 yr-old man
3-day history of diarrhoea
Altered mental status
Dry mucous membranes
Serum Na+ is 168 mmol/L
Management?
Correct water deficit - 5% dextrose
Correct extracellular fluid volume depletion - 0.9% saline
Serial sodium measurements - every 4-6 hours
What are the effects of diabetes mellitus on serum sodium
Variable
Hyperglycaemia draws water out of the cells leading to hyponatraemia
Osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatraemia
