Chem path 3: Sodium and fluid balance Flashcards
What is the commonest abnormality in hospitalised patients?
Hyponatraemia
What is the underlying pathogenesis in hyponatraemia?
Excess extracellular water
What hormone is responsible for water balance and how does it act on the kidney?
ADH, acts on V2 receptors in collecting duct cells by inserting AQ2 channels
Where are V1 receptors found and how does ADH act on them?
Vascular smooth muscle, ADH –> vasoconstriction at higher concentrations
What are the two main stimuli for ADH secretion?
Increased osmolality and reduced blood volume/pressire
What are serum osmolality and blood pressure/volume mediated by, respsectively?
Osmoreceptors and baroreceptors
Where are osmoreceptors and baroreceptors found, respectively?
Osmoreceptors - hypothalamus
Baroreceptors - Carotids, aorta, atria
What is the first step in the clinical assessment of someone with hyponatraemia?
Assess their volume status
What is the most reliable clinical sign of hypovolaemia?
Low urine Na+ (<20)
In which cohort of patients is urine Na+ not a reliable clinical marker of hypovolaemia?
patients on diuretics as they will have a high urine sodium regardless
What are the clinical features of hypervolaemia?
Raised JVP
Bibasal crackles
Peripheral oedema
Causes of hypovolaemic hyponatraemia
Diarrhoea/vomiting
Diuretics
Salt losing nephropathy
Euvolaemic hypoantraemia
Hypothyroidism
SIADH
Adrenal insufficiency
Hypervolaemic hyponatraemia
Cirrhosis
Nephrotic syndrome/renal failure
Heart failure
What does a urine sodium <20 indicate and what does one >20 indicate?
if <20, suggests the hyponatraemia is not due to renal causes >20 suggests due to renal causes
How does cardiac failure cause hypervolaemic hyponatraemia?
Low pressure is detected by the baroreceptors –> ADH release
How does cirrhosis cause hypervolaemic hyponatraemia?
Release of vasodilators e.g. NO which results in low pressure which is detected by baroreceptors –> ADH release
How does renal failure cause hypervolaemic hyponatraemia?
Not excreting enough water. CKD results in urinary protein loss and hence oedema. This lower circuilating volume activates RAAS which results in sodium retention. This in turn causes ADH release –> water retention and hypervolaemic hyponatraemia
How does hypothyroidism cause euvolaemic hyponatraemia?
Reduced cardiac contractility –> reduced BP –> ADH release
How does SIADH cause hyponatraemia?
Excess AQ2 insertion in collecting duct –> water retention and increased blood volume –> suppresses RAAS –> less aldosterone –> less Na+ reabsorption
Causes of SIADH
Lung cancer, pneumonia (legionella), CNS pathology, stroke, drugs (SSRIs, TCA, PPI, carbamazepine), surgery
Why does SIADH cause a euvolaemic hyponatraemia and not hypervolaemic?
Excess ADH = excess water = volume expansion → secretion for BNP → naturesis –> euvolaemic
What are two causes of pseudohyponatraemia?
Hyperlipidaemia and hyperproteinaemia
What happens to the urinary sodium in patients with heart failure?
Urinary sodium low as there will be secondary hyperaldosteronism causing sodium retention
