Sodium and water balance Flashcards
How are electrolytes distributed in the intracellular and extracellular spaces
-Have same osmolality but different makeups
Intracellular - Na 10mmol/L K 160mmol/L Osmolality 280-290mosmkg
More K in intracellular
Extracellular -Na 135-145 mol/L K 4mmol/L Osmolality 280-290mosmKg
More Na in extracellular
How is plasma sodium regulated
- Controlled by volume reception and osmoreceptors (notice chance in plasma conc)
- Plasma sodium regulated by amount of salt and water
- Volume receptors regulate sodium via renal sodium excretion (sodium lost and gained in kidney) - notes the change in circulating volume
- Osmoreceptors - regulate water via thirst and water reabsorption in kidneys
What are the two factors that maintain sodium homeostasis
Atrial Natriuretic peptide (ANP)
-Release due to atrial stretch and reduces Na reabsorption and aldosterone/ angiotensin to decrease sodium reabsorbed
Sympathetic activity
- By stretch receptors in thoracic veins, aortic arch and area of circulation
- Activity increases with decreased circulating volume and increased renin secretion to promote sodium reabsorption
Describe the features of hyponatraemia
-Reduced sodium concentration (surgical wards= common - elderly ortho ladies)
- Causes: Sodium loss -Sodium and water loss -Excess water
- Can sometimes be caused by hyperglycaemia- Na shifts into cells*
- Symptoms start at 125-130mmol/L - if they fall below 120 = seizures + coma
- Symptoms= nausea, malaise, headache. disorientation , coma, seizures
- To find the cause of hyponatraumie determine fluid balance
- Water excess -Deydration -Normal balance
What are the causes of hypervolaemic hyponatraemia
- Congestive heart failure, cirrhosis and nephrotic syndrome
- Clinically obvious (oedema)
- Urinary sodium will be less than 20mmol/L (can’t check this if patient is on diuretics)
- Investigations- CXR, liver enzymes, albumin (cirrhosis/ nephritic syndrome), urine protein
What are the steps involved in hypervolaemic hyponatraemia
- Fluid lost into 3rd space
- Reduced intravascular volume (decreased circulating volume)
- This will stimulate osmoreceptors and volume receptors to secrete ADH to retain water and thus dilutes plasma
- This increases ADH
- This causes water retention which dilutes plasma and causes hyponatraemia
Discuss the causes and features of hypovolaemic hyponatraemia
- Loss of salt and water but salt is lost in excess of water
- Increased skin turgor, dry mucous membranes, postural drop in BP
2 places we can loose salt from
1. GI losses- diarrhoea, vomiting, fistula, urinary sodium is less than 20 moll/L
Kidneys try to hold as much sodium as possible so urinary sodium will be low
- Renal losses - hyperglycaemia, thiazide diuretics- urinary sodium is greater than 20mmol/L - high urinary sodium
What are the causes and features of euvolaemic hyponatraemia
- Normal sodium and water balance
- Urinary sodium will be more than 20mmol/L
- Addison’s disease (cortisol deficiency), Hypothyroidism, SIADH)
Discuss the features of SIADH
SIADH- retention of water unnecessarily
- Retention will dilute the plasma and reduce its osmolality thus increasing urine osmolality >100mmols/g
- Urinary sodium will be much greater than 20
- Only diagnosed if renal, adrenal and pituitary function are all normal
- Features: Pain, nausea, drugs (carbamazepine and SSRIs ), paraneoplastic
What are the investigations carried out to find the cause of hyponatraemia
- Measure lipids, protein and glucose
- Assess fluid balance
- Measure urine sodium (esp if euvolaemic)
- Urine and plasma osmolality
- Syacthen tests (Addison’s-give ACTH)
- TFTs
- Imaging
What are the steps involved in managing hyponatraemia
- Slow Na correction
- No more than 12mmol/L of sodium per 24 hrs
- Initially aim for increase of 1-2 mol/L per hour
- Treat cause
- If sodium is depleated (gut/ renal loss) then replace with oral (slow sodium tablets) or IV (Normal saline)
A rapid increase will cause water to shift out of cells in brain and will cause brain shrinkage - cerebral haemorrhage
What are the steps involved in managing SIADH
- Fluid restriction to 1L per 24 hrs (monitor sodium during this time)
- Demeclocyline if not responding/ need rapid treatment- it causes nephrogenic diabetes insidious- blocks ADH in the kidneys
- Aqauporin receptor antagonists- blocks sodium reabsorption
Discuss the features and symptoms of hypernatraemia
- Watere deficit= main cause
- Loss of water in excess of sodium
- Occasionally due to excess sodium if - hypertonic saline is given or - given antibiotics with high sodium content given
- Symptoms start at over 150mmol/L
- Anorexia, nausea, vomiting
- Altered mental status
- Cerebral bleeding, subarachnoid haemorrhage
- Brain shrinkage due to water leaving cells*
-Assess volume status - hypovolaemic -Hypervolaemic -Euvolaemic
What are the causes of hypovolaemic hypernatraemia
- Dermal losses- burns, sweating (fever)
- GI loss- vomiting, diarrhoea, fistula
- Renal loss- diuretics (loop)
- post- obstruction (renal obstruction- diuretic phase)
- acute and chronic renal disease (usually causes fluid retention)
- hyper osmolar non-ketotic coma (excess water lost in kidneys)
What are the causes of hypervolaemic hypernatraemia
- Excess hypertonic saline
- antibiotic infusions with increased sodium content
- Usually iatrogenic
