CHEMPATH: Sodium and Fluid Balance Flashcards
What is the most common electrolyte abnormality in hospitalised patients?
Hyponatraemia i.e. sodium <135 mmol/L
What is the main pathogenesis of hyponatraemia?
Increased extracellular water
What hormone regulates warer balance?
Regulated by ADH
- Synthesised in hypothalamus
- Secreted from posterior pituitary, acts on CD in the kidney
- Water retention through insertion of aquaporin 2 (AQA2)
NB: It also acts on V1 receptor to cause vasoconstriction. Then ADH is called vasopressin.
Summarise the effecs of ADH on different receptors.
- Acts on V2 receptors in collecting duct à insertion of AQA2
- V1 receptors
- Vascular smooth muscle
- Vasoconstriction – higher concentrations
- “Vasopressin”
What are the main 2 stimuli for ADH secretion?
- 2 main stimuli for ADH secretion:
- Serum osmolality (high) – mediated by hypothalamic osmoreceptors
- Blood volume/pressure (low) – mediated by baroreceptors in carotids, atria, aorta
Why does vomiting cause hyponatraemia?
Loss of blood volume leads to baroreceptors detecting this and increasing ADH secretion
What is the first step in the assessment of a patient with hyponatraemia?
Clinical assessment of volume status to see whether hypo/eu/hypervolaemic
What are the clinical signs of hypovolaemia? What is the most reliable?
- Tachycardia
- Postural hypotension
- Dry mucous membranes
- Reduced skin turgor
- Confusion or drowsiness
- Reduced urine output
MOST RELIABLE INDICATOR:
- Low urine Na+ (<20)
- Normal range = 40-220 mEq/L [<20 non-renal loss; >20 in renal loss)
- Kidneys are the best detector of hyponatraemia
Which drugs will affect your ability to use the urine sodium as the most reliable indicator of hypovolaemia?
Patients on diuretics may have urine Na that is not reliable (hypovolemic, but no hyponatraemia)
List some clinical signs of hypervolaemia.
- Clinical Signs of hypervolemia
- Raised JVP
- Bi-basal crackles
- Peripheral oedema
What are the causes of hypovolaemic hyponatraemia?
- Causes of a hyponatraemia in a hypovolaemic patient (this requires more sodium than water loss)
- Diuretics
- Diarrhoea and vomiting
- Salt losing nephropathy
What are the causes of hypervolaemic hyponatraemia?
Causes of hyponatraemia in a hypervolemic patient → cases are excess water, excess ADH
- Cardiac failure → low pressure → detected by baroreceptors → ADH release
- Cirrhosis → vasodilated due to excess NO → low BP → baroreceptors → ADH release
- Renal failure → not excreting enough water
What are the causes of euvolaemic hyponatraemia?
- Hypothyroidism –> reduced contractility –> reduced BP –> ADH release
- Adrenal insufficiency –> less aldosterone –> less Na+ reabsorptio
- SIADH –> AQA2 water retention –> inc. volume –> suppress RAAS –> less aldosterone –> less Na+ reabsorption
What are the causes of SIADH?
- CNS pathology – stroke, haemorrhage, tumour
- Lung pathology – pneumonia (Legionella), pneumothorax
- Drugs – SSRI, TCA, PPI, carbamazepine, opiates
- Tumours
- Surgery
What investigations would you order in a patient with euvolemic hyponatraemia?
- Hypothyroidism → thyroid function tests
- Adrenal insufficiency → short SynACTHen test
- SIADH → plasma and urine osmolality → low plasma and high urine osmolality
- Excess ADH = excess water = volume expansion → secretion for BNP → naturesis –> euvolaemic
- If sodium is high**, it’s a **pseudohyponatraemia (i.e. hyperlipidaemia, hyperproteinaemia)
What are the important factors for allowing a diagnosis of SIADH to be made?
- No hypovolaemia (euvolaemia)
- No hypothyroidism
- No adrenal insufficiency
- Reduced plasma osmolality (resorbing lots of water) AND
- Increased urine osmolality (>100 is high) (concentrating the urine)
How do you manage hypovolaemic hyponatraemia?
volume replacement with 0.9% saline
How do you manage a patient with euvolaemic hyponatraemia?
fluid restrict (<750ml/day + ABx infusions) + treat underlying cause
How do you manage hypervolaemic hyponatraemia?
fluid restrict (<750ml/day + ABx infusions) + treat underlying cause
What is fluid restriction?
0.5L-1L over 24 hours
What is the cause of hyponatraemia in hypothyroidism?
Reduced effects of thyroid hormone on the heart causing more ADH release due to lower contractility
Or the two could be unrelated and just both common conditions not linked
How do you recognise severe hyponatraemia? What is the management?
- Reduced GCS
- Seizures
Mangement:
- Seek expert help – treat with hypertonic 3% (2.7%) saline [2 or 3 boluses of 150ml]
What is a complication of treating hyponatraemia too quickly?
- Central Pontine Myelinolysis
What is the limit for increasing sodium in mmol/L over the first 24 hours?
8-10mmol/L in the first 24 hours
CPM will occur a few days later i.e. it is not sudden so you must correct the sodium ASAP if this has happened
What are the signs and symptoms of CPM?
- Signs/symptoms: Quadriplegia, dysarthria, dysphagia, seizures, coma, death
If fluid restriction is insufficient, what drugs can be used to treat SIADH?
-
Demeclocycline -
- Induce nephrogenic diabetes insipidus
- Reduces responsiveness of collecting tubule cells to ADH
- Monitor U&Es as risk of nephrotoxicity
- Tolvaptan – V2 receptor antagonist
reduced blood volume and increased serum osmolality
Reduced** plasma osmolality and **increased urine osmolarity [>20]
Which type of GI loss can cause hypernatraemia?
GI losses not accompanied by drinking water e.g. in children and older patients
What are the 2 main GENERAL causes of hypernatraemia?
Increase in sodium
Loss of water
What causes of increase in sodium can lead to hypernatraemia? (including 3 endocrine)
- Medical high intake (hypertonic saline, sodium bicarbonate)
- Dietary high intake (salty infant formula, high dietary salt)
- Conn’s syndrome (high aldosterone: renin ratio), BAH (high aldosterone: renin ratio)
- Renal artery stenosis (low GFR from RAS à low BP at JGA à high renin à high aldosterone)
- Cushing’s syndrome (overactivation of MR by cortisol à aldosterone-like effect)
What are the causes of water loss which can cause hypernatraemia?
-
Renal losses:
- Osmotic diuresis
-
Diabetes insipidus (less ADH action / release)
- Insensitivity/lack to/of ADH –> solitary water losses à hypovolaemia
- Body compensates by resorbing more Na+ to reduce the water loss
- Water loss persists and so you get a hypovolaemic hypernatraemia
-
Non-renal losses:
- GI loss
- Sweat loss
What are the causes of nephrogenic DI?
Nephrogenic DI
- Hypercalcaemia
- Hypokalaemia
- Lithium
- Sickle cell anaemia
What 5 investigations should you do for suspected diabetes insipidus?
- Serum glucose – exclude diabetes mellitus à osmotic diuresis
- Serum potassium – exclude hypokalaemia à nephrogenic DI
- Serum calcium – exclude hypercalcaemia à nephrogenic DI
- Plasma and urine osmolality – exclude hyperaldosteronism (high plasma osmolality, low urine osmolality)
- Water deprivation test (normal = concentrated urine, no ADH = carry on passing water – dilute urine)
How is hypernatraemia treated?
- Fluid replacement → dextrose (if the patient is also hypovolemic, then 0.9% saline and 5% dextrose water)
- Treat underlying cause
E.G. 70-year old man, 3-day history of diarrhoea, altered mental status, dry mucous membranes. Serum Na is 168mmol/L
What are the effects of DM on sodium?
How do you correct water deficit in hypernatraemia?
How do you correct ECF volume depletion?
How often are serial Na+ measurements taken?
- Correct water deficit → 5% dextrose
- Correct ECF volume depletion → 0.9% saline
- Serial Na+ measurements → every 4-6 hours
Why do you use dextrose and not saline in hypernatraemia?
Dextrose is water
Would need 10L of saline to correct hypernatraemia because of the amount of sodium that is in normal saline
How do you correct sodium if you have made it too high?
Dextrose and desmopressing injections
