Disorders of sodium concentration Flashcards
(36 cards)
What is Hyponatraemia?
low plasma sodium (Na+) concentration less than 135 mmol/L.
What is the normal range for sodium?
135-145 mmol/L
How is mild, moderate and sever hyponatraemia classified?
Mild hyponatraemia: serum Na+ 130-135 mmol/L.
Moderate hyponatraemia: serum Na+ 125-129 mmol/L.
Severe hyponatraemia: serum Na+ < 125 mmol/L.
What is the difference between acute and chronic hyponatraemia?
Acute: duration < 48 hours.
Chronic: duration > 48 hours
Which hormone is the key hormone responsible for osmoregulation? Where is this hormone produced?
Antidiuretic hormone
Produced by the magnocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus.
It is stored and released by the posterior pituitary in response to rising plasma osmolality.
How does Antidiuretic hormone regulate plasma osmolality?
Normal plasma osmolality is approximately 275-295 mOsm/L.
ADH release begins at a plasma osmolality of around 280 mOsm/kg
Thirst begins at 290 mOsm/kg
ADH acts on the distal convoluted tubule and collecting duct to increase water reabsorption
ADH stimulates the insertion of aquaporin-2 channels onto the luminal membrane of kidneys, allowing the free entry of water.
ADH also causes vasoconstriction of arterioles.
What are the 3 main categories of hypovolemia based on fluid status?
hypovolaemia (reduction in extracellular fluid volume), euvolaemia, hypervolaemia
What are the causes of hyponatraemia with hypovolaemia
Extrarenal causes (urinary sodium <20 mmol/L)
- Vomiting
- Diarrhoea
- Haemorrhage
- Burns
- sweating
- Pancreatitis
- Sepsis
- Cerebral salt wasting
Kidney (urinary sodium >20 mmol/L)
- Osmotic diuresis (e.g. hyperglycaemia, severe uraemia)
- Diuretics (mainly but not only thiazide diuretics)
- Adrenocortical insufficiency (Addison’s)
- Tubulointerstitial renal disease
- Unilateral renal artery stenosis
- Recovery phase of acute tubular necrosis
What are the signs and symptoms associated with someone who is hypovolaemic? (9)
- Thirsty
- Hypotensive
- Low urine output
- Tachycardic (hypovolaemia can active a vagal mechanism that can cause bradycardia)
- Reduced skin turgor
- Dry mucous membranes
- Capillary refill time > 2 seconds
- Dizziness
- Postural drop in systolic blood pressure- Low JVP (can only be seen if patient is lying completely flat)
- Cold peripheries
- Early warning signs >3
How does vomiting lead to Hypovolaemia hyponatraemia?
Vomiting causes water loss HCL acid and electrolytes are also lost. Loss of HCL causes alkalaemia Kidneys compensate by excreting bicarbonates with sodium This increases sodium excretion
How does diarrhoea lead to hyponatraemia?
There is a loss of water and electrolytes in the form of sodium bicarbonate. Loss of bicarbonate = Academia
The kidneys as a result will excrete hydrogen ions to increase the serum PH
What are some causes of hypoaldosteronism and how does it cause Hypovolaemia hyponatraemia?
Addison’s disease
Spironolactone
Aldosterone is realised from the adrenal cortex. Aldosterone acts on the kidneys to reabsorb sodium in exchange for potassium
Low aldoserone will decrease the effect aldosterone has on the distal tubules and collecting ducts in the kidneys
There will be decreased expression of the sodium/ potassium ATP pump and aldosterone dependant sodium channels
Aldosterone can not retain sodium, increasing sodium and water excretion (water follows sodium)
What are the causes of hyponatraemia with Euvolaemia?
Abnormal antidiuretic hormone release
- Vagal neuropathy (failure of inhibition of ADH release)
- Deficiency of adrenocorticotrophic hormone (ACTH) or glucocorticoids (Addison’s disease)
- Hypothyroidism
- Severe potassium depletion
Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
Psychiatric illness
- Psychogenic polydipsia’
- Antidepressant therapy
Increased sensitivity to ADH
- Chlorpropamide
- Tolbutamide
ADH like substances
- Oxytocin
- Desmopressin
osmotically active substances stimulating osmotic ADH release
- Glucose
- Chronic alcohol use
- Mannitol
- Sickle cell syndrome
High water, low solute intake (i.e. primary polydipsia, anorexia nervosa)
What are the causes of hyponatraemia with hypervolaemia?
Heart failure - congestive heart failure
Liver failure - Cirrhosis
Oliguric kidney injury
Hypoalbuminaemia - nephrotic syndrome
What are some cause of pseudo hyponatraemia?
High glucose hyperlipidaemia hyperproteinaemia Paraoriteins Arm being infused with fluid containing low sodium Mannitol
Which drugs can cause postural hypotension?
Tricyclic antidepressants Nitrates Calcium-channel blockers α-Adrenoceptor-blocking drugs Beta blockers
How is Hyponatraemia with hypovolaemia managed?
In healthy patients -
- oral electrolyte–glucose mixtures
- Increase salt intake with slow sodium 60–80 mmol/day
In a patient with vomiting or severe volume depletion -
- Give intravenous fluid with potassium supplements, i.e. 1.5–2 L 5% glucose (with 20 mmol K+) and 1 L 0.9% saline over 24 h plus measurable losses.
Correction of acid–base abnormalities is usually not require
How is Hyponatraemia with euvolaemia managed?
Most cases are simply managed by restriction of water intake (to 1000 or even 500 mL/day) with review of diuretic therapy. Magnesium and potassium deficiency must be corrected. In mild sodium deficiency, 0.9% saline given slowly (1 L over 12 h) is sufficient.
What is the commonest cause of acute onset hyponatraemia?
Postoperative iatrogenic hyponatraemia
What are common causes of hyponatraemia?
Postoperative iatrogenic
hyponatraemia
Excessive water intake associated with psychosis,
Marathon running
Use of Ecstasy
Medical emergency with aggressive treatment
If someone is suffering from acute hyponatremia with neurological symptoms, which symptoms would you expect to see and how is this condition treated?
fits, coma or cerebral oedema
hypertonic saline (3%, 513 mmol/L)
It must be given very slowly (not more than 70 mmol/h), the aim being to increase the serum sodium by 4–6 mmol/L in the first 4 hours, but the absolute change should not exceed 15–20 mmol/L over 48 hours.
Which neurological complication can occur is a patients extracellular osmolality rises too quickly?
osmotic demyelination syndrome
When treating someone with hyponatremia, how much should their plasma sodium concentration rise daily?
should not rise by more than 8 mmol/L per day
The rate of rise of plasma sodium should be even lower in patients at higher risk for osmotic demyelination syndrome (ODS)
What are the risk factors for a patient to develop osmotic demyelination syndrome?
- alcohol excess,
- cirrhosis,
- malnutrition
- hypokalaemia
- pre-existing hypoxaemia
- central nervous system radiation
