Electrolyte Disturbances Flashcards by Izzie Setford

What is more important to the body in terms of body fluid compartments, osmolality or volume?

Osmolality is always maintained at the expense of volume

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Which body fluid compartment are you more likely to find Potassium and sodium respectively?

Potassium - intracellular fluid

Sodium - extracellular fluid (interstitial and intravascular)

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What determines serum sodium concentration, and how it this regulated?

The amount of water in the extracellular fluid compartment

- regulated by changing intake or output of water

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What are the main mechanisms that regulate water metabolism?

Thirst

Anti-diuretic hormone

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What is ADH produced in repose to?

Decreased plasma volume (sensed by baroreceptors in the carotid sinus, aortic arch and atria)
Increased plasma osmolality (sensed by osmoreceptors in the hypothalamus)

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Which mechanisms regulate the effective arterial volume?

Renin-angiotensin system
- reduced volume sensed by JGA of the kidneys (secrete renin)
- angiotensin II is a vasoconstrictor and promotes aldosterone release
Carotid/aortic baroreceptors
- increase sympathetic nervous system activity
Cardiac receptors
- atrial natriuretic peptide release in response to high sensed volume

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What is the function of atrial natriuretic peptide?

Reduced sodium, water and adipose loads of circulatory system

reducing blood pressure
generates sodium loss in the kidneys

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How does aldosterone exert its effect on the kidney?

Aldosterone enters the tubule cell, and binds to internal mineralocorticoid receptors, and move into the nucleus
This then stimulates the ENaC channels on the apical membrane to remove sodium from the filtrate
It also stimulates the potassium/sodium exchanger on the basolateral membrane, increasing potassium loss

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What sodium serum concentrations is classed as hyponatraemia?

Less than 135mmol/l

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Which electrolyte imbalance is the most common?

Hyponatraemia

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What is the most common cause of hyponatraemia?

Disorder of water balance

inability to suppress ADH - so there is too much water retention (SIADH)
renal impairment

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Name the causes/types of hyponatraemia.

With reduced extracellular fluid - sodium and water loss
With normal extracellular fluid - water gain
With increased extracellular fluid - sodium and water gain

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What things can cause hyponatraemia with reduced extracellular fluid?

Loss of sodium via the GI tract - vomiting/diarrhoea

Loss of sodium via the kidneys - diuretics/renal tubular disorders

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What things can cause hyponatraemia with normal extracellular fluid?

Syndrome of inappropriate ADH (SIADH)

Glucocorticoid deficiency

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What things can cause hyponatraemia with increased extracellular fluid?

Heart failure, hepatic cirrhosis, nephrotic syndrome

Pathological fluid retention lowers plasma sodium

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What are the main causes of SIADH?

Cancer - lung/leukaemia
Chest disease - pneumonia
CNS disorders - infections/injury
Drugs - opiates, thiazides, anticonvulsants, PPI, antidepressants

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How would SIADH be diagnosed?

Hyponatraemia - inappropriately low serum osmolality
Urine osmolality>plasma osmolality
- urine sodium is greater than 20mmol/l
Absense of adrenal, thyroid, pituitary and renal insufficiency
No recent diuretic use

How does hyponatraemia cause cerebral oedema?

Low sodium levels in the blood mean water tries to enter cells to balance the osmolality
The body can normally adapt to this, but if it’s happens to fast, then it can’t do so as effectively
In chronic hyponatraemia, the brain cells adapt, and the intracellular osmolality decreases

Which disease can be caused by a sudden rise in plasma sodium?

Osmotic demyelination syndrome

Clinical features of hyponatraemia. (In order of increasing severity)

Often asymptomatic 
Mild confusion
Gait instability 
Marked confusion 
Drowsiness 
Seizures

When are you allowed to try and quickly increase plasma sodium?

If it drops suddenly (after endurance exercise or neurosurgery)
If it is very severe (unconscious or seizures)
- give infusions of hypertonic (3%) saline

How should less severe hyponatraemia be treated?

Try to establish a cause first
Fluid restriction (normally)
Don’t often need second line treatment
- can consider AVPR2 antagonists

What is hypernatraemia normally due to?

Water loss (dehydration) and an inability to access water
Water loss normally due to
- insensible/sweat losses (severe burns/sepsis)
- GI loss
- DI
- osmotic diuresis due to hyperglycaemia

Can you get hypernatraemia from sodium overload

Yes - but it is very rare

How is hypernatraemia managed ?

Treatment of the underlying cause Estimate total body water deficit to guide fluid regimen Avoid overly rapid correction - aim for 10mmol/l drop in 24hours (concern is cerebral oedema) - use IV 5% dextrose

What are the sources of calcium in the body?

GI tract - dietary calcium absorbed throughout small intestine - vitamins D dependent Bones - calcium reservoir - regulates plasma calcium via the actions of osteoblasts and osteoclasts Kidney - free calcium is filtered by the glomerulus - 97-99% is reabsorbed

Effects of vitamin D on calcium levels

Vitamin D - increase GI absorption - increases bone resorption - increases renal reabsorption

Role of PTH in calcium regulation

Decrease in plasma leads to increased PTH PTH increases bone resorption - releasing calcium and phosphate In the kidneys, PTH also increases phosphate excretion, calcium reabsorption and vitamins D formation - vitamin D increases GI absorption of calcium - can also increase bone resorption even more

Clinical features of hypercalcaemia

Bones- bone pain, osteoporosis, fractures Stones - kidney stones, flank pain, frequent urination Moans - confusion, dementia, memory loss, depression Abdominal groans - nausea, vomiting, anorexia, pancreatitis, peptic ulcer disease, constipation

ECG changes in hypercalcaemia

Shortened QTc interval | Bradycardia

Causes of hypercalcaemia

Primary hyperparathyroidism - single parathyroid adenoma (normally) - increased bone resorption and GI absorption Malignancy - usually due to secretion of PTH-related peptide - breast, lung and multiple melanoma are the most common tumour

What test distinguishes between primary hyperparathyroidism or malignancy as a cause of hypercalcaemia?

PTH levels - decreased means malignancy is likely - normal or increased means primary hyperparathyroidism

What do you need to know before treating hypercalcaemia?

``` Accurate interpret ion of serum calcium - 45% bound to albumin - total vs ionised calcium Severity - mild is less than 3mmol/l - moderate is between 3 and 3.5mmol/l - severe is greater than 3.5mmol/l ```

How is hypercalcamia managed?

Rehydration Bisphosphonate therapy Calcitonin - increases renal calcium excretion + decreases bone resorption Glucocorticoids - inhibits vitamin D production Parathyroidectomy if treatment doesn't work

Describe rehydration treatment for hypercalcaemia.

Patients are often hypovolaemic Isotonic 0.9% saline infusion corrects hypovolaemia (careful of fluid overload) Don't try to normalise calcium if only mildly elevated

Describe bisphosphonate therapy for hypercalcaemia.

Inhibits bone resorption by inhibiting osteoclasts Agents of choice for treating hypercalcaemia of malignancy - zone droning acid is most commonly used Delayed effect (best 2-4 days after treatment starts)

How does the body respond to an increase in plasma calcium normally?

Thyroid gland released calcitonin into the blood Calcitonin improves the uptake of calcium and phosphate into the bones Blood calcium levels drop

What problems can acute hypocalcaemia cause?

Tetany - increased neuromuscular excitability - peri-oral numbness, muscle cramps and tingling of hands/feet - if severe can cause capopedal spasm, laryngospasm and seizures Cardiac complications - dysrhythmia - hypotension

ECG changes in hypocalcaemia

Reduction in QTc interval - (dysrhythmia)

Possible causes of hypocalcaemia

Low PTH - after parathyroid surgery or if you have autoimmune hypoparathyroidism High PTH - cam cause vitamin D deficiency, chronic renal failure and subsequent loss of calcium Drugs Hyomagnesaemia - leads to PTH resistance

Treatment for hypocalcaemia.

``` IV calcium replacement for tetany or cardiac problems May need magnesium infusion Chronic management - vitamins D - oral calcium salts Treat the underlying cause ```