Chemical Pathology 13 - Potassium & electrolytes

Question 1

What is the serum concentration of potassium?

Answer 1

3.5 - 5

Question 2

What are the hormones involved in renal potassium regulation?

Answer 2

Angiotensin II

Aldosterone

Question 3

Where is renin released from?

Answer 3

Juxtaglomerular apparatus

Question 4

Where is angiotensinogen released from?

Answer 4

Liver

Question 5

Where is angiotensin I converted to angiotensin II?

Answer 5

Lung

Question 6

What is the action of angiotensin II

Answer 6

Secretion of aldosterone from adrenals

Question 7

What are the two stimuli for aldosterone release?

Answer 7

Angiotensin II

Hyperkalaemia

Question 8

How does aldosterone work?

Answer 8

Acts on cortical collecting tubule (distal tubule) of nephron

Causes sodium retention and potassium excretion:
Aldosterone –> synthesis of sodium channels –> lumen becomes negative –> potassium moves down electrochemical gradient into lumen

Question 9

What are the main causes of hyperkalaemia?

Answer 9

First go thorough RAAS:

1. Renal failure (as reduced GFR –> less renin –> less aldosterone)
2. Reduced renin (rare)
3. ACE inhibitor (less ACE –> less angiotensin II –> less aldosterone)
4. AgII receptor blocker (–> less aldosterone)
5. Addison’s disease (less aldosterone)
6. MR receptor blockers = aldosterone antagonists

Then:

7. Rhabdomyolysis (damaged cells leak out potassium)
8. Acidosis (H+ enters cell, K+ moves into lumen following electrochemical gradient)

Question 10

Which 3 drug classes are associated with hyperkalaemia?

Answer 10

ACE inhibitors
ARBs
Spironolactone

Question 11

How can diabetic nephropathy cause hyperkalaemia?

Answer 11

Type 4 renal tubular acidosis - causes reduced renin which in turn reduces aldosterone –> less K+ excretion

Question 12

What ECG change is associated with hyperkalaemia?

Answer 12

Peaked T waves

Question 13

Why is hyperkalaemia not seen until late renal failure?

Answer 13

Initially reduced GFR –> increased aldosterone to compensate –> hyperkalaemia only occurs when compensatory mechanism falls

Question 14

How do you manage a patient with hyperkalaemia?

Answer 14

10mls 10% calcium gluconate (ONLY when potassium is over 6.5 or there are ECG changes)

20% dextrose + 10 units insulin (give dextrose alongside insulin to prevent hypoglycaemia)

Nebulised salbutamol (if hyperkalaemia is severe, it puts potassium into cells)

Treat the underlying cause

Question 15

What are the 3 broad causes of hypokalaemia?

Answer 15

GI loss
Renal loss
Redistribution into the cells

Question 16

Why is calcium gluconate used in hyperkalaemia?

Answer 16

It stabilises the membrane of cells in the myocardium (doesn’t actually reduce potassium)

Question 17

Why is insulin given in hyperkalaemia?

Answer 17

Drives potassium into cells

Question 18

When would you give fluids in a hyperkalaemic patient, as well as calcium gluconate + insulin?

Answer 18

If their hyperkalaemia is secondary to renal failure that is caused by hypovolaemia

Question 19

How does heart failure cause hyponatraemia?

Answer 19

Reduced CO –> reduced BP –> detected by baroreceptors –> Excess ADH –> sodium loss
ALSO
Lower BP –> lower GFR –> ADH –> sodium wasting

Question 20

What is reabsorbed in the ascending limb of the loop of henle?

Answer 20

Na
K
Cl

Question 21

What is reabsorbed in the DCT of the loop of henle?

Answer 21

Na

Cl

Question 22

Give 2 causes of impaired sodium resorption in the ascending limb of the loop of henle

Answer 22

Loop diuretics

Barter syndrome

Question 23

Give 2 causes of impaired sodium resorption in the DCT of the loop of henle

Answer 23

Thiazide diuretic

Gitelman syndrome

Question 24

What is the effect on potassium of impaired sodium resorption in the nephron?

Answer 24

More sodium delivered to distal nephron –> sodium goes from urine into cell, making urine more negative –> potassium drawn out into urine

Question 25

How does excess aldosterone affect potassium?

Answer 25

Causes increased sodium in cells –> exchanged with potassium –> hypokalaemia

Question 26

What are the 3 mechanisms of renal loss of potassium?

Answer 26

Hyperaldosteronism

Increased sodium delivery to distal nephron

Osmotic diuresis

Question 27

How does alkalosis affect potassium?

Answer 27

Hypokalaemia

Question 28

What are the top 3 methods of potassium loss?

Answer 28

GI loss (vomiting)
Renal loss
Redistribution into cells (insulin/ beta agonists/ alkalosis - treatments for hyperkalaemia!)

Question 29

What are the clinical features of hypokalaemia?

Answer 29

Muscle weakness
Cardiac weakness (arrhythmia)
Polyuria and polydipsia (somehow potassium affects ADH, causing nephrogenic DI)

Question 30

What screening test would you order in a patient with hypokaleamia and hypertension?

Answer 30

Aldosterone: renin ratio

Question 31

How does Cushing’s affect potassium?

Answer 31

Excess cortisol acts like excess aldosterone –> hypokalaemia

Question 32

Where is hyperkalaemia detected?

Answer 32

Adrenal cortex

Question 33

How does osmotic diuresis affect potassium?

Answer 33

Drags potassium out with water loss –> hypokalaemia

Question 34

How would you manage a patient with hypokalaemia?

Answer 34

Potassium between 3-3.5: oral KCl

Potassium <3: IV KCl, max rate = 10mmol/hour

Treat the underlying cause

Question 35

How does cirrhosis cause hyponatraemia?

Answer 35

Cirrhosis causes vasodilators to be released (nitric oxide)

BP reduced

ADH released

Question 36

How come SIADH doesn’t cause hypervolaemia?

Answer 36

Initial hypervolaemia is followed by NP release –> natiuresis –> euvolaemia –> euvolaemic hyponatraemia