W6 - K+ & electrolytes

Question 1

What is the most abundant intracellular cation?

Answer 1

Potassium

Question 2

What is the range for potassium serum concentration?

Answer 2

3.5-5.0 mmol/L

Question 3

Which hormones are involved in renal regulation of potassium?

Answer 3

1. Angiotensin II
2. Aldosterone

Question 4

Describe the renin-angiotensin-aldosterone system (RAAS)

Answer 4

Renin is released from the juxtaglomerular apparatus => angiotensinogen (from liver) becomes angiotensin I => Ag I is converted to Ag II in the lungs by ACE => Ag II stimulates the release of aldosterone from the adrenal glands => aldosterone causes Na+ and H2O retention, K+ excretion into urine = hence BP and BV go up

Question 5

What are the stimulants for the production of aldosterone?

Answer 5

1. Angiotensin II (via RAAS)
2. High potassium

Question 6

Where in the kidney does aldosterone act?

Answer 6

On principal cells in the cortical collecting tubule

Question 7

Describe the normal physiology of potassium excretion in collecting tubule?

Answer 7

Na reabsorption through ENac (epithelial sodium channels) leads to tubular lumen negative electrical potential => potassium moved down the electrical gradient into the negatively charged lumen (urine) => K+ Excretion

Question 8

How does aldosterone induce K+ excretion in the collecting tubule?

Answer 8

Aldosterone binds the MR (mineralocorticoid receptor) => increased sodium channels => increased Na+ reabsorption from lumen => lumen negatively charged => K+ moves down its electrical gradient to lumen, and out into urine

Question 9

What are the main causes (4 categories) of hyperkalaemia?

Answer 9

1. Renal impairment – i.e., renal failure
2. Drugs – i.e., ACEi (ramipril), ARBs (losartan), aldosterone antagonist (spironolactone)
3. Low aldosterone – Addison’s disease, type 4 renal tubular acidosis (diabetic nephropathy; low renin => low aldosterone)
4. Release from cells – i.e., rhabdomyolysis, acidosis (maintain electroneutrality)

Question 10

What is the main ECG change associated with hyperkalaemia?

Answer 10

Peaked T waves

Question 11

How would you manage a patient with hyperkalaemia?

Answer 11

1) 10 ml 10% calcium gluconate
2) 100 ml 20% dextrose + 10 units of insulin*
3) Nebulised salbutamol

+ Treat the underlying cause

*dextrose given with insulin to ensure the insulin alone does not make patient hypoglycaemic

Question 12

What are the causes (4 main categories + more specific ones) of hypokalaemia?

Answer 12

1. GI loss
2. Renal loss
   - Hyperaldosteronism (excess cortisol)
   - increased sodium delivery to distal nephron
   - Osmotic diuresis (thiazide diuretics, loop diuretics)
3. Redistribution into the cells
   - insulin, beta-agonists, alkalosis
4. Rare causes:
   - renal tubular acidosis type 1 & 2, hypomagnesaemia

Question 13

Explain the MOA of renal potassium loss

Answer 13

Reabsorption of sodium can be blocked anywhere along the nephron:

Loop of Henle blockage => loop diuretics, Barter syndrome (deficiency of Na+/K+/Cl- channel)

Distal tubule => thiazide diuretics, Gitelman syndrome

Increased sodium delivery to collecting tubule => potentiates electrical gradient => K+ loss

Question 14

What are the clinical features (3) of hypokalaemia?

Answer 14

1. Muscle weakness
2. Cardiac arrhythmia
3. Polyuria & polydipsia (nephrogenic DI)

Question 15

Why does hypokalaemia cause polyuria and polydipsia?

Answer 15

Hypokalemia leads to resistance to ADH => will NOT reabsorb water => polyuria + polydipsia (nephrogenic DI)

Question 16

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

Answer 16

Aldosterone:renin ratio (as you suspect Conn’s disease)

NB: ratio would be HIGH in Conn’s!

Question 17

How would you manage a patient with hypokalaemia?

Answer 17

Serum K+ 3.0-3.5 mmol/L

• oral potassium chloride (2 SandoK tablets tds for 48 hours)
• Recheck serum potassium

Serum K+ <3.0 mmol/L

• IV potassium chloride
• maximum rate 10 mmol/hr
• Treat the underlying cause, i.e. spironolactone

Question 18

Why is IV potassium chloride only given at max rate 10 mmol/hr?

Answer 18

Rates >20 mmol/hr are highly irritating to peripheral veins

Question 19

Hyperkalaemia is a side effect of which drug?
A) Furosemide
B) Bendroflumethiazide
C) Salbutamol
D) Ramipril

Answer 19

D) rampiril

Question 20

Hypokalaemia is a side-effect of which of the following drugs?
A) Spironolactone
B) Indomethacin
C) Perindopril
D) Furosemide

Answer 20

d) furosemide