K & Mg physiology

Question 1

role of Mg

Answer 1

• bone formation
• enzyme co-factor = ATP metabolism, muscle contraction/relaxation, neurological function
• regulation of vascular tone
• cardiac rhythm
• platelet activation

Question 2

Mg reabsorption

Answer 2

glomerulus
- freely filtered
proximal tubule
- 15% reabsorption
Loop of Henle
- 70% reabsorption in thick ascending limb
- paracellular channel formed by claudin
Distal tubule
- 10% reabsorption
- by transient receptor potential channels (TRPC6)
- mutations give hypomagnesaemia & hypercalcaemia

Question 3

Assessing Mg

Answer 3

• serum Mg
• red cell Mg
• 24 hour excretion
• Mg retention test
• isotope analysis

Question 4

causes of hypomagnesemia

Answer 4

1. decreased dietary intake
2. GI malabsorption
3. endocrine (hyperaldosterone, SIADH)
4. renal loss (congenital, drug-induced)
   - proton pump inhibits, amphotericin B

Question 5

symptoms of hypomagnesemia

Answer 5

• weakness, fatigue
• fasciculations, cramps, tetany
• numbness
• seizures
• arrhythmias

Question 6

treatment for hypomagnesemia

Answer 6

• oral (most cases)

- IV magnesium sulphate

Question 7

hypermagnesaemia

Answer 7

• less common
• cause = excessive oral administration of Mg / Mg-containing drugs (in GORD drugs)
  - advanced CKD possible, less common

Question 8

homeostasis of K

Answer 8

• initial changes buffered by movement in/out of skeletal m (regulated by insulin, catecholamines)
• hyperglycemia & acidosis cause K efflux from cell
• serum K affected by intake, losses, redistribution from ECF/cells

Question 9

K reabsorption

Answer 9

glomerulus
- freely filtered

proximal tubule

- 60% reabsorption
- passive paracellular movement with water (due to Na/K exchanger)

Loop of Henle

- 30% reabsorption in thick ascending limb
- active reabsorption by NKCC2 channel
- ROMK channel & Na/K/ATPase also involved 

Distal tubule & CD

- variable reabsorption/secertion
- influenced by aldosterone, angiotensin

Question 10

hypokalaemia and its symptoms

Answer 10

• serum K <3.5mmol/L

- symptoms = muscle weakness, paralysis, cramps, cardiac conduction abnormalities, constipation

Question 11

causes of hypokalaemia

Answer 11

GI loss
-vomiting, diarrhea, laxatives, ileostomy

renal loss

• hyperaldosterone
• diuretics (e.g. loop diuretics - furosemide)
• renal tubular acidosis
• liquorice (produces enzymes w/ aldosterone-like effect)

Question 12

treatment for hypokalaemia

Answer 12

• treat underlying cause
• mild hypokalemia (3-3.5) = use oral K
• sever (<3) = use IV replacement

Question 13

hyperkalemia and its symptoms

Answer 13

• serum K > 5mmol/L
• symptoms = fatigue, weakness, paresthesia, nausea, vomiting, dyspnoea, palpitations
• pseudo-hyperkalemia = breakdown of RBCs in sample raises K

Question 14

causes of hyperkalemia

Answer 14

• increased intake (usually in kidney disease)
• disruption of cell intake
  - acidosis
  - beta blockers
  - rhabdomyolysis
• decreased excretion
  - renal failure
  - hypoaldosteronism (spironolactone)
  - drugs = ACEi, AIIA

Question 15

Addison’s disease

Answer 15

• deficient adrenal cortex hormones (aldosterone, cortisol)
• gives characteristic hyper-pigmentation (excessive ACTH & MSH production)
• lethargy, weakness, weight loss, low BP
• electrolytes = low Na, high K
• diagnosis by synacthen test
• treated w/ synthetic hormones (e.g. dexamethasone)

Question 16

ECG changes with hyperkalemia

Answer 16

• prolonged PR, wide QRS

- progressing to peaked T waves, loss of QRS complex -> VF

Question 17

treatment for hyperkalemia (if abnormal ECG)

Answer 17

short term = drive K into cells to stabilise AP:

• IV Ca
• salbutamol nebuliser
• insulin w/ dextrose

after that:

• reduce K absorption
• increase elimination (K losing diuretics e.g. furosemide, dialysis)

fix underlying problem
- acidosis = give oral bicarbonate tablets or IV in emergency