Lecture 21: Mg and K Flashcards
Describe the distribution of Mg:
- ECF contains 1%
- Mostly in Bone, muscle and soft tissue
What is the role of Mg in the bodies physiology?
- Bone formation
- Cofactor for many enzymatic reactions
- Regulation of vascular tone
- Cardiac rhythm
- Platelet activated thrombosis
Whats the role of the kidneys in Mg regulation?
- Serum Mg is controlled by renal excretion
- 95% reabsorbed ~5% secreted - throughout the kidney but mainly the ascending LOH (usually paracellularly) DCT has transporter.
How do you assess Mg status?
- Serum Mg (poor indicator of upstream stores)
- Red cell Mg
- 24hr excretion
- Mg retention test
- Isotope analysis
What are the causes of hypomagnesamia?
1) Decrease diet
2) Gi malsorption or loss
3) Endocrine - Hypoaldosteroneism, DM, SIADH
4) Renal loss
What drugs can induce renal loss of Mg?
- Aminoglycosides
- Omeprazole
- Pentamidine
What are the Sx of hypomagnesaemia?
- Weakness and fatigue
- Fasciculations/cramps
- Tetany
- Seizures
- Arrhythmias
How do you treat hypomagnesium?
- Oral tabs
- Could use IV
When is hypermagnesaemia seen?
Rare
- CKD compensatory mechanisms fail might cause it
- Addisons or hypothyroidism
Sx of hypermagnesium?
- Hypotension
- Cutaneous flushing
- N&V
Describe the normal K levels in the body:
3.5->5.0mmol/L in plasma
ICF ~150mmol.L
What does daily K homeostasis rely on?
Daily K oral intake, most of which is excreted in the kidneys
Describe the internal balance of K:
Initial changes in ECF K are intially buffered by movement of K into or out of skeletal muscle regulated by insulin and catecholamines.
Describe how tonicity influences K:
Hyperglyceamia will lead to a K efflux from the cell.
Describe how pH influences K:
- Acidosis can also drive K efflux
- Alkalosis will lead to K influx
Where is K reabsorbed? and secreted
- 60% PCT (Paracellular)
- 30% Asc. LoH (Paracellular) but ROMK also pumping it out.
- Aldo promotes Na/KATPase, K in cell is secreted) (and ENAC for Na reabsorption)
What affects serum K?
- K intake
- K losses
- K redistributed from ECF in/out of cells
What are the Sx of hypokalaemia?
Serum <3.5mmol
- Muscle weakness if not paralysis
- Cramps
- Constipation
- Cardiac conduction abnormalities
What is hypokalaemic periodic paralysis?
- Autosomal dominant or thyrotoxic
- Abnormal K channels on cell membrane
- High carb meal (Inuslin) or SNS activation i.e exercise or anxiety
= Excessive movement of K into cells = extreme weakness
What are the causes of K loss?
Renal:
- Hyperaldosteronism (tumor)
- Licorice
- Diuretics that arent K sparring
- Renal tubular acidosis
Gut
- V&D
- Laxitives
- Ileostomy
Whats conns syndrome?
Adrenal adenoma
- Secretes aldosterone
Presents with:
- Hypertension
- Hypokalaemia
How does licorice cause hypokalaemia?
Acts on mineralcorticoids receptor = psuedohyperaldosteroneism
Which diuretics influence K?
PCT: Mannitol
Asc LOH: Fruosemide
DCT: Metolozone
CD: Spironolactone or amiloride
How is hypokalaemia treated?
- Treat the underlying problem
- Use oral K for mild and IV for severe
What are the Sx of hyperkalaemia?
K > 5.2mmol/L
Symptoms
- Fatigue or weakness
- Paraesthesia
- Nausea and vomiting
- Dyspnoea
- Palpitations
What is the risk of hyperkalaemia?
Ventricular fibrillation and cardiac arrest
Prolonged P-P
What can cause psuedohyperkalamia?
When a lab sample is heamolysed
What are the causes of hyperkalaemia?
Increased intake
Disruption of cell intake
- Beta blockers
- Acidosis
- Rhabdomyolysis
Decreased excretion
- Renal failure
- Hypoaldosteronism
- ACEi/ARB
Whats happening in addisons?
- Deficient secretion of aldosterone and cortisol
Bronzed because heightened ACTH causes melanocyte sitmulation
How is addisons diagnosed and treated?
Lab: Hyperkalaemia, Hyponatrreamia
Diagosis: Short synacthen test
Treated: Dexamethasone, fludrocortisone
How is hyperkalaemia treated?
VIP
- Stabilise AP
- Push K into cells (beta agonists or insulin (need dextrose too)
- Reduce K absorption (Cation exchange i.e Ca resonium binds K in gut and eliminates)
- Treat acidosis (Bicarb)
- Increase elimination (K losing diuretic or dialysis)
- Fix underlying casues
How is the AP stabilised?
- Ca stabilises AP
- Normalises membrane excitability
- Lasts 30 mins
- Prevents cardiac arrest while you sort the rest out
What are the cells of the kidney mainly involved with K secretion?
Principle (ROMK) and intercalated cells (K/H exchanger)
