Regulation of potassium Flashcards
What is the normal range for potassium?
- 3.5-5.5 mmol/L
What happens if extracellular K+ rises?
- Resting membrane potential is decreased i.e. becomes less negative (depolarised)
- Can lead to asystole
What happens if extracellular K+ falls?
- Resting membrane potential is increased - i.e. becomes more negative (hyperpolarised)
- Can result in ventricular fibrillation
What features are seen on an ECG if someone has hypokalaemia?
- Slightly prolonged PR interval
- Slightly peaked P wave
- ST wave depression
- Shallow T wave
- Prominent U wave
What features are seen on an ECG if someone has hyperkalaemia?
- Wide, flat P wave
- Prolonged PR interval
- Widened QRS
- Decreased R wave amplitude
- Depressed ST segment
- Tall, tented T wave
How is potassium handled in the nephron?
- K+ is brought into the cell by Na+/K+/2Cl- transporter
- K+ secreted into lumen of nephron by ROMK - then excreted in urine
What are the clinical features of hyperkalaemia?
- Can be asymptomatic so only picked up on ECG
- Muscle weakness
- Cardiac arrhythmias
What can hyperkalaemia result from?
- Lack of excretion
- Release from cells e.g. during rhabdomyolysis
- Excess administration (too much K+ in fluid)
What is the emergency treatment for hyperkalaemia?
- Calcium gluconate
- Insulin
- Calcium resonium
How does calcium gluconate treat hyperkalaemia?
- Ca2+ stabilises the myocardium
- Prevents arrhythmias
How does insulin treat hyperkalaemia?
- Drives K+ into cells
- Lowers plasma concentration
- Given with glucose to avoid hypoglycaemia
How does calcium resonium treat hyperkalaemia?
- Removes K+ by increasing excretion from the bowels
- Only way to remove K+ without renal replacement therapy
How do we treat hyperkalaemia after the emergency treatment?
- Low potassium diet
- Stop offending medications
- Furosemide enhances potassium loss in urine
- Dialysis?
What causes hypokalaemia?
- Reduced dietary intake
- Increased entry into cells
- Increased GI losses
- Increased urine loss
What are the clinical effects of hypokalaemia?
- Muscle weakness, cramps, tetany starting in lower extremities
- Vasoconstriction and cardiac arrhythmias
- Impaired ADH action causing thirst, polyuria, and no concentration of urine
- Metabolic alkalosis due to increase in intracellular H+ concentration
How do we treat hypokalaemia?
- Treat the cause - diuretics, diarrhoea, poor oral intake of potassium
- Give potassium replacement
- Oral: bananas, oranges, sando-K
- IV - add KCl to IV bags
- Potassium sparing diuretics - spironolactone, amiloride
Where is most body potassium found?
- Intracellularly
How is K+ excreted from the body?
- Kidneys excrete 80% of K+
- Bowel excretes 20%
How does the kidney respond to acidosis?
- Alpha cells (of collecting duct)
- Carbonic anhydrase convert H2O and CO2 from respiration into H2CO3
- H2CO3 breaks down to H+ and HCO3-
- H+ is exchanged for K+ by H+/K+ ATPase
- K+ brought into cells and H+ excreted in urine
- HCO3- goes into blood and buffers H+, making H2CO3
- K+ also moves into blood
How does the kidney respond to alkalosis?
- Beta cells (of collecting duct)
- H2O and CO2 from respiration are converted to HCO3- and H+ by carbonic anhydrase
- HCO3- exchanged with Cl- (HCO3- excreted in urine)
- K+ brought into beta cells and exchanged for H+
- H+ moves into blood
How can acidosis affect potassium levels in the blood?
- Can cause hyperkalaemia
How can alkalosis affect potassium levels in the blood?
- Can cause hypokalaemia
How can acidosis lead to hyperkalaemia?
- H+ is taken up into cells in exchange for K+ ions which are released into the blood
How can alkalosis lead to hypokalaemia?
- H+ is released from cells in exchange for K+ ions which are taken up into cells
