Electrolytes

Master answer

Serum Potassium

• Normally 3.5~5mmol/L
• Hyperkalaemia [K+] >5mmol/L
• Severe hyperkalaemia [K+] >7mmol/L

ECG Changes

• Peaked T waves at [K+] > 5.5mmol/L
• Loss of P wave at [K+] > 6.5 mmol/L
• QRS widening at [K+] > 7.5mmol/L
• VF or asystole

Consequences of Acute hyperkalaemia

• CVS
  • ECG changes
    • Tenting of T wave
    • Flattening of P wave and increase PR interval
    • Widening of QRS
    • VF
  • Arrythmia
    • VF
• CNS
  • Lethargy
• Other
  • Flaccid paraylis
  • Muscle weakness
  • Respiratory failure

Management

Drugs that eliminate body K+

• Loop diuretics e.g. frusemide
  • 40mg IV
  • Inhibition of the Na+/K+/2Cl- cotransporter on luminal membrane of the thick ascending Loop of Henle
  • This causes loss of normal positive charge in lumen, leading to the loss of paracellular resorption of K+
  • Net result is K+ excretion
  • Elimination half-life 45-9 mins
• Cation exchange resins e.g. sodium polystyrene sulfonate
  • 15~45gm
  • Not absorbed in GI tract
  • Prevents potassium reabsorption in the GI tract (predominantly in the colon where K+ secretion occurs)
  • Onset of action: 1 hr PR, 4-6hrs PO
  • Duration: variable
  • No effect acutely

Drugs causing increased intracellular uptake of K+

• Insulin/glucose
  • 10 units IV (with 50ml 50% dextrose)
  • Insulin receptor agonism → transcellular shift of K from extracellular to intracellular space via upregulation of Na/K ATPase → transient reduction in serum [K+]
  • Onset: 20-30 mins
  • Duration: 2-6 hours (prolonged if i.v. infusion)
• Sodium bicarbonate
  • Alkalosis increases the activity of the Na+/K+ ATPase pump, increasing K+ uptake
  • Onset: 30-60 mins
  • Duration: 2-3 hours
• β2 agonists e.g. salbutamol
  • 5~20mg via nebulizer, repeated (or 6-12 puffs via MDI, repeated)
  • β2 agonism → increased intracellular [cAMP] → transcellular shift of K from extracellular to intracellular space → transient reduction in serum [K+]
  • Onset: 30 mins
  • Duration: 2-3 hours
• I.V. fluid
  • Causes haemodilution
  • Can increase renal excretion of K+ by increasing renal perfusion and increasing urine output

Other Treatments in Hyperkalaemia:

• Calcium (No effect on K+ level)
  • 10ml 10% CaCl (6.8mmol) or 10% CaGluconate (2.2mmol)
  • Immediate onset
  • Stabilizes cardiac membrane by reducing threshold potential
• Dialysis
  • via continous Veno-Venous Dialysis or intermittent haemodialysis
  • Dialysis fluid adjusted to create large concentration gradient for potassium
    extraction via dialysis of filtration

JC / Sakurai 2019

Answer by JC, Sakurai