Electrolytes

Master answer

Potassium

• Predominany intracellular cation
• Total Stores: Approx 3200mmol (50mmol/kg)
• Key Functions:
  • Main determinant of ICF osmolality and tonicity
  • Responsible for RMP of excitable cells via Goldmann-Hodgkin-Katz due to ↑↑gK relative to other species
  • Role in action potential → repolarisation phase
  • Secretion of insulin and multiple other KATP dependant processes
  • Regulation of IC processes (protein/glycogen synthesis)
  • Involved in Na+/K+ ATPase in cell membranes

Potassium balance / Determinants of Serum Potassium

• Intake
  • Oral – very variable
  • Approx. 50-200mmol/day

• Distribution
  • Exchangable Pool
    • ICF (90%): [K+] 150 mmol/L (1° ICF cation)
    • ECF (2%): [K+] 3.5-5 mmol/L
  • Non-exchangable Pool
    • Bone (8%)

• Transcellular balance

• Elimination
  • Faecal – 8mmol/day
  • Renal – 92mmol/day (See next heading for details)
    • Urinary K+ excretion = [K+ filtered by glomerulus] + [K+ secreted by CCD/LDCT] – [K+ reabsorbed by renal tubules]
    • Glomerular filtration: Freely filtered = 756mmol/day (180L x 4.2mmol/L)
    • PCT: 65% reabsorption
    • LoH: 25~30% reabsorption
    • DCT/Collecting Ducts – variable
      • Determined by Aldosterone, Plasma [K+], Tubular Flow rate
      • Secreted by principal cells
      • Reabsorbed by intercalating cells

JC / Gladwin / Sakurai 2020

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

JC / Sakurai 2019

Answer by JC, Sakurai