• Plasma potassium level <3.5 mmol/L or below normal level defined by local laboratory
• Symptoms may occur if potassium level <3 mmol/L 
• May be a late sign of total body potassium depletion due to mobilisation of intracellular potassium stores 

• Muscle weakness and paralysis
• Arrhythmias (premature atrial and ventricular beats, sinus bradycardia, paroxysmal atrial or junctional tachycardia, atrioventricular block, and ventricular tachycardia or fibrillation)
• ECG changes
  • increased amplitude and width of P wave
  • prolongation of PR interval
  • T wave flattening and inversion
  • ST depression
  • prominent U waves (best seen in precordial leads)
  • apparent long QT interval due to fusion of T and U waves

T wave inversion and prominent U wave

Apparently long QT interval (actually T-U fusion)

• Low oral intake/potassium concentration in IV fluids
• Renal loss
  • diuretics
  • bicarbonate administration
  • renal tubular acidosis
  • inherited salt-losing tubulopathies e.g. Bartter syndrome
• Diarrhoea (Note: potassium content of lower GI loss is greater than upper GI loss)
• Alkalosis (approximately 0.4 mmol/L fall in potassium for every 0.1 unit rise in pH)
• Insulin administration
• Salbutamol administration (high-dose, nebuliser/IV)
• Liposomal-amphotericin (prolonged use)
• Doxapram
• Increased mineralocorticoid activity, due to:
  • hypovolaemia
  • 11- beta-hydroxylase deficiency (rarer form of congenital adrenal hyperplasia – presents with virilization, hypertension, and hypokalemia)
  • primary hyperaldosteronism

• Confirm value on venous laboratory sample (Note: ‘normal’ value on capillary sample may be falsely reassuring if sample has haemolysed and true value is lower)
• ECG
• Cardiac monitor, if ECG changes present
• Mild hypokalaemia (serum level 3–3.5 mmol/L) does not require investigation 
  • if hypokalaemia persists or >2 mmol/kg/day maintenance is required – investigate as for significant hypokalaemia (below)
• If significant hypokalaemia (serum level <3 mmol/L) and no obvious cause check:
  • acid/base balance and bicarbonate level on blood gas
  • urinary potassium level. Level >20 mmol/L suggests excess renal potassium losses 
  • if baby is hypertensive, measure plasma renin and aldosterone 
• If hypokalaemia not responding well to replacement, check magnesium level

• Supplement feeds/fluids
  • normal maintenance potassium requirement is 2 mmol/kg/day
  • higher amounts will be needed to correct hypokalaemia
• Review medications
  • if baby is on insulin infusion, consider stopping

Symptomatic babies

• Give rapid potassium supplementation
  • 'Strong potassium' solution
  • contains 20 mmol/10 mL 
  • must be diluted at least 50-fold with sodium chloride 0.9% or a mixture of sodium chloride 0.9% in glucose before administration
  • maximal peripheral concentration 40 mmol/L (1 mmol in 25 mL)
  • maximal central concentration 80 mmol/L (1 mmol in 12.5 mL)
  • rate 0.2 mmol/kg/hr (maximum 0.5 mmol/kg/hr if severe potassium depletion) 
• Continuous cardiac monitoring necessary
• Recheck potassium 2–4 hr and assess need for continuing infusion

Asymptomatic babies

• Potassium replacement given according to how baby is being fed:
  • orally fed babies
    • oral supplement e.g. potassium chloride 1 mmol/kg 12-hrly. Titrate dose according to response
  • babies on IV fluids
    • add potassium chloride 3−5 mmol/kg/day to IV fluid, depending on electrolyte levels and titrate according to response
  • babies receiving parenteral nutrition (PN) 
    • increase potassium content of the PN to 3–5 mmol/kg/day 
    • if modified PN not available, run separate potassium infusion 3–5 mmol/kg/day alongside current PN 

• Monitor potassium levels according to clinical need:
  • well babies receiving oral potassium check level once to twice weekly 
  • well babies on IV fluids or PN with mild hypokalaemia (potassium 3–3.5 mmol/L) check level daily
  • check more frequently if:
    • significant hypokalaemia (serum level <3 mmol/L)
    • symptomatic hypokalaemia 
    • concentrations of potassium >5 mmol/kg/day are being given
• Once plasma/serum potassium level is in normal range, continue potassium supplementation to allow replenishment of total body potassium (intracellular) stores:
  • orally fed – continue for a further week 
  • IV fluids/PN − reduce potassium to 2 mmol/kg/day as maintenance
  • recheck potassium level following these changes to ensure hypokalaemia does not recur