Potassium - Hypokalaemia
Background
Hypokalaemia is defined as a serum potassium of < 3.5mmol/L
Can be further subclassified as per below:
MILD – 3.0-3.5mmol/L
MODERATE – 2.5-3.0mmol/L
SEVERE <2.5mmol/L
Hypokalaemia generally leads to hypomagnesaemia, so it is important to measure and replace Mg2+ as required.
Hypokalaemia mechanism | Cause |
Increased urine loss | Hypomagnesaemia |
Increased gastrointestinal loss | Diarrhoea and vomiting |
Reduced potassium intake | Poor nutrition |
Hyperaldosteronism | Primary |
Shift of potassium into cells | Phaeochromocytoma Anabolic state Insulin overdose Beta adrenergic drugs |
(Adapted from eTG hypokalaemia table 5.21)
Assessment
History and examination
- Can be asymptomatic
- Weakness, lethargy, paralysis, tetany
- Cardiac dysrhythmias if severe: VT, VF, heart block, asystole
Management
Investigations
EUC, CMP, Albumin
ECG done early on suspicion of electrolyte abnormalities:
- T wave flattening/inversion (earliest sign)
- Increased P wave amplitude
- Prolongation of PR interval
- Widespread ST depression
- Prominent U waves (Best seen in the precordial leads V2-V3)
- Apparent long QT interval due to fusion of T and U waves (= long QU interval)
Treatment
Cardiac monitoring and treatment of cause are the underlying principles. The rate of replacement and methods are highly dependent on the symptoms and signs and degree of deficit.
MILD Hypokalaemia (3-3.5mmol/L)
Oral is usually best but can lead to GI upset
- ORAL (eg slowK): potassium chloride sustained release 1200 to 3600mg (16-48mmol) daily in divided doses
OR
- Effervescent (eg chlorvescent): Potassium 548 to 1096mg (14-28mmol) dissolved in water and taken orally, 2 to 3 times daily.
MODERATE TO SEVERE Hypokalaemia (2.5- 3.0mmol/L and asymptomatic)
Replace as per mild if can take oral
Consider IV replacement as for severe – if unable to take oral, associated with symptoms muscle paralysis or cardiac rhythm disturbance with ECG evidence of hypokalaemia
MODERATE TO SEVERE Hypokalaemia (<3.0mmol/L and symptomatic)
Potassium chloride 20 to 40mmol/L IV according to premixed infusion bag:
- Peripheral access: 20mmol KCl in 1L 0.9% sodium chloride over 2hrs
- Central access: 20mmol KCL in 0.9% sodium chloride over 1 hour
Rapid intravenous administration or overdose may cause cardiac arrest.
Rate of infusion through a peripheral IV line should not exceed 10mmol/hour
Rate of infusion through a central line should not exceed 40mmol/hour
Refer to critical care for higher rates
Cardiac monitoring whilst infusing and serum potassium concentration measured every 2 hours
Concurrent oral can be used if conscious
If magnesium low (<0.8mmol/L) give MgSO4 2.5g (10mmol) over 4hrs IV
Once the hypokalaemia is no longer severe, the rate of intravenous potassium repletion should be reduced or changed to oral therapy. Patients should be treated until the serum potassium concentration is persistently above 3.0 to 3.5 mmol/L and symptoms or signs attributable to hypokalaemia have resolved
Consider consultation with critical care when
Severe hypokalaemia requiring iv replacement