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Nephrology

Question 18 of 300

You have been asked to give a teaching session to a group of final year medical students. The topic of the session is hypokalaemia. Which of the following abnormalities is commonly associated with hypokalaemia?

Answer:

Many patients who are potassium deficient are also deficient in magnesium. Magnesium is important for potassium uptake and for the maintenance of intracellular potassium levels, particularly in the myocardium. Repletion of magnesium stores will facilitate more rapid correction of hypokalaemia.

Hypokalaemia

Nephrology

Last Updated: 4th October 2022

Hypokalaemia is the most common electrolyte disorder in clinical practice. It is seen in up to 20% of hospitalised patients. Hypokalaemia increases the incidence of arrhythmias and sudden cardiac death. The risk is increased further in patients with pre-existing heart disease and in those treated with digoxin.

Definition

Hypokalaemia is defined as a serum potassium level < 3.5 mmol/L.

Severe hypokalaemia is defined as a serum potassium level < 2.5 mmol/L and may be associated with symptoms.

Causes

  • Increased potassium loss
    • Gastrointestinal loss e.g. diarrhoea, vomiting, ileostomy, intestinal fistula
    • Drugs e.g. diuretics, laxatives, steroids
    • Renal disorders e.g. renal tubular disorders, diabetes insipidus, dialysis
    • Endocrine disorders e.g. Cushing's syndrome, hyperaldosteronism
  • Transcellular shift
    • Metabolic alkalosis (when serum pH increases, serum potassium decreases because potassium shifts from the vascular to the cellular space)
    • Insulin/glucose therapy
    • Salbutamol and other beta-agonists
    • Theophylline
  • Magnesium depletion
  • Poor dietary intake

Clinical Features

  • As serum potassium concentration decreases, the nerves and muscles are predominantly affected causing:
    • Fatigue
    • Muscle weakness
    • Leg cramps
    • Constipation
  • In severe cases, with serum potassium < 2.5 mmol/L, clinical features include:
    • Rhabdomyolysis
    • Ascending paralysis
    • Respiratory difficulties
    • Cardiac arrest
  • ECG features include:
    • U waves
    • Prolongation of PR interval
    • T wave flattening
    • ST segment changes
    • Arrhythmias
    • Cardiac arrest (VF/pVT, PEA, asystole)

Treatment

  • This depends on the severity of hypokalaemia and the presence of symptoms and ECG abnormalities.
  • The potential cause should be identified and addressed.
  • Gradual replacement of potassium (via oral route) is preferable, but in an emergency IV potassium is required.
  • The maximum recommended IV dose of potassium is 20 mmol/h, but more rapid infusion (e.g. 2 mmol/min for 10 min, followed by 10 mmol over 5 - 10 min) is indicated for unstable arrhythmias when cardiac arrest is imminent. 0.9% sodium chloride is the preferred infusion fluid as 5% glucose may cause transcellular shift of potassium into cells.
  • Continuous ECG monitoring is essential during IV infusion and the dose should be titrated after repeated sampling of serum potassium levels.
  • Many patients who are potassium deficient are also deficient in magnesium. Magnesium is important for potassium uptake and for the maintenance of intracellular potassium levels, particularly in the myocardium. Repletion of magnesium stores will facilitate more rapid correction of hypokalaemia.

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l
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