Nephrology
Question 158 of 180
A 29 year old man presents to the Emergency Department complaining of fatigue and muscle cramps. He is found to be hypokalaemic. A 12-lead ECG is arranged. Which of the following ECG changes is associated with hypokalaemia?
Answer:
ECG features of hypokalaemia include:- U waves
- Prolongation of PR interval
- T wave flattening
- ST segment changes
- Arrhythmias
- Cardiac arrest (VF/pVT, PEA, asystole)
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 |
