Physiology
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This was previously featured in an exam
Renal
Question 61 of 180
A 44 year old woman is sent into ED by her GP with an abnormal ECG and blood test results following some routine tests. Which of the following electrolyte disorders is most likely to cause these changes:
Answer:
This ECG shows the classical ECG changes of hypokalaemia: ST segment depression, inverted T waves, large U waves, and a slightly prolonged PR interval.Hypokalaemia
Physiology / Renal / Potassium Balance
Last Updated: 21st April 2019
RCEM defines normal values for total serum potassium as 3.0 - 4.5 mmol/L.
Causes
Causes of hypokalaemia can be divided into:
- Increased loss
- Renal loss:
- Thiazide or loop diuretics
- Renal tubular acidosis
- Hypomagnesaemia
- Hyperaldosteronism
- Transporter mutations (rare)
- Drugs such as penicillins, aminoglycosides, amphotericin
- GI loss:
- Diarrhoea, vomiting
- Renal loss:
- Transcellular shift of K+ into cells
- Metabolic alkalosis
- Insulin excess or overdose
- Catecholamines in acute stress e.g. acute MI
- Beta-2 sympathomimetics
- Decreased intake
- Inadequate replacement whilst NBM or on TPN, malnutrition, anorexia nervosa, hypocaloric diets
Clinical Features
Clinical features of hypokalaemia may include:
- Asymptomatic
- Muscle weakness
- Muscle cramps, rhabdomyolysis and myoglobinuria
- Ascending paralysis resulting in respiratory failure
- Constipation
- Gut ileus with distension, anorexia, nausea and vomiting
- Impaired ADH action with polyuria and polydipsia
- ECG changes and cardiac arrhythmias
ECG Changes
Hypokalaemia increases automaticity and delays repolarisation of cardiac cells. This predisposes the heart to dysrhythmias such as ectopic beats, atrioventricular block and atrial and ventricular fibrillation.
Classic ECG changes:
- Progressive lengthening of the PR interval
- ST segment depression
- Flattening of the T wave
- Prominent U waves
- Prolongation of QT interval
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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 |
