ELECTROLYTE DISORDERS

Question 1

What abnormality does this ECG indicate? Point out the three main abnormalities. Which muscular symptoms would you expect in this patient?

Answer 1

Displays many of features of hyperkalaemia:

• Prolonged PR
• Broad, bizarre QRS complexes: merge with preceding P and subsequent T waves
• Peaked T waves

Hyperkalaemia causes muscle paralysis: starts at lower extremities then trunk then upper extremities. Note that very wide QRS complexes are almost always due to hyperkalaemia

Question 2

What is the most common cause of hyperkalaemia?

Answer 2

Acute and chronic kidney disease

Question 3

Explain the mechanism by which insulin Deficiency and beta-blockers both cause hyperkalaemia

Answer 3

Sodium/potassium ATPase is activated by adrenaline and insulin

When there is lack of insulin or during beta-blockade the pump is less active → more potassium outside cells

Question 4

How does digoxin affect potassium levels? Explain why

Answer 4

Blocks sodium/potassium ATPase: causes hyperkalaemia

Question 5

Explain why tumour lysis syndrome causes hyperkalaemia. Explain why hyperosmolarity causes hyperkalaemia

Answer 5

• Tumour lysis syndrome: cell contents released
• Hyperosmolarity: water driven out of cells, potassium follows

Question 6

What does this ECG show? What muscle symptoms would you expect in a patient with this ECG?

Answer 6

• ST depression
• T wave inversion
• U waves
• Long QU interval

Indicative of hypokalaemia: expect muscle weakness/paralysis (lower extremities then trunk then upper extremities)

Question 7

What is the origin of the U wave?

Answer 7

Origin unclear

May represent repolarisation of Purkinje fibres but can be normal

Question 8

What are the two most common causes of hypokalaemia?

Answer 8

• Increased renal losses: diuretics, types 1 and 2 renal tubular acidosis
• Increased GI losses: vomiting, diarrhoea

Question 9

A patient with hypokalaemia that does not respond to potassium therapy is likely to have which electrolyte imbalance?

Answer 9

Hypomagnesaemia: cannot correct potassium until magnesium is corrected!

Magnesium promotes urinary potassium loss

Question 10

Acute hypercalcaemia is often caused by what? What is the main symptom? Explain why this occurs

Answer 10

Malignancy

Polyuria + polydipsia

High calcium ⇒ loss of ability to concentrate urine due to down regulation of aquaporins → AKI. Calcium also acts like loop diuretic

Question 11

What are the four main symptoms of chronic hypercalcaemia? How do you remember them? Why is this no longer seen?

Answer 11

• Stones: kidney
• Bones: increased calcium resorption, bone pain
• Groans: abdominal pain, constipation
• Psychiatric overtones: anxiety, altered mental status

Question 12

What are the two most common causes of hypercalcaemia?

Answer 12

1. Hyperparathyroidism
2. Malignancy

Question 13

Somebody presents with twitching of the face. Exhibit the following sign on tapping of facial nerve. What is likely diagnosis and what is the sign? Explain why this occurs

Answer 13

Hypocalcaemia

Chvostek’s sign: facial contraction tetany on tapping of the nerve

♦ Calcium blocks sodium channels in neurons: low calcium causes easy depolarisation and spontaneous contractions

Question 14

You noticed a patient hand spasms on inflation of the blood pressure cuff. What is the sign called and what is it indicative of?

Answer 14

Trousseau’s sign: indicative of hypocalcaemia

♦ Calcium blocks sodium channels in neurons: low calcium causes easy depolarisation and spontaneous contractions

Question 15

Most common cause of hyperphosphataemia and symptoms

Answer 15

Acute and chronic kidney disease

Mostly asymptomatic: symptoms usually from hypocalcaemia caused by phosphate precipitating serum calcium

Question 16

How does hypoparathyroidism affect phosphate levels?

Answer 16

• PTH inhibits sodium/phosphate co-transporter in PCT → increased phosphate excretion
• Hypoparathyroidism increases levels as transporter will be more active

Question 17

What can long-standing untreated hyperphosphataemia cause?

Answer 17

Calciphylaxis AKA calcific uremic arteriolopathy (CUA)

• phosphate deposition & calcification of small blood vessels in fat, skin
• associated with kidney disease
• results in non-healing wounds
• usually fatal

Question 18

Describe and explain how diabetic ketoacidosis affects phosphate levels

Answer 18

Affected much the same way as potassium (extracellular shift but depleted total body levels)

Question 19

What electrolyte imbalance should you consider in an alcoholic who is malnourished and in hospital and starts experiencing hypoventilation and respiratory muscle weakness?

Answer 19

Hypophosphataemia caused by refeeding syndrome (increased food intake causes increased metabolism and a further decrease in phosphate levels)

Main symptom of low phosphate is weakness due to ATP depletion, often presents as respiratory muscle weakness

Question 20

Magnesium blocks the channels of which 2 electrolytes? What then are the symptoms of high magnesium? What is the main cause of hypermagnesaemia?

Answer 20

Calcium and potassium channels

Raise magnesium will cause the symptoms of decreased calcium and potassium

​main cause: renal insufficiency

Question 21

Hypomagnesaemia will cause a reduction in which 2 electrolytes?

List some causes of low magnesium

Answer 21

• Calcium
• Potassium

♦ GI losses, pancreatitis, renal losses, alcohol induced tubular dysfunction, drugs e.g. omeprazole (impaired absorption)

Question 22

Most common cause of hypercalcaemia in outpatients

Answer 22

Primary hyperparathyroidism

Question 23

Which anti-viral drug is famous for causing a high number of electrolyte imbalances including renal wasting of magnesium?

Answer 23

Foscarnet