Serum Electrolyte Levels

Question 1

What is the equation for anion gap?

Answer 1

Anion gap = (Na+) - (Cl- + HCO3-)

Question 2

What is the anion gap in a healthy person?

Answer 2

Anion gap: 8-16

* the negatively charged plasma proteins account for most of the anion gap

Question 3

An increased anion gap is referred to as … ?

Answer 3

Metabolic acidosis (H+ neutralises HCO3-)

Question 4

Metabolic acidosis with a normal anion gap is called?

Answer 4

Hyperchloraemic acidosis, because the reductin in HCO3- is balanced by the increased CL- (e.g. in chronic diarrhoea, kidney tubular acidosis)

Question 5

Metabolic acidosis with a normal anion gap is called?

Answer 5

Hyperchloraemic acidosis, because the reductin in HCO3- is balanced by the increased CL- (e.g. in chronic diarrhoea, kidney tubular acidosis)

Question 6

What is hypernatraemia?

Answer 6

Na+ > 145 mmol/L

Question 7

What are the main causes of hypernatraemia?

Answer 7

a. Water depletion (diabetes inispidus)
b. Water and sodium depletion (diarrhoea)
c. Corticosteroid excess (e.g. Cushing’s syndrome, Conn Syndrome - both associated with increase mineralcorticoid)
d. Excess IV hypertonic Na solutions

Question 8

What are the clinical features of hypernatraemia?

Answer 8

• Thirst, confusion, oliguria
• Orthostatic hypotension
• Muscle twitching or cramps
• Severe: seizures, delirium, hyperthermia, coma

Question 9

What is hyponatraemia?

Answer 9

Na+

Question 10

What are the main causes of hyponatraemia?

Answer 10

a. Water retention (CCF, hypoalbuinaemia)
b. Kidney failure to conserve salt (e.g. nephritis, DM)
c. GI losses of Na+ (e.g. diarrhoea, vomiting)
d. Drugs (e.g. diuretics, ACE inhibitors)

Question 11

What are the clinical features of hyponatraemia?

Answer 11

• Lethargy, confusion, mental changes (e.g. in personality)

- Severe: convulsions, coma, death

Question 12

What is hyperkalaemia?

Answer 12

K+ >5mmol/L

Question 13

What may the first sign of hyperkalaemia be?

Answer 13

The first sign of hyperkalaemia (>6mmol/L) can be cardiac arrest

Question 14

What are the main causes of hyperkalaemia?

Answer 14

a. Kidney failure
b. Acidosis (especially metabolic)
c. Mineralcorticoid deficiency (Addison’s, aldosterone antagonists)
d.

Question 15

What are the main causes of hyperkalaemia?

Answer 15

a. Kidney failure
b. Acidosis (especially metabolic)
c. Mineralcorticoid DEFICIENCY: (Addison’s, aldosterone antagonists)
d. Excessive intake of K+ (e.g. IV fluids with K)
e. Drugs (e.g. spiranolocatone, ACE inhibitors (opposes aldosterone), NSAIDs)

Question 16

What are the clinical features of hyperkalaemia?

Answer 16

• Muscle weakness, flaccid paralysis (rare)
• May be asymptomatic until cardiac toxicity
• May cause cardiac arrest - asystole versus fibrillation
• ECG: peaked T waves, decrease QT, increase PR interval
• -> arrhythmias

Question 17

What is hypokalaemia?

Answer 17

K+

Question 18

What are the causes of hypokalaemia?

Answer 18

a. Kidney disease
b. GIT loss: vomiting, diarrhoea
c. Alkalosis
d. Mineralocorticoid EXCESS: Cushing syndrome, increase aldosterone, Conn syndrome
e. Loss of extracellular fluid to intracellular (e.g. burns, other trauma)
f. decrease intake of K+
g. Drugs (diuretics - frusemid, thiazides)

Question 19

What are the clinical features of hypokalaemia?

Answer 19

• Lethargy, muscle weakness, cramps, mental lethargy, confusion
• Severe flaccid paralysis, tetany, coma
• ECG prominent U waves, depressed ST segment, decrease T waves, arrhythmias