Electrolytes

Question 1

What is the normal reference range for sodium?

Answer 1

135-145mmols/L

Values less than 120 or more than 150 are potentially dangerous

Question 2

What can contribute to sodium abnormalities?

Answer 2

Intake
Loss or retention (gut, sweat, renal)
Water intake/loss/retention

Question 3

What are the clinical features of hypernatraemia?

Answer 3

H2O depletion (Vol decrease):
Unconscious/confused pts
Infants
Thirst centre damage
Dehydration

Conn’s/Cushings (Aldo increase)

DI (ADH Deacrease):
Cranial – no ADH
Nephrogenic – ADH ineffective

Addison’s (Adrenal insufficiency) (Aldo )

Question 4

What are the clinical features of hyponatraemia?

Answer 4

H2O excess (vol increase):
infusion of hypotonic fluid
psychogenic DI

Na+ depletion (Na+ decrease from gut/skin/sweat)

Heart failure (Renal blood flow decrease)

SIADH (ADH increase)

Diuretics (Vol decrease)

Loss of Na+ and H2O (Vol decrease):
Osmotic diuresis (eg diabetes)

Question 5

What are the common causes of hyponatraemia?

Answer 5

Iatrogenic eg fluid mismanagement
Drugs especially diuretics
Renal failure
Addison’s disease
Hypothyroidism
Liver failure 
Heart failure
SIADH

Question 6

What is the reference range for potassium?

Answer 6

3.5-5mmol/L

Values less than 3 or more than 6 are potentially dangerous

Question 7

What are the normal intracellular and extracellular concentrations of potassium?

Answer 7

Intra- 150mmol/L

Extra- 5mmol/L

Question 8

Describe acidosis

Answer 8

ATPase slows
Less potassium into cells
Hyperkalaemia

Question 9

Describe alkalosis

Answer 9

ATPase speeds up
More potassium into cells
Hypokalaemia

Question 10

How is ATPase usually regulated in the red blood cells?

Answer 10

Catecholamines (e.g. Adrenaline)

Insulin

Question 11

How is ATPase usually regulated in the kidney?

Answer 11

Mineralocorticoid activity:
Aldosterone
(Cortisol)

Diuretics:
Spironolactone
Amiloride

Question 12

What are some of the common causes of potassium abnormalities?

Answer 12

Intake

Distribution

Loss/Retention:
Renal
Gut
Oral

Question 13

What are the clinical features of hyperkalaemia?

Answer 13

Spurious (common):
Haemolysis – release from cells
Delay – release from cells
EDTA contamination – direct from K+ salt of EDTA
Drip arm – K+ drip

Mineralocorticoid deficiency – decreased Na+  K+/H+ exchange:
Addison’s disease
RTA IV (hyporeninaemic hypoaldosteronism)

Acidosis – Distribution:
Diabetic ketoacidosis
Lactic acidosis
Chronic respiratory acidosis

Hypoxia & related – ineffective pumps:
Hypoxia
Renal failure

Potassium sparing diuretics:
Spironolactone – aldosterone inhibitor
Amiloride - Na+  K+/H+ pump inhibitor

Question 14

What are the clinical features of hypokalaemia?

Answer 14

Mineralocorticoid excess – increased Na+  K+/H+ exchange:
Conn’s syndrome
Cushing’s syndrome
Licorice
Secondary hyperaldosteronism

Increased cellular uptake:
Insulin excess
Alkalosis: plus increased renal loss
Increased catecholamines

Renal tubular acidosis (RTA):
Type I – H+ excretion impaired
Type II – HCO3- reabsorption impaired

Excess Na+ for exchange:
Prolonged saline infusion – more Na+ to reabsorb in exchange for K+
Loop/thiazide diuretics – inhibit Na reabsorption earlier in tubule

Extra-renal causes – may also cause secondary hyperadosteronism due to fluid loss:
Diarrhoea
Laxative abuse
GI infection
Prolonged vomiting

Question 15

How do you avoid leaky red cells giving a spurious potassium result in the lab?

Answer 15

Check using heparinised sample & arrange for rapid analysis by the laboratory.

Question 16

Why is magnesium important in electrolyte balance?

Answer 16

Mg is essential for K+ reabsorption in the kidney & for normal intracellular distribution of K+
Decreased Mg is often associated with hypocalcaemia as it is a co-factor for PTH release