Clinical chemistry of disordered fluid and electrolyte balance

Question 1

When is ADH released

Answer 1

IN response to high osmolality and low volume

Question 2

Role of ADH

Answer 2

increased permeability of kidney distal tubule

Question 3

Role of aldosterone

Answer 3

Stimulate Na+ reabsorption in exchange for either K+ or H+ ions

Result: Na+ retention and loss of K+ and H+

Question 4

What is hydrostatic pressure

Answer 4

Pushes fluid form vessel to interstitial fluid

Question 5

What is oncotic pressure

Answer 5

Pushes fluid from interstitial space to vessel

Question 6

What is oncotic pressure determined by

Answer 6

ALbumin

Question 7

Why do patients with nephrotic syndrome have oedema

Answer 7

They lose albumin in urine so have albuminuria

This causes low oncotic pressure so hydrostatic pressure moves fluid into interstitial space

Question 8

How does liver cirrhosis cause ascites

Answer 8

Increased hydrostatic pressure because of portal hypertension

Low oncotic pressure because liver is not producing albumin

Question 9

How does heart failure cause oedema

Answer 9

High hydrostatic pressure

Question 10

Causes of hyponatremia

Answer 10

Sodium depletion

• Renal loss (osmotic diuresis, mineralocorticoid excess)
• GI loss (vomiting diarrhoea)
• Skin loss (excessive sweating)

Excess body water

• Psychogenic polydipsia
• Oedema
• SIADH

Question 11

Renal causes of sodium depletion

Answer 11

Osmotic diuresis

Mineralocorticoid deficiency

Question 12

Dilutional causes of excess body water

Answer 12

Cirrhosis
Cardiac Failure
Nephrotic syndrome
SIADH

Question 13

How do diagnose SIADH

Answer 13

High urine osmolality

High urine sodium (because of inhibition of RAAS)

Question 14

When should SIADH not be diagnosed

Answer 14

When patient has oedema

No renal, cardiac, thyroid or adrenal problems

Question 15

What does SIADH respond to

Answer 15

Water restriction

Question 16

Causes of SIADH

Answer 16

Malignancy
Cerebral (e.g. infection)
Pulmonary (e.g. pulmonary embolus, pneumonia)
Drugs

Question 17

Which drugs can cause hyponatraemia

Answer 17

Diuretics
ADH potentiation
Stimulate ADH release e.g. nicotine

Question 18

What are the two causes of artifactual hyponatraemia

Answer 18

Drip arm sample

Pseudohyponatremia

Question 19

How to treat hyponatremia when the cause is fluid and sodium depletion

Why should you be careful

Answer 19

IV saline

Slow correction to minimise risk of central pontine myelinosis

Question 20

How to treat acute dilution hyponatremia (severe symptoms and acute onset)

Answer 20

Hypertonic saline to correct serum Na and reduce risk of cerebral oedema

Question 21

Causes of hypernatremia

Answer 21

Inadequate fluid intake
Water depletion
Excessive sodium intake
Reduced sodium excretion

Question 22

Causes of water depletion which may cause hypernatremia

Answer 22

Renal- polyuria e.g. diabetes insipidus

Non renal- vomiting, diarrhoea

Question 23

What can cause reduced sodium excretion which may cause hypernatremia

Answer 23

Mineralocorticoid excess

Question 24

Artifactual causes of hypernatremia

Answer 24

Contamination from preservative

Drip arm

Question 25

Symptoms of hypernatremia

Answer 25

Dehydration- thirst, oliguria, confusion, weakness, convulsion and death

Question 26

investigations in hypernatremia

Answer 26

Clinical history

Polyuria: 24h urine volume, urine osmolality, serum glucose, calcium, potassium. Water deprivation test

Mineralocorticoid excess: spot urine K+, renin and aldosterone

Question 27

If the patient has mineralocirtocoid excess, what are the first line tests that you do

Answer 27

Spot urine K

Renin and aldosterone