Sodium and Water Balance

Question 1

What percentage of body weight is water? (adult male)

Answer 1

60% (42 L)

Question 2

Main electrolyte in cells?

Answer 2

K 160 mmol

Question 3

Main electrolyte present in extracellular fluid?

Answer 3

Na 135-145

Question 4

Regulation of plasma sodium (2)

Answer 4

Volume receptors regulate sodium via renal sodium excretion

Osmoreceptors regulate water through thirst and water absorption in kidneys

Question 5

What is atrial natriutetic peptide/ANP?

Answer 5

Released due to atrial stretch

Directly reduces Na reabsorption

Reduces angiotension/aldosterone to reduce heart strain

Question 6

How does sympathetic activity affect sodium balance?

Answer 6

Stretch receptors in atrium, thoracic veins and in aortic arch increase sympathetic activity when stimulated by a lack of volume

THis increases renin angiotensin, aldosterone and sodium reabsorption

Question 7

What causes pseudohyponatraemia?

Answer 7

Increased triglycerides and proteins can give falsely low Na levels

Not pseudohyponatraemia, but high glucose can draw Na into cells. THis is reversed when glucose is corrected

Question 8

Clinical symptoms of hyponatraemia

Answer 8

Initialy asymptomatic

Nausea, headache, confusion

Seizures, coma, brain damage, respiratory failure, death

Question 9

Causes of hypervolaemic hyponatraemia? What will urine osmolality be?

Answer 9

Congestive heart failure
Cirrhosis
Nephrotic syndrome

If no diuretics, urinary sodium will be <20 as there is no increased loss

Question 10

Investigations for hypervolaemic hyponatreamia?

Answer 10

CXR
Liver enzymes
Albumin
Urine protein

Question 11

Pathophysiology of hypervolaemic hyponatraemia? (recognise)

Answer 11

Excess fluid is lost to the tissues

Therefore low circulating volume

This stimulates osmoreceptors and volume receptors. ADH released so lots of water is retained and plasma becomes diluted

Question 12

Urine Na when a patient is dehydrated? Hypovolaemic hyponatraemia

Answer 12

Na lost from gut, urine Na will be LOW as kidneys are holding onto it

Na lost from kidneys, urine Na will be high

Question 13

Causes of euvolaemic hyponatraemia? IMPORTANT (3)

Answer 13

Addison’s disease
Hypothyroidism

SIADH (plasma diluted but not hypervolaemic. Diagnosis of exclusion, can be due to pain, nausea or drugs or SCC of LUNG!)

Question 14

How to determine cause of hyponatraemia? (6)

Answer 14

Assess fluid balance
Measure urine Na
Urine and plasma osmolality
Measure lipids, proteins, glucose

Synacthen test, TFTs

CXR!!

Question 15

Rate at which Na should be replaced?

Answer 15

SLOW

Not more than 12mmol/L increase in 24 hours

Initally aim for 1-2 mmol per hour

Rapid increase causes brain damage

Question 16

Treatment of SIADH? (recognise)

Answer 16

FLuid restriction to 1 litre/24 hours

Demeclocycline

Aquaporin receptor antagonists

Question 17

Main cause of hypernatraemia?

Answer 17

Usually a water deficit

Occasionially iatrogenic is hypertonic saline used

Question 18

Symptoms of hypERnatraemia?

Answer 18

Asymptomatic until over 150 mmols

Anorexia, nausea and vomiting

Confusion, coma

CEREBRAL BLEEDING, sub arach!!

Question 19

Causes of hypovolaemic hypernatraemia? (fure fluid losses)

Answer 19

Burns/sweating

GI loss: V + D, fistulas

Renal: Diuretics, CKD

Question 20

Causes of hypervolaemic hypernatraemia?

Answer 20

Iatrogenic due to hypertonic saline or antibiotic infusions

Question 21

Causes of euvolaemic hypernatraemia? (3)

Answer 21

(Loss of fluid in excess of salt but not enough to affect circulating volume)

Diabetes insipidus

Fever

Mechanical ventilation

Question 22

Hypernatraemia management?

Answer 22

Treat cause

Fluid replacement

Correct Na by not more than 1-2 mmol/L and 10-12 mmol/L in 24 hours

Monitor fluid balance