Hyponatraemia

Question 1

What is normal serum sodium?

Answer 1

135-145mmol/l

Question 2

What is hyponatraemia?

Answer 2

Low serum sodium

Question 3

What are three important facts about hyponatraemia?

Answer 3

1. It is the commonest electrolyte abnormality you will see (20-30% hospital admissions)
2. It can kill
3. It is usually caused by an excess of water and is therefore dilutional

Question 4

What is the total body water percentage?

Answer 4

Men: 60% water, 42 litres
Women: 55% water, 38 litres

Question 5

What are the compartments in which fluids will spread into?

Answer 5

-Intracellular (30L)
=locked in by cell membranes
-Interstitial (9L)
=water that bathes cells and peritoneal space (pericardial sac)
-Vascular (3L)
=in vessels/ free water in blood
=Kidneys, guts, (skin, lungs= insensible losses can be up to half a litre of fluid)

Question 6

How does water move between compartments?

Answer 6

• Ions so water moves to keep osmolality equal on both sides

- Sodium-potassium ATPases= potassium intracellular cation and sodium is extracellular cation

Question 7

What forces contribute to water movement?

Answer 7

• Internal hydrostatic pressure (out of compartment)
• External hydrostatic pressure in
• Internal and external osmotic pressures (pulling into compartments)
• sodium= external osmotic pressure

Question 8

How does cerebral oedema occur?

Answer 8

• Dilute sodium-containing fluids (4% dextrose, 0.18% saline)
• Dilutes extracellular space= hyponatraemia
• Fluid moves to hypertonic cellular space (osmotic and hydrostatic forces)
• Cerebral oedema occurs (=confusion)

Question 9

What is the volume status of hyponatraemia?

Answer 9

• Most people are normal with 55-60% total body water= euvolaemic/ normovolaemic
• Some people are volume deplete= hypovolaemic
• Some people are volume overloaded= hypervolaemic

Question 10

What are the types of hyponatraemia?

Answer 10

-Normovolaemic
-Hypovolaemic
-Hypervolaemic
=Clinical signs are generated when there is a difference in the relative size of fluid compartments from normal (extracellular compartment)

Question 11

What are the clinical signs of hypovolaemia?

Answer 11

• Postural hypotension (dizzy as less perfusion to head)
• Tachycardia (to maintain cardiac output)
• Absence of jugular venous pulse at 45 degrees
• Reduced skin turgor/ dry mucosae (pinch test)
• Supine hypotension
• Oliguria (passing smaller volumes of urine)
• Organ failure

Question 12

What are the clinical signs of hypervolaemia?

Answer 12

• Hypertension
• Tachycardia
• Raised jugular venous pulse at 45 degrees
• Gallop rhythm (between the two normal)
• Peripheral and pulmonary oedema
• Third space gains= peritoneal, pleural and joint spaces
• Organ failure

Question 13

What happens during gastroenteritis to blood volume?

Answer 13

-Salt-rich diarrhoea
=water loss cannot compensate for Na+ loss so hyponatraemia
-Hydrostatic forces favour water movement from cells to the extracellular space
-Osmotic forces favour the reverse
-Vascular depletion causes hypotension
-Standing up causes postural hypotension and collapse
-Hypovolaemic hyponatraemia

Question 14

Why does hypovolaemic hyponatraemia occur?

Answer 14

• Common conditions
• Excessive sodium losses; water losses are insufficient to concentrate sodium back up
• Depends of volume of water lost and concentration of sodium therein

Question 15

What are the causes of hypovolaemic hyponatraemia?

Answer 15

-Haemorrhage
-Vomiting
-Diarrhoea
-Burns (more evaporation of water)
-Diuretic states (kidneys losing lots of water like diabetes mellites, hypercalcaemia)
-Sequestration (inflammation in compartment draws water in, pneumonia)
-Misc. renal disease
-Heat exposure
-Addison’s disease
-Iatrogenic
=Diuretics
=Stromas/ fistulae
=Gastric aspiration

Question 16

What are the causes of euvolaemic hyponatraemia?

Answer 16

-Water being evenly distributed across all compartments
-Hyponatraemia is dilutional
-Causes:
=Hypotonic intravenous fluids
=hypothyroidism?
=SIADH

Question 17

What are the causes of hypervolaemic hyponatraemia?

Answer 17

-Water gains exceed sodium gains
=Heart failure
=Liver failure
=Nephrotic syndrome

Question 18

How does heart failure lead to hypervolaemic hyponatraemia?

Answer 18

-Enlarged, pleural effusions
-A reduced cardiac output
=Reduced effective circulating volume
=Reduced organ perfusion
=Physiological correcting mechanism kicks in
=Hypovolaemia wins over tonicity
=Renin/ angiotensin/ aldosterone stimulation
=ADH stimulation
-

Question 19

What do the correcting mechanisms in heart failure result in?

Answer 19

• Sodium retention (aldosterone)
• Water retention (aldosterone and ADH)
• Hyponatraemia (from dilution)
• Fluid overload worsens LV function
• Hypovolaemia continues to win over hyponatraemia
• Maladaptive response (cycle worsens)

Question 20

What is SIADH?

Answer 20

-Syndrome of inappropriate ADH secretion
-ADH secretion is excessive
=Not suppressed by reduced tonicity (anti-diuretic)
=water reabsorption is excessive
=sodium diluted
-hyponatraemia
-Clinically euvolaemic

Question 21

What is the aetiology of SIADH?

Answer 21

-Pituitary hypersecretion/ direct effect:
=Neurological (meningitis, encephalitis, head injury, stroke)
-Ectopic secretion (malignancy):
=SCLC, pancreas, bladder, prostate
=Pulmonary (non-malignant)= TB, pneumonia
-Potentiation of action
=Drugs (thiazide diuretic, carbamazepine, amitriptyline)

Question 22

What is the treatment for hypovolaemia?

Answer 22

-Restoration of volume state
=Blood if necessary
=crystalloid (0.9% saline)
-Cessation of diuretics
-Steroids for Addison's

Question 23

What is the treatment for hypervolaemia?

Answer 23

-Diuretics
=usually loop diuretics (furosemide/ bumetanide)
-Fluid restriction
-Treatment of underlying cause
=heart attack

Question 24

What is the treatment for euvolaemic hyponatraemia?

Answer 24

-Treat underlying cause
=stop IV fluids
-thyroxine replacement
-Fluid restriction
=down to 500ml/day
-Rarely
=demeclocycline (decreases sensitivity to ADH)