Water and Salt

Normal serum sodium range

135-145 mmol/L

Hyponatraemia

Low serum sodium

What is the most common electrolyte abnormality?

Hyponatraemia (20-30% od hospital admissions)

What can cause hyponatraemia?

• Sodium loss

• Water gain

Why are women made up of less water than men (55% compared to 60%)?

Women have more adipose tissue.

What are the separate fluid compartments in the body called?

• Intracellular (30 L)

• Interstitial (9 L)

• Vascular (3 L)

What does movement of water in the body respond to?

Tonicity (a measure of the effective osmotic pressure gradient)

In which compartments is sodium found?

Extracellular and vascular compartments

Euvolaemic / Normovolaemic

A normal volume of blood

Hypovolaemic

Blood volume is depleted

Hypervolaemic

Blood volume is too high.

What are the different types of hyponatraemia?

• Normovolaemic hyponatraemia

• Hypovolaemic hyponatraemia

• Hypervolaemic hyponatraemia

What happens to sodium levels if we lose water?

Sodium becomes concentrated - hypernatraemia.

What happens to sodium levels if we gain water?

Sodium becomes diluted - hyponatraemia.

What happens to water in the body when we lose salt?

Water is also lost - hypovolaemia.

What happens to water in the body when we gain salt?

Water is gained - hypervolaemia

What diseases can cause fluid loss?

• Haemorrhage

• Vomiting

• Diarrhoea

• Burns

• Diuretic states

• Sequestration

What are the iatrogenic causes of fluid loss?

• Diuretics

• Stomas / fistulas

• Gastric aspiration

• Surgical drains

What diseases can cause fluid gain (salty)?

• Heart failure

• Liver failure

• Renal failure

What diseases can cause fluid gain (more water than salt)?

• Hypothyroidism

• Psychogenic

• ADH excess

What are the iatrogenic causes of fluid gain?

• IV fluids (saltiness variable)

• Supplemental nutrition (usually salty)

What can cause pure water loss hypernatraemia?

• Fever (sweating too much)

• Hyperventilation

• Diabetes insipidus

Why might some patients have a reduced water intake?

• Iatrogenic

• Psychosocial - elderly, infants, apathetic etc.

• Stroke, coma, confusion etc.

What causes pure Na+ gain hypernatraemia?

This is rare.

• Iatrogenic:

  • Concentrated feeds

  • Emetics

Hypersecretion of ADH can cause what?

• SIADH

• Hyponatraemia

Failure of ADH causes what?

• Diabetes insipidus

• Hypernatraemia

How does excessive ADH secretion cause hyponatraemia?

• No suppressed by reduced tonicity

• Free water reabsorption is excessive

• Sodium is diluted and hyponatraemia results

Does a person with SIADH have a normal blood volume?

Yes.

What causes diabetes insipidus?

ADH is either not produced in sufficient amounts, or non-functional.

What might cause non/reduced synthesis of ADH?

• Pituitary tumour

• Head injury

• Meningitis

• Genetic

• Idiopathic

When investigating a patient on a renal ward, what should you inspect in the environment?

• Charts

• Drug kardex

• Drains and drips

• Psychiatric disease?

• Chest disease?

• Neurological disease?

• Volume state: hyper/normo/hypovolaemic

What recent events should ask a renal patient when taking a history?

• Surgery

• Trauma

• Diagnoses

• Diarrhoea

• Chest diseases

• Drugs

• Drips

What questions should you ask a renal patient when taking a past medical history?

• Polyuria

• Polydipsia

• Hypovolaemiae

What non-specific investigations should be considered for a renal patient?

• Other electrolytes

• FBC

• Infection screen

• Chest X-ray

• Serum cortisol

• CT brain

How should a patient be given water?

Orally - cannot give a patient water through a drip unless it is in a 5% dextrose solution (typical concentration).

What are the symptoms of dehydration / volume depletion?

• Thirst

• Dizziness

• Dysphagia

• Weakness

• Confusion / aggression

• Coma

What are the clinical signs of volume depletion / dehydration?

• Postural hypotension

• Tachycardia

• Absence of JVP at 45 degrees

• Reduced skin turgor / dry mucosae

• Supine hypotension

• Oliguria

• Organ failure

Why do burns victims lose more water?

The skin cannot prevent water evaporating.

What happens to the volume of the blood when there is pure water gain?

It remains normal / there is mild hypervolaemia.

What happens to the volume of the blood when there is Na⁺ and water gain?

Interstitial oedema / hypervolaemia.

What happens to the volume of the blood when there is Na⁺ and water loss?

Hypovolaemia

Hypovolaemic hyponatraemia

Any case in which salt and water loss occur, but water loss is insufficient to concentrate the sodium.

Why does heart failure lead to hypervolaemic hyponatraemia?

• Reduced effective circulating volume

• Reduced organ perfusion

• Physiological correcting mechanisms kick in:

  • RAAS - sodium retention

  • ADH - water retention

• Hyponatraemia results from dilution

• Fluid overload worsens LV function, creating a worsening cycle

When there is abnormalities in [Na⁺], what is usually the cause: water or Na⁺ movement?

Water

What are some other names for SIADH?

AVP and SIAD

SIADH aetiology

Excessive secretion of ADH, which can be caused by:

• Pituitary hypersecretion

  • Meningitis

  • Encephalitis

  • Head injury

  • Stroke

• Ectopic secretion

  • Malignancy (eg. bladder, prostate…)

  • Pulmonary (eg. TB)

• Drugs (many)

SIADH causes…

Excessive water resorption and therefore normovolaemic hyponatraemia.

Diabetes insipidus causes…

Excessive renal water loss, and therefore hypovolaemic hypernatraemia.

What causes diabetes insipidus?

A condition caused by failure of the ADH mechanism:

• Cranial / central causes - non / reduced synthesis

  • Pituitary tumour

  • Head injury

  • Meningitis

  • Genetic

  • Idiopathic

• Nephrogenic causes - reduced tubular response

  • Inherited

  • Drugs (eg. lithium)

  • Metabolic (hyperkalaemia, hypercalcaemia)

What are the clinical signs of volume excess / overhydration?

• Hypertension

• Tachycardia

• Raised JVP at 45 degrees

• Gallop rhythm

• Oedema

• Organ failure

Is SIADH common?

No

Is diabetes insipidus common?

No.

Central pontine myelinolysis

Damage to the myelin sheath covering the pons - related to rapid correction of Na⁺.

What is the recommended rate of Na+ correction?

• 4-10 mmol/L/day if asymptomatic

• 8-12 mmol/L/day if symptomatic

What specific investigations are there for water and salt balance?

• Plasma osmolality

• Urine osmolality

• Urine (and other) sodium concentration

What is a normal plasma osmolality?

280-300

What plasma osmolality indicates diabetes insipidus?

>300

What plasma osmolality indicates SIADH?

<280

What urine osmolality indicates diabetes insipidus?

<300

What urine osmolality indicates SIADH?

>300

When a patient has hyponatraemia, how is this managed?

Usually with restriction of water intake.

When a patient has hypernatraemia, how is this managed?

Giving the patient water (typically orally).

Aside from restricting / giving water, how else are conditions affecting water and salt balance managed?

• Treat the primary cause

• Stop / change any drugs that are causing it (remember to look at drips and feeds too)

• Allow time

What are some specific treatments for cranial / central diabetes insipidus?

Synthetic ADH - desmopressin.

What are some specific treatments for nephrogenic diabetes insipidus?

• Supraphysiological ADH

• Diuretics (paradoxically)

• NSAIDs