Sodium and Fluid Balance Flashcards by Georgia Leggett

What is normal sodium?

135-145 mEq/L

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What is the underlying pathogenesis of hyponatraemia?

Increased extracellular water

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Which hormone controls water balance? How?

ADH: acts on V2 receptors in the collecting duct, leads to insertion of aquaporin-2

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What is the difference between V1 and V2 receptors?

V2: collecting duct

V1: vascular smooth muscle (Vasoconstriction at high conc.)

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What do you examine in a fluid exam?

Pulse

L/S BP

Mucous membranes

Tissue turgor

Confusion

JVP

Urine output

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7 clinical signs of hypovolaemia

Tachycardia

Postural hypotension

Dry mucous membranes

Reduced skin turgor

Confusion/drowsiness

Reduced urine output

Low urine Na+ (<20)

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How long do you need to wait before measuring urine Na+ for a patient who is on diuretics?

48h after stopping the diuretics

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3 clinical signs of hypervolaemia?

Raised JVP

Bibasal crackles

Peripheral oedema

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3 causes of hyponatraemia in a hypovolaemic patient?

D+V (losing salt + water, ADH only causes retention of water)

Diuretics (within weeks not years)

Salt losing nephropathy

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What are causes of hyponatraemia in a hypervolaemic patient?

Cardiac failure

Cirrhosis

Nephrotic syndrome/ renal failure

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List 3 causes of euvolaemic hyponatraemia. How do you test for each of these?

Hypothyroidism: TFTs (low T4)
Adrenal insufficiency: Short synACTHen test
SIADH: LOW plasma + HIGH urine osmolality >100

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What are causes of SIADH?

CNS: Stroke, tumours, abscess/ infection

Lung: Pneumonia, SCLC, pneumothorax

Drugs

Tumours

Surgery

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What is the management for hypovolaemic hyponatraemia?

Volume replacement with 0.9% saline

Remove stimulus for ADH secretion as restore blood volume

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What is the management for euvolaemic hyponatraemia?

Fluid restriction

Treat underlying cause e.g. test for SIADH etc.

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What is the management for hypervolaemic hyponatraemia?

Fluid restriction

Treat the underlying cause (HF)

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What are symptoms of severe hypovolaemia?

Reduced GCS

Seizures

Seek expert help (give hypertonic 3% saline)

Why must sodium be corrected slowly?

Serum Na+ must NOT be corrected >8-10 mmol/L in the 1st 24 hours

Risk of osmotic demyelination (central pontine myelinolysis) if too rapid

What are 6 symptoms of central pontine myelinolysis?

Quadriplegia

Dysarthria

Dysphagia

Seizures

Coma

Death

What drugs can be used to treat SIADH if water restriction is insufficient?

Demeclocycline: reduces responsiveness of collecting tubule cells to ADH (risk of nephrotoxicity so monitor U+Es)

Tolvaptan: V2 receptor antagonist

What are the main causes of hypernatraemia?

Unreplaced water loss
GI losses
Sweat losses (only if you don’t drink)
Renal losses: osmotic diuresis, diabetes insipidus
Patient not drinking e.g. baby/ elderly

What are tests for diabetes insipidus?

Serum glucose: exclude DM
Serum K: exclude hypokalaemia
Serum Ca: exclude hypercalcaemia
Plasma + urine osmolality
Water deprivation test

What are treatments for hypernatraemia?

Fluid replacement: 5% dextrose
Treat the underlying cause

What is the commonest electrolyte imbalance in hospital?

Hyponatraemia

Na <135

What stimulates ADH release?

Increased osmolality: mediated by hypothalamic osmoreceptors. (+ stimulates thirst)

Decreased pressure on the baroreceptors in the carotids, atria + aorta

What is the effect of increased ADH secretion on serum sodium?

Hyponatraemia

What is the most useful indicator of hypovolaemia?

Low urine Na \<20 (diuretics confound)

How does cardiac failure cause hypervolaemic hyponatraemia?

Reduced contractility Less renal perfusion Low BP Detected by baroreceptors + juxtaglomerular cells Activates RAAS Causes Aldosterone + ADH secretion Water reabsorption

List 5 drugs that can cause SIADH

SSRI TCA Opiates PPIs Carbamazepine

How does cirrhosis cause hypervolaemic hyponatraemia?

Excess NO stimulates vasodilation Low BP detected by baroreceptors Stimulates ADH + water reabsorption

How does renal failure cause hypervolaemic hyponatraemia?

Not excreting water (not excreting Na optimally but water worse)

How does hypothyroidism cause euvolaemic hyponatraemia?

Reduced cardiac contractility Lowers BP Detected by baroreceptors Stimulates ADH release

How does adrenal failure cause euvolaemic hyponatraemia?

Less/ no aldosterone Less/ no cortisol

How does lung disease e.g. COPD or pneumothorax cause SIADH?

Hypercapnia + respiratory acidosis stimulates ADH release

Why does SIADH lead to euvolaemia (rather than hypervolaemia)?

To become hypervolaemic would need Na + water reabsorption In SIADH excess water is absorbed causing stretching of the atria Stimulates release of natriuritic peptide (ANP) Leads to increased Na excretion in urine

How could giving someone with hyponatraemia saline cause harm?

If patient had SIADH They would retain the water, exacerbating the hyponatraemia

Why should you exclude hypokalaemia and hypercalcameia when testing for DI?

Low K or high Ca can cause nephrogenic diabetes insipidus, interfere with ADH action

How does the water deprivation test work?

If dont drink water all day, urine should be concentrated as ADH will be secreted to retain water In DI will continue to have dilute urine as not secreting ADH

What are the effects of diabetes mellitus on serum sodium?

Variable Hyperglycaemia draws water out of cells leading to hyponatraemia Osmotic diuresis in uncontrolled diabetes leads to loss of water + hypernatraemia

Which pathogen can cause SIADH in a pneumonia?

Legionella