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YR5 Chemical Pathology > Sodium and Fluid Balance > Flashcards

Sodium and Fluid Balance Flashcards

1
Q

What is normal sodium?

A

135-145 mEq/L

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2
Q

What is the underlying pathogenesis of hyponatraemia?

A

Increased extracellular water

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3
Q

Which hormone controls water balance? How?

A

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

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4
Q

What is the difference between V1 and V2 receptors?

A

V2: collecting duct

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

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5
Q

What do you examine in a fluid exam?

A

Pulse

L/S BP

Mucous membranes

Tissue turgor

Confusion

JVP

Urine output

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6
Q

7 clinical signs of hypovolaemia

A

Tachycardia

Postural hypotension

Dry mucous membranes

Reduced skin turgor

Confusion/drowsiness

Reduced urine output

Low urine Na+ (<20)

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7
Q

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

A

48h after stopping the diuretics

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8
Q

3 clinical signs of hypervolaemia?

A

Raised JVP

Bibasal crackles

Peripheral oedema

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9
Q

3 causes of hyponatraemia in a hypovolaemic patient?

A

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

Diuretics (within weeks not years)

Salt losing nephropathy

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10
Q

What are causes of hyponatraemia in a hypervolaemic patient?

A

Cardiac failure

Cirrhosis

Nephrotic syndrome/ renal failure

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11
Q

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

A

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

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12
Q

What are causes of SIADH?

A

CNS: Stroke, tumours, abscess/ infection

Lung: Pneumonia, SCLC, pneumothorax

Drugs

Tumours

Surgery

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13
Q

What is the management for hypovolaemic hyponatraemia?

A

Volume replacement with 0.9% saline

Remove stimulus for ADH secretion as restore blood volume

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14
Q

What is the management for euvolaemic hyponatraemia?

A

Fluid restriction

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

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15
Q

What is the management for hypervolaemic hyponatraemia?

A

Fluid restriction

Treat the underlying cause (HF)

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16
Q

What are symptoms of severe hypovolaemia?

A

Reduced GCS

Seizures

Seek expert help (give hypertonic 3% saline)

17
Q

Why must sodium be corrected slowly?

A

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

18
Q

What are 6 symptoms of central pontine myelinolysis?

A

Quadriplegia

Dysarthria

Dysphagia

Seizures

Coma

Death

19
Q

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

A

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

Tolvaptan: V2 receptor antagonist

20
Q

What are the main causes of hypernatraemia?

A

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

21
Q

What are tests for diabetes insipidus?

A

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

22
Q

What are treatments for hypernatraemia?

A

Fluid replacement: 5% dextrose
Treat the underlying cause

23
Q

What is the commonest electrolyte imbalance in hospital?

A

Hyponatraemia

Na <135

24
Q

What stimulates ADH release?

A

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

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

25
Q

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

A

Hyponatraemia

26
Q

What is the most useful indicator of hypovolaemia?

A

Low urine Na <20

(diuretics confound)

27
Q

How does cardiac failure cause hypervolaemic hyponatraemia?

A

Reduced contractility

Less renal perfusion

Low BP

Detected by baroreceptors + juxtaglomerular cells

Activates RAAS

Causes Aldosterone + ADH secretion

Water reabsorption

28
Q

List 5 drugs that can cause SIADH

A

SSRI
TCA
Opiates
PPIs
Carbamazepine

29
Q

How does cirrhosis cause hypervolaemic hyponatraemia?

A

Excess NO stimulates vasodilation

Low BP detected by baroreceptors

Stimulates ADH + water reabsorption

30
Q

How does renal failure cause hypervolaemic hyponatraemia?

A

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

31
Q

How does hypothyroidism cause euvolaemic hyponatraemia?

A

Reduced cardiac contractility

Lowers BP

Detected by baroreceptors

Stimulates ADH release

32
Q

How does adrenal failure cause euvolaemic hyponatraemia?

A

Less/ no aldosterone

Less/ no cortisol

33
Q

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

A

Hypercapnia + respiratory acidosis stimulates ADH release

34
Q

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

A

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

35
Q

How could giving someone with hyponatraemia saline cause harm?

A

If patient had SIADH

They would retain the water, exacerbating the hyponatraemia

36
Q

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

A

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

37
Q

How does the water deprivation test work?

A

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

38
Q

What are the effects of diabetes mellitus on serum sodium?

A

Variable

Hyperglycaemia draws water out of cells leading to hyponatraemia

Osmotic diuresis in uncontrolled diabetes leads to loss of water + hypernatraemia

39
Q

Which pathogen can cause SIADH in a pneumonia?

A

Legionella

YR5 Chemical Pathology (16 decks)

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