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a path 3 > CHEMPATH: Sodium and Fluid Balance > Flashcards

CHEMPATH: Sodium and Fluid Balance Flashcards

1
Q

What is the most common electrolyte abnormality in hospitalised patients?

A

Hyponatraemia i.e. sodium <135 mmol/L

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2
Q

What is the main pathogenesis of hyponatraemia?

A

Increased extracellular water

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3
Q

What hormone regulates warer balance?

A

Regulated by ADH

  1. Synthesised in hypothalamus
  2. Secreted from posterior pituitary, acts on CD in the kidney
  3. Water retention through insertion of aquaporin 2 (AQA2)

NB: It also acts on V1 receptor to cause vasoconstriction. Then ADH is called vasopressin.

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4
Q

Summarise the effecs of ADH on different receptors.

A
  • Acts on V2 receptors in collecting duct à insertion of AQA2
  • V1 receptors
    1. Vascular smooth muscle
    2. Vasoconstriction – higher concentrations
    3. “Vasopressin”
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5
Q

What are the main 2 stimuli for ADH secretion?

A
  • 2 main stimuli for ADH secretion:
    1. Serum osmolality (high) – mediated by hypothalamic osmoreceptors
    2. Blood volume/pressure (low) – mediated by baroreceptors in carotids, atria, aorta
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6
Q

Why does vomiting cause hyponatraemia?

A

Loss of blood volume leads to baroreceptors detecting this and increasing ADH secretion

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7
Q

What is the first step in the assessment of a patient with hyponatraemia?

A

Clinical assessment of volume status to see whether hypo/eu/hypervolaemic

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8
Q

What are the clinical signs of hypovolaemia? What is the most reliable?

A
  • Tachycardia
  • Postural hypotension
  • Dry mucous membranes
  • Reduced skin turgor
  • Confusion or drowsiness
  • Reduced urine output

MOST RELIABLE INDICATOR:

  • Low urine Na+ (<20)
    • Normal range = 40-220 mEq/L [<20 non-renal loss; >20 in renal loss)
    • Kidneys are the best detector of hyponatraemia
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9
Q

Which drugs will affect your ability to use the urine sodium as the most reliable indicator of hypovolaemia?

A

Patients on diuretics may have urine Na that is not reliable (hypovolemic, but no hyponatraemia)

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10
Q

List some clinical signs of hypervolaemia.

A
  • Clinical Signs of hypervolemia
    • Raised JVP
    • Bi-basal crackles
    • Peripheral oedema
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11
Q

What are the causes of hypovolaemic hyponatraemia?

A
  • Causes of a hyponatraemia in a hypovolaemic patient (this requires more sodium than water loss)
    • Diuretics
    • Diarrhoea and vomiting
    • Salt losing nephropathy
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12
Q

What are the causes of hypervolaemic hyponatraemia?

A

Causes of hyponatraemia in a hypervolemic patient → cases are excess water, excess ADH​

  • Cardiac failure → low pressure → detected by baroreceptors → ADH release
  • Cirrhosis → vasodilated due to excess NO → low BP → baroreceptors → ADH release
  • Renal failure → not excreting enough water
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13
Q

What are the causes of euvolaemic hyponatraemia?

A
  • Hypothyroidism –> reduced contractility –> reduced BP –> ADH release
  • Adrenal insufficiency –> less aldosterone –> less Na+ reabsorptio
  • SIADH –> AQA2 water retention –> inc. volume –> suppress RAAS –> less aldosterone –> less Na+ reabsorption
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14
Q

What are the causes of SIADH?

A
  • CNS pathology – stroke, haemorrhage, tumour
  • Lung pathology – pneumonia (Legionella), pneumothorax
  • Drugs – SSRI, TCA, PPI, carbamazepine, opiates
  • Tumours
  • Surgery
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15
Q

What investigations would you order in a patient with euvolemic hyponatraemia?

A
  • Hypothyroidism → thyroid function tests
  • Adrenal insufficiency → short SynACTHen test
  • SIADH → plasma and urine osmolality → low plasma and high urine osmolality
    • Excess ADH = excess water = volume expansion → secretion for BNP → naturesis –> euvolaemic
    • If sodium is high**, it’s a **pseudohyponatraemia (i.e. hyperlipidaemia, hyperproteinaemia)
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16
Q

What are the important factors for allowing a diagnosis of SIADH to be made?

A
  • No hypovolaemia (euvolaemia)
  • No hypothyroidism
  • No adrenal insufficiency
  • Reduced plasma osmolality (resorbing lots of water) AND
  • Increased urine osmolality (>100 is high) (concentrating the urine)
17
Q

How do you manage hypovolaemic hyponatraemia?

A

volume replacement with 0.9% saline

18
Q

How do you manage a patient with euvolaemic hyponatraemia?

A

fluid restrict (<750ml/day + ABx infusions) + treat underlying cause

19
Q

How do you manage hypervolaemic hyponatraemia?

A

fluid restrict (<750ml/day + ABx infusions) + treat underlying cause

20
Q

What is fluid restriction?

A

0.5L-1L over 24 hours

21
Q

What is the cause of hyponatraemia in hypothyroidism?

A

Reduced effects of thyroid hormone on the heart causing more ADH release due to lower contractility

Or the two could be unrelated and just both common conditions not linked

22
Q

How do you recognise severe hyponatraemia? What is the management?

A
  • Reduced GCS
  • Seizures

Mangement:

  • Seek expert help – treat with hypertonic 3% (2.7%) saline [2 or 3 boluses of 150ml]
23
Q

What is a complication of treating hyponatraemia too quickly?

A
  • Central Pontine Myelinolysis
24
Q

What is the limit for increasing sodium in mmol/L over the first 24 hours?

A

8-10mmol/L in the first 24 hours

CPM will occur a few days later i.e. it is not sudden so you must correct the sodium ASAP if this has happened

25
Q

What are the signs and symptoms of CPM?

A
  • Signs/symptoms: Quadriplegia, dysarthria, dysphagia, seizures, coma, death
26
Q

If fluid restriction is insufficient, what drugs can be used to treat SIADH?

A
  • Demeclocycline -
    • Induce nephrogenic diabetes insipidus
    • Reduces responsiveness of collecting tubule cells to ADH
    • Monitor U&Es as risk of nephrotoxicity
  • Tolvaptan – V2 receptor antagonist
27
Q
A

reduced blood volume and increased serum osmolality

28
Q
A

Reduced** plasma osmolality and **increased urine osmolarity [>20]

29
Q

Which type of GI loss can cause hypernatraemia?

A

GI losses not accompanied by drinking water e.g. in children and older patients

30
Q

What are the 2 main GENERAL causes of hypernatraemia?

A

Increase in sodium

Loss of water

31
Q

What causes of increase in sodium can lead to hypernatraemia? (including 3 endocrine)

A
  • Medical high intake (hypertonic saline, sodium bicarbonate)
  • Dietary high intake (salty infant formula, high dietary salt)
  • Conn’s syndrome (high aldosterone: renin ratio), BAH (high aldosterone: renin ratio)
  • Renal artery stenosis (low GFR from RAS à low BP at JGA à high renin à high aldosterone)
  • Cushing’s syndrome (overactivation of MR by cortisol à aldosterone-like effect)
32
Q

What are the causes of water loss which can cause hypernatraemia?

A
  • Renal losses:
    • Osmotic diuresis
    • Diabetes insipidus (less ADH action / release)
      • Insensitivity/lack to/of ADH –> solitary water losses à hypovolaemia
      • Body compensates by resorbing more Na+ to reduce the water loss
      • Water loss persists and so you get a hypovolaemic hypernatraemia
  • Non-renal losses:
    • GI loss
    • Sweat loss
33
Q

What are the causes of nephrogenic DI?

A

Nephrogenic DI

  • Hypercalcaemia
  • Hypokalaemia
  • Lithium
  • Sickle cell anaemia
34
Q

What 5 investigations should you do for suspected diabetes insipidus?

A
  • Serum glucose – exclude diabetes mellitus à osmotic diuresis
  • Serum potassium – exclude hypokalaemia à nephrogenic DI
  • Serum calcium – exclude hypercalcaemia à nephrogenic DI
  • Plasma and urine osmolality – exclude hyperaldosteronism (high plasma osmolality, low urine osmolality)
  • Water deprivation test (normal = concentrated urine, no ADH = carry on passing water – dilute urine)
35
Q

How is hypernatraemia treated?

A
  • Fluid replacement → dextrose (if the patient is also hypovolemic, then 0.9% saline and 5% dextrose water)
  • Treat underlying cause

E.G. 70-year old man, 3-day history of diarrhoea, altered mental status, dry mucous membranes. Serum Na is 168mmol/L

36
Q

What are the effects of DM on sodium?

A
37
Q

How do you correct water deficit in hypernatraemia?

How do you correct ECF volume depletion?

How often are serial Na+ measurements taken?

A
  • Correct water deficit → 5% dextrose
  • Correct ECF volume depletion → 0.9% saline
  • Serial Na+ measurements → every 4-6 hours
38
Q

Why do you use dextrose and not saline in hypernatraemia?

A

Dextrose is water

Would need 10L of saline to correct hypernatraemia because of the amount of sodium that is in normal saline

39
Q

How do you correct sodium if you have made it too high?

A

Dextrose and desmopressing injections

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