CHEMPATH: Sodium and Fluid Balance

Question 1

What is the most common electrolyte abnormality in hospitalised patients?

Answer 1

Hyponatraemia i.e. sodium <135 mmol/L

Question 2

What is the main pathogenesis of hyponatraemia?

Answer 2

Increased extracellular water

Question 3

What hormone regulates warer balance?

Answer 3

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.

Question 4

Summarise the effecs of ADH on different receptors.

Answer 4

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

Question 5

What are the main 2 stimuli for ADH secretion?

Answer 5

• 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

Question 6

Why does vomiting cause hyponatraemia?

Answer 6

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

Question 7

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

Answer 7

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

Question 8

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

Answer 8

• 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

Question 9

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

Answer 9

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

Question 10

List some clinical signs of hypervolaemia.

Answer 10

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

Question 11

What are the causes of hypovolaemic hyponatraemia?

Answer 11

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

Question 12

What are the causes of hypervolaemic hyponatraemia?

Answer 12

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

Question 13

What are the causes of euvolaemic hyponatraemia?

Answer 13

• 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

Question 14

What are the causes of SIADH?

Answer 14

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

Question 15

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

Answer 15

• 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)

Question 16

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

Answer 16

• 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)

Question 17

How do you manage hypovolaemic hyponatraemia?

Answer 17

volume replacement with 0.9% saline

Question 18

How do you manage a patient with euvolaemic hyponatraemia?

Answer 18

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

Question 19

How do you manage hypervolaemic hyponatraemia?

Answer 19

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

Question 20

What is fluid restriction?

Answer 20

0.5L-1L over 24 hours

Question 21

What is the cause of hyponatraemia in hypothyroidism?

Answer 21

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

Question 22

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

Answer 22

• Reduced GCS
• Seizures

Mangement:

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

Question 23

What is a complication of treating hyponatraemia too quickly?

Answer 23

• Central Pontine Myelinolysis

Question 24

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

Answer 24

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

Question 25

What are the signs and symptoms of CPM?

Answer 25

* **Signs/symptoms**: Quadriplegia, dysarthria, dysphagia, seizures, coma, death

Question 26

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

Answer 26

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

Question 27

Answer 27

reduced blood volume and increased serum osmolality

Question 28

Answer 28

**_Reduced**_ plasma osmolality and _**increased_** urine osmolarity [\>20]

Question 29

Which type of GI loss can cause hypernatraemia?

Answer 29

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

Question 30

What are the 2 main GENERAL causes of hypernatraemia?

Answer 30

Increase in sodium Loss of water

Question 31

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

Answer 31

* _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)

Question 32

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

Answer 32

* **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

Question 33

What are the causes of nephrogenic DI?

Answer 33

***Nephrogenic DI*** * Hypercalcaemia * Hypokalaemia * Lithium * Sickle cell anaemia

Question 34

What 5 investigations should you do for suspected diabetes insipidus?

Answer 34

* 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)

Question 35

How is hypernatraemia treated?

Answer 35

* Fluid replacement → **_dextrose_** (if the patient is also hypovolemic, then 0.9% saline and 5% dextrose water) * Treat underlying cause ## Footnote *E.G. 70-year old man, 3-day history of diarrhoea, altered mental status, dry mucous membranes. Serum Na is 168mmol/L*

Question 36

What are the effects of DM on sodium?

Answer 36

Question 37

How do you correct water deficit in hypernatraemia? How do you correct ECF volume depletion? How often are serial Na+ measurements taken?

Answer 37

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

Question 38

Why do you use dextrose and not saline in hypernatraemia?

Answer 38

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

Question 39

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

Answer 39

Dextrose and desmopressing injections