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Y5 Path: Chem Path > Sodium & Fluid Balance > Flashcards

Sodium & Fluid Balance Flashcards

1
Q

Osmolality vs Osmolarity?

A

Osmolality = mOsm/kg

  • More accurate, as it’s weight temp. doesn’t affect it
  • Calculated in a lab machine

OsmolaRity = mOsm/litRe

  • More practical
  • This is volume rather than weight
  • Calculated from blood test
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2
Q

How do you calculate osmolality? What’s the normal range?

A

2 (Na + K) + Glucose + Urea

Normal range = 275 - 295 mOsmol/kg

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3
Q

What causes an osmolar gap of >10mOsmol/kg?

A

Alcohol: methanol, ethanol
Sugars: Mannitol, sorbitol
Lipids: Hypertriglyceridaemia
Proteins: Hypergammaglobulinaemia

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4
Q

How do you calculate osmolar gap?

A

Measured osmolality - Calculated osmolality

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5
Q

Which ion is the largest contributor to plasma osmolality?

A

Sodium, as it is the extracellular cation of highest conc.

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6
Q

Rank the following from highest to lowest osmolality:
DI, DKA, HHS, Pneumonia, SIADH

A

DKA and HHS both have high glucose in blood.
However, HHS by definition has osmolality >320, and also typically DKA doesn’t have glucose conc as high as HHS.

  1. HHS
  2. DKA

DI is lack of ADH or insensitivity to ADH. Poor water resorption -> High serum osmolality

SIADH causes excess water resorption so would have the lowest osmolality

Pneumonia only sometimes causes SIADH.

Therefore:

  1. HHS
  2. DKA
  3. DI
  4. Pneumonia
  5. SIADH
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7
Q

What 2 factors affect sodium regulation

A
  1. Blood volume
  2. Serum osmolality
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8
Q

How does increased blood volume affect Na+ regulation?

A

Increased blood volume -> Atrial stretch -> Increased Atrial Natriretic Peptide (ANP)

ANP decreases release of:

  • Aldosterone (Adrenal cortex)
  • ADH (hypothalamus)
  • Renin (kidneys)

Overall DECREASES sodium conc + blood volume

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9
Q

How does increased osmolality affect Na+ regulation?

A

High osmolality -> Thirst + ADH release -> Decreased Na+ conc.

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10
Q

How does decreased osmolality affect Na+ regulation?

A

Low osmolality -> ADH suppression -> Increased Na+ conc.

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11
Q

What is more important regarding the role of ADH: maintaining blood volume or maintaing osmolality?

A

Maintaining blood VOLUME overrides osmolality.

ADH has conflicting role:

Low blood volume -> Increased ADH

Low osmolality -> Decreased ADH

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12
Q

What are the steps you need to take when patient comes in with hyponatraemia?

A

Step 1: Check plasma osmolality to exclude pseudohyponatraemia
(low Na+ with normal/high plasma osmolality)

Causes of pseudohyponatraemia:
Normal osmolality - high lipids, proteins
High osmolality - high sugars, mannitol, alcohols

Step 2: Assess volume status

  • Check BP, HR, CRT
  • Leg oedema
  • Pulmonary oedema

Step 3: Check urine Na+

  • If <20mmol/L = Kidneys are working, extra-renal cause (D+V, burns)
  • If >20mmol/L = RENAL problem (renal disease, diuretics, cerebral salt wasting)
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13
Q

Causes of hypovolaemic hyponatraemia?

A
  • D+V
  • Diuretics
  • Salt losing nephropathy
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14
Q

Management of hypovolaemic hyponatraemia?

A

IV 0.9% NaCl (Saline)
Treat underlying cause

IF not resolving, involve seniors and consider slow IV hypertonic 3% NaCl

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15
Q

Causes of Hypervolaemic hyponatraemia?

A
  • Cardiac failure
  • Cirrhosis
  • Nephrotic syndrome

Check urine Na+:

  • <20 = Extra-renal (CCF, cirrhosis, nephrotic syndrome)
  • >20 = RENAL cause (CKD)
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16
Q

What are the causes of hypervolaemic hyponatraemia if patient has HIGH or LOW urnary sodium?

A

Urinary sodium:

<20mmol/L = Extra-renal (CCF, cirrhosis, nephrotic syndrome)

>20mmol/L = RENAL cause (CKD)

17
Q

Management of hypervolaemic hyponatraemia?

A

Fluid restrict
Treat underlying cause

18
Q

Causes of euvolaemic hyponatraemia?

A
  • Hypothyroidism
  • Adrenal insufficiency
  • SIADH

Check urine Na+:

  • <20 = Psychogenic polydipsia, tea/toast diet in elderly
  • >20 = Hypothyroidism, adrenal insufficiency, SIADH
19
Q

Investigations for euvolaemic hyponatraemia?

A
  1. TFTs (?hypothyroidism)
  2. Short synacthen test (?adrenal insufficiency)
  3. Plasma + urine osmolality
20
Q

Management of euvolaemic hyponatraemia?

A

Fluid restrict
Treat underlying cause

Demeclocycline or Tolvaptan for resistant SIADH

21
Q

Diagnostic criteria for SIADH?

A
  1. NO hypovolaemia
  2. NO hypothyroidism (normal TFTs)
  3. NO adrenal insufficiency (normal short synacthen test)
  4. Reduced plasma osmolality (<270) AND
  5. Increased urine osmolality (>100)

Low plasma Na+ (<135)
Low plasma osmolality (<270)

High urinary Na+ (>20)
High urinary osmolality (>100)

22
Q

Causes of SIADH?

A
  • BRAIN (tumour, bleeds, etc..)
  • LUNG (pneumothorax, PEs, etc..)
  • DRUGS (SSRI, TCA, opiates, PPIs, carbamazepine)
  • Tumours (any)
  • Surgery
23
Q

Causes of hypernatraemia? (hypo/hyper/euvolaemic)

A
  1. Unreplaced water loss
    • D+V, sweat loss, burns
    • Renal loss: osmotic diuresis (diabetics), reduced ADH (Diabetes Insipidus)
  2. Patient can’t drink enough (e.g. children or elderly)​
24
Q

Management of hypernatraemia

A

Oral intake of water

Slow IV 5% dextrose 1L/6hrs (since glucose draws water back in) guided by urine output + plasma Na+

25
Q

What are 2 types of Diabetes Insipidus?

A
  • Central DI = Due to reduced ADH
  • Nephrogenic DI = due to ADH resistance
26
Q

Causes & management of both types of Diabetes Insipidus?

A

Central: Pituitary surgery, irradiation, tumour, trauma
Mx = desmopressin

Nephrogenic: hypokalaemia, hypercalcaemia, drugs (lithium, demeclocycline)
Mx = thiazides

27
Q

Key investigations for DI?

A
  1. Serum glucose - exclude DM (can cause osmotic diuresis which can similarly lead to hypernatraemia)
  2. Serum potassium - exclude hypokalaemia
  3. Serum calcium - exclude hypercalcaemia
  4. Plasma + urine osmolality (urine:plasma osmolality <2:1 = DI)
  5. Water deprivation test
28
Q

How do you interpret water deprivation test to diagnose DI?

A
  1. Urine concentrates after fluid restriction = NORMAL or primary polydipsia
  2. Urine concentrates after giving desmopressin (ADH) = CENTRAL DI
  3. Urine remains DILUTE after desmopressin = NEPHROGENIC DI

Diagnostic for DI - despite raised plasma osmolality, urine is DILUTE with urine:plasma osmolality of <2:1

You can EXCLUDE DI f urine:plasma osmolality ratio is >2:1

Y5 Path: Chem Path (2 decks)

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