Sodium & Fluid Balance Flashcards
Osmolality vs Osmolarity?
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
How do you calculate osmolality? What’s the normal range?
2 (Na + K) + Glucose + Urea
Normal range = 275 - 295 mOsmol/kg
What causes an osmolar gap of >10mOsmol/kg?
Alcohol: methanol, ethanol
Sugars: Mannitol, sorbitol
Lipids: Hypertriglyceridaemia
Proteins: Hypergammaglobulinaemia
How do you calculate osmolar gap?
Measured osmolality - Calculated osmolality
Which ion is the largest contributor to plasma osmolality?
Sodium, as it is the extracellular cation of highest conc.
Rank the following from highest to lowest osmolality:
DI, DKA, HHS, Pneumonia, SIADH
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.
- HHS
- 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:
- HHS
- DKA
- DI
- Pneumonia
- SIADH
What 2 factors affect sodium regulation
- Blood volume
- Serum osmolality
How does increased blood volume affect Na+ regulation?
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
How does increased osmolality affect Na+ regulation?
High osmolality -> Thirst + ADH release -> Decreased Na+ conc.
How does decreased osmolality affect Na+ regulation?
Low osmolality -> ADH suppression -> Increased Na+ conc.
What is more important regarding the role of ADH: maintaining blood volume or maintaing osmolality?
Maintaining blood VOLUME overrides osmolality.
ADH has conflicting role:
Low blood volume -> Increased ADH
Low osmolality -> Decreased ADH
What are the steps you need to take when patient comes in with hyponatraemia?
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)
Causes of hypovolaemic hyponatraemia?
- D+V
- Diuretics
- Salt losing nephropathy
Management of hypovolaemic hyponatraemia?
IV 0.9% NaCl (Saline)
Treat underlying cause
IF not resolving, involve seniors and consider slow IV hypertonic 3% NaCl
Causes of Hypervolaemic hyponatraemia?
- Cardiac failure
- Cirrhosis
- Nephrotic syndrome
Check urine Na+:
- <20 = Extra-renal (CCF, cirrhosis, nephrotic syndrome)
- >20 = RENAL cause (CKD)
What are the causes of hypervolaemic hyponatraemia if patient has HIGH or LOW urnary sodium?
Urinary sodium:
<20mmol/L = Extra-renal (CCF, cirrhosis, nephrotic syndrome)
>20mmol/L = RENAL cause (CKD)
Management of hypervolaemic hyponatraemia?
Fluid restrict
Treat underlying cause
Causes of euvolaemic hyponatraemia?
- Hypothyroidism
- Adrenal insufficiency
- SIADH
Check urine Na+:
- <20 = Psychogenic polydipsia, tea/toast diet in elderly
- >20 = Hypothyroidism, adrenal insufficiency, SIADH
Investigations for euvolaemic hyponatraemia?
- TFTs (?hypothyroidism)
- Short synacthen test (?adrenal insufficiency)
- Plasma + urine osmolality
Management of euvolaemic hyponatraemia?
Fluid restrict
Treat underlying cause
Demeclocycline or Tolvaptan for resistant SIADH
Diagnostic criteria for SIADH?
- NO hypovolaemia
- NO hypothyroidism (normal TFTs)
- NO adrenal insufficiency (normal short synacthen test)
- Reduced plasma osmolality (<270) AND
- Increased urine osmolality (>100)
Low plasma Na+ (<135)
Low plasma osmolality (<270)
High urinary Na+ (>20)
High urinary osmolality (>100)
Causes of SIADH?
- BRAIN (tumour, bleeds, etc..)
- LUNG (pneumothorax, PEs, etc..)
- DRUGS (SSRI, TCA, opiates, PPIs, carbamazepine)
- Tumours (any)
- Surgery
Causes of hypernatraemia? (hypo/hyper/euvolaemic)
-
Unreplaced water loss
- D+V, sweat loss, burns
- Renal loss: osmotic diuresis (diabetics), reduced ADH (Diabetes Insipidus)
- Patient can’t drink enough (e.g. children or elderly)
Management of hypernatraemia
Oral intake of water
Slow IV 5% dextrose 1L/6hrs (since glucose draws water back in) guided by urine output + plasma Na+
What are 2 types of Diabetes Insipidus?
- Central DI = Due to reduced ADH
- Nephrogenic DI = due to ADH resistance
Causes & management of both types of Diabetes Insipidus?
Central: Pituitary surgery, irradiation, tumour, trauma
Mx = desmopressin
Nephrogenic: hypokalaemia, hypercalcaemia, drugs (lithium, demeclocycline)
Mx = thiazides
Key investigations for DI?
- Serum glucose - exclude DM (can cause osmotic diuresis which can similarly lead to hypernatraemia)
- Serum potassium - exclude hypokalaemia
- Serum calcium - exclude hypercalcaemia
- Plasma + urine osmolality (urine:plasma osmolality <2:1 = DI)
- Water deprivation test
How do you interpret water deprivation test to diagnose DI?
- Urine concentrates after fluid restriction = NORMAL or primary polydipsia
- Urine concentrates after giving desmopressin (ADH) = CENTRAL DI
- 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
