ChemPath MedEd Flashcards
Normal Range for Sodium
135-145 mmol/L
What kind of ion is Na+ in the body and how are its levels maintained?
Predominantly an extracellular cation
largely maintained by active pumping from ICF -> ECF
by Na+/K+ ATPase
What % of Na+ in the body is freely exchangeable? What is going on with the rest?
70% freely exchangeable, the rest complexed in bone
mild hyponatraemia sodium levels
130-135 mmol/L
red common in hospital
severe hyponatraemia sodium levels
<125 mmol/L
How do we treat hyponatraemia?
treat the underlying cause unless severe (<125 mmol/L) and symptomatic
Hyponatraemia that is compensated (usually chronic) is rarely an emergency to treat: even with
sodium in the 110-120 range that are asymptomatic, it is more dangerous to correct them too fast
than to leave the patient at that level.
Sx of symptomatic hyponatraemia
- Nausea + vomiting (<134 mmol/L)
- Confusion (<131 mmol/L)
- Seizures, non-cardiogenic pulmonary oedema (<125 mmol/L)
- Coma (<117 mmol/L) and eventual death
Reduced GCS, Seizures -> Seek expert help (Treat with Hypertonic 3% saline)
Is Symptomatic hyponatraemia a medical emergency?
Yes
What is the commonest electrolyte abnormality seen in hospitalised patients?
hyponatraemia
Na <135 mmol/L
What is the underlying pathogenesis of hyponatraeamia?
increased EC water
How does ADH mediated water retention occur?
ADH acts on V2 receptors in the collecting ducts
Insertion of aquaporin-2
channels
-> more water reabsorbed (??)
see if want to add more details here
What are the effects of VP?
Acts on V2 receptors (collecting duct) -> Insertion of aquaporin-2
V1 receptors (on vascular smooth muscle)
- vasoconstriction (higher concentrations)
-> alternative name ‘vasopressin’
What controls ADH secretion?
- high osmolality measured by hypothalamic osmoreceptors stimulates release
- low blood volume/pressure detected by baroreceptors in carotids, atria and aorta
What is the effect of increased ADH secretion on serum sodium concentration?
lowers sodium concentration
HypOnatraemia
increased water reabsorption -> decreased sodium concentration
TURP syndrome
- Transurethral resection of the prostate (TURP) syndrome is fluid overload and iso-osmolar hyponatraemia during TURP from large volumes of irrigation fluid being absorbed through venous sinuses
- Irrigation fluid is required to maintain visibility despite bleeding tissue beds
- TUPR syndrome can also occur in other procedures requiring large volumes of irrigation, such as hysteroscopy
- True TURP syndrome is now rare, particularly as glycine-based irrigation fluids are less commonly used
60-40-20 rule
The 60-40-20 rule:
- 60% total body weight = water
- 40% of body weight = intracellular
- 20% of body weight = extracellular
Osmolality vs Osmolarity
Osmolality = total number of particles in solution - measured with an osmometer, units = mmol/kg.
Osmolarity = calculated, units = mmol/l
Osmolality and osmolarity should roughly equate
Osmolarity formula
2(Na + K) + urea + glucose
normal range for serum osmolality
275 – 295 mmol/kg
Difference between osmolarity and osmolality term
osmolar gap
can be useful in metabolic acidosis cases
This is because if the osmolarity is lower than the osmolality, we can
assume there are extra (unmeasured) solutes that are dissolved in the serum
What impacts the osmolality?
Physiological
Na+
K+
Cl-
HCO3-
urea
glucose
Pathological
Endogenous (i.e. glucose), Exogenous (ethanol, mannitol)
osmolality in true and in pseudohyponatraemia
true: low
pseudo: normal/high
clinical signs of hypovolaemia
Tachycardia
Postural hypotension
Dry mucous membranes
Reduced skin turgor
Confusion/drowsiness
Reduced urine output
Low urine Na+ (<20)
clinical signs of hypervolaemia
Raised JVP
Bibasal crackles (on chest examination)
Peripheral oedema
Pseudohyponatraemia causes according to osmolality:
normal:
- Spurious
- Drip arm sample
- Pseudohyponatraemia (hyperlipidaemia/ paraproteinaemia)
high:
- Glucose (e.g. HHS)/mannitol infusion
- toxic alcohols
Hyponatraemia with elevated plasma osmolality is due to an excess of osmotically active solutes in the plasma. Often this is glucose (in HHS) but can also be mannitol. This draws water from cells into the plasma, which dilutes down the sodium. This is technically a true hyponatraemia however it is due to another chemical in the blood.
What are the causes of hyponatraemia in a hypovolaemic patient?
Renal: diuretics
Extra-renal: diarrhoea, vomiting
-> think: fluid is being lost somewhere
What are the causes of hyponatraemia in a hypervolaemic patient?
Cardiac failure
Cirrhosis
Renal failure
-> think failure
What are the causes of hyponatraemia in a euvolaemic patient?
Hypothyroidism
Adrenal insufficiency
Syndrome of inappropriate ADH (SIADH)
-> think endocrine
Causes of SIADH
CNS pathology
Lung pathology
Drugs (SSRI, TCA, opiates, PPIs, carbamazepine)
Tumours
Surgery
What investigations would you order in a patient with euvolaemic hyponatraemia?
? Hypothyroidism: Thyroid function tests
? Adrenal insufficiency: Short Synacthen test
? SIADH: Plasma & urine osmolality (low plasma & high urine osmolality)
Findings consistent with SIADH dx
No Hypovolaemia
No Hypothyroidism
No Adrenal insufficiency
Reduced plasma osmolality AND
Increased urine osmolality (>100)
How would you manage a hypovolaemic patient with hyponatraemia?
Volume replacement with 0.9% saline
How would you manage a hypervolaemic patient with hyponatraemia?
Fluid restriction
Treat the underlying cause (e.g. HF, cirrhosis, primary polydipsia, nephrotic syndrome; renal failure (urine Na >20))
Mx of severe hyponatraemia (<125)
i.e. Reduced GCS, Seizures
Seek expert help (Treat with Hypertonic 3% saline)
Central pontine myelionlysis
damage to the myelin sheath of the white matter in the CNS caused by a sudden rise in serum osmolality
-> quadriplegia, dysarthria, dysphgia, seizures, coma, death
therefore do not correct serum sodium >8-10 mmol/L in the first 24h
What is the most important point to remember while correcting hyponatraemia?
Serum Na must NOT be corrected > 8-10 mmol/L in the first 24h
Risk of osmotic demyelination (central pontine myelionlysis)
quadriplegia, dysarthria, dysphgia, seizures, coma, death
