Disturbed electrolytes - low Na+ Flashcards
normal Na+?
how are Na+ levels maintained?
- 135-144 mmol/L
* hypothalamic osmoreceptors and hypovolaemia regulate ADH and thirst
most common cause of ↓Na+ ?
- impaired water excretion - therefore Na+ more dilute
* (less commonly from hypertonic urine loss or excess water ingestion)
types of ↓Na+?
- Hypotonic (aka hypoosmotic). Commonest. Sub-classified into hypo-, eu-, and hyper-volaemic
- Hypertonic: ↑glucose or mannitol pull H2O into ECF
- Isotonic ‘pseudo-hyponatraemia’: artefact of measurement due to ↑proteins/lipids e.g. in multiple myeloma
simple way to divide causes of hypotonic hyponatraemia?
- renal causes: ↑Na+ in urine → ↓Na+ in serum
* non-renal causes: ↓Na+ in serum → ↓Na+ in urine
what happens in hypovolaemic hypotonic hyponatraemia? (dry patients)
↓H2O
↓↓Na+
EXTRA-RENAL Na+ loss/deficiency (urine Na+ <20):
• GI: ↓oral fluid intake, D, V
• 3rd space: burns, pancreatitis, peritonitis
RENAL Na+ loss (urine Na+ >20):
• excess diuretics, especially thiazides (may also be euvolaemic)
• mineralocorticoid deficiency (Addison’s)
• renal salt losing disease e.g. interstitial nephritis, polycystic kidney disease
• cerebral salt wasting, post trauma or surgery
euvolaemic hypotonic hyponatraemia?
↑H2O
↔Na+
EXCESS H2O intake (urine Na+ <20):
• excess IV fluid. A common cause in hospital, especially post-surgery
• excess oral fluid (polydipsia) athletes, psych - primary polydipsia, MDMA
FAILURE of renal H2O excretion (urine Na+ >20):
• SIADH
• hypothyroidism (sometimes via SIADH), secondary adrenal insufficiency
• NSAIDs: reduces prostaglandin-mediated suppression of ADH’s renal effects
hypervolaemic (oedematous) hypotonic hyponatraemia?
↑↑H2O
↑Na+
ORGAN FAILURE (urine Na+ <20):
• heart failure
• cirrhosis
• nephrotic syndrome (rare as a cause of ↓Na+)
URINE Na+ >20:
• advanced renal failure (AKI or CKD)
Sx + Ex ↓Na+?
- headache
- N, V, anorexia
- muscle cramps
- tired, dizzy, disorientated
- severe: seizures, coma, cerebral oedema.
- ↓BP
- peripheral oedema
- gradual onset: mild, due to cerebral adaptation, with neurons releasing inorganic (Na+, Cl-, K+) and organic osmolytes to preserve osmolality
- a lot of apparently ‘asymptomatic’ hyponatraemia in fact leads to an increased risk of FALLS due to subtle neurological effects
Ix ↓Na+?
BEDSIDE
• lying-standing BP
• urine (once hypotonic ↓Na+ confirmed)
BLOODS • Na+ • serum osmolality - rule out isotonic pseudo-hyponatraemia (280-295 mOsmol/kg) - rule out hypertonic hyponatraemia - otherwise must be hypotonic • FBC, U&E, LFT, cortisol, and TFT
how to interpret findings in ↓Na+?
1) URINE NA+ Marker of aldosterone and RAS activity. ↑aldosterone retains Na+. Na+ <20 in hypovolaemia. Na+ <20 in excess H2O intake. Na+ >20 in euvolaemia. Na+ >20 in renal Na+ loss.
2) URINE OSMOLALITY Marker of ADH activity. >100 when ADH is actively concentrating urine. Either appropriately in hypovolaemia. Or inappropriately in SIADH. <100 less common. (In excess H2O intake.)
URINE Ix findings in hypovolaemic ↓Na+?
↓Na+ <20
↑osmo >100 (urine being concentrated)
body retains salt and water
URINE Ix findings in SIADH and Addison’s ↓Na+?
↑osmo >100 (inappropriately concentrated)
↑Na+ >20
what is the following a marker of in urine Ix for ↓Na+?
- urine Na+?
- urine osmolality?
- urine Na+ = aldosterone activity (↑aldosterone retains Na+ in blood to ↑BP)
- urine osmolality = ADH activity (↑ADH retains water in blood and ↑urine osmolality by concentrating urine)
URINE Ix findings in excess H2O intake (polydipsia) ↓Na+?
↓Na+
↓osmo (dilute)
Tx ↓Na+?
ASx/mild?
• Ix cause
• hypovolaemic? saline, 2L/24hr challenge if unsure (Na+ rise = hypo)
• euvolaemic? fluid restrict
Severe symptoms? (coma/seizures)
“SALTY:”
• “SA” line 3 percent, 1-2 ml/kg/hr, HDU/ICU transfer
• “L”oop diuretic (furosemide) if not hypovolaemic
• ↑”T”en 10 mmol/24hr, re-check 2 hrly, until 125/well
• “Y” is it happening?
