Ch5 Sodium Flashcards
What is the equation for osmolality?
= 2x ([Na] + [K]) + urea + glucose in mmol/l
What is the [Na] in normal saline?
0.9% which means 9g per litre
Define SIADH
Hypotonic hyponatraemia with serum osmo <275 (dilute) and inappropriately concentrated urine with urine osmo >100.
What is the difference between CSW and SIADH?
CSW has extracellular fluid depletion due to renal sodium loss. Renal [Na] >20.
Why should you measure TSH in a patient with hypoNa?
To rule out hypothyroidism
What is the risk of rapid Na correction?
Central pontine myelinolysis
What is the classification of the severity of hypoNa?
Mild <135, Moderate <130 and Severe <125
What is the cause of a low Na in a dry patient i.e. Na <135 and serum osmo >295?
Hyperglycaemia or mannitol administration
What is the cause of a low Na in a hypotonic patient i.e Na <135 and serum osma <275?
If urine osmo dilute (urine osmo <100) then psychogenic polydipsia or low Na intake. If urine osmo conc (urine osmo >100) then check volume status. If dry and urine Na >20 then CSW / diuretics / Addison’s disease. If dry and urine Na <20 then extrarenal losses of Na such as GI tract or burns etc. If euvolemic then SIADH If hypervolaemic and urine Na >20 then renal failure or hypothyroidism. If hypervolaemic and urine Na <20 then CHF and cirrhosis.
What is the treatment for SIADH?
Fluid restriction
What is the treatment for CSW?
Volume replacement and Na replacement
Fludrocortisone can be used for low Na if refractory
What is the most common cause of hypoNa in Neurosurgery?
SIADH
What is the incidence of SIADH and CSW in SAH?
SIADH 35% and CSW 20%
What are the causes of isotonic hyponatraemia?
Osmo 275-295: pseudohyponatraemia due to hyperlipidaemia or hyperparaproteinaemia;
What are the causes of hypertonic hyponatraemia?
Osmo >295: Hyperglycaemia / mannitol administration
What are the causes of hypotonic hyponatraemia?
Osmo <275: Urine osmo <100 - psychogenic polydipsia Urine osmo >100 - depends on fluid status Hypervolaemic: Renal failure if urine Na >20 and CHF / cirrhosis if urine Na <10 Euvolaemic: SIADH Hypovolaemic: CSW / Addisons if urine Na >20 and GI tract or skin losses if urine Na <10
What are the causes of SIADH?
Malignancy Infection (meningitis / encephalitis / TB) Pulmonary disorders Endocrine disturbances (adrenal insufficiency / hypothyroidism) Drugs
What are the diagnostic criteria for SIADH?
Serum osmo <275 (hypotonic) Urine osmo >100 (inappropriately conc urine) Clinically euvolaemic Urinary Na >40 Normal thyroid and adrenal function No diuretic use
What is the most rapid recommended correction of Na?
8 mmol/24 hours
What is the solute ratio?
= Urine [Na] + Urine [K} / plasma [Na}
How does the solute ratio guide you?
A solute ratio >1 means fluid restriction of <500ml/day; if <1 then 1l/day
What urine osmo is suggestive of SIADH?
>100 mOsm/kg
What is the definitive test for SIADH?
Water load test: Give a water load of 20ml/Kg (max 1.5L). Urine output should be 2/3 of the water load at 4 hours, otherwise the patient has SIADH.
What is the contraindication to the water load test?
Na<125 or symptomatic hyponatraemia
How would you treat a patient with acute (<48 hours) and severe hypoNa (Na<125)?
ICU transfer 3% saline (1-2 ml/h/kg body weight) with 20mg Frusemide Check Na every 2 hours and adjust 3% saline infusion rate Replace K Max correction 8mEq/24 hours
How would you treat a patient with chronic (>48 hours) and severe hypoNa (Na<125)?
Fluid restriction based on the solute ratio For refractory cases consider demeclocycline (partial antagonist to ADH in the renal tubules) or conivaptan (vasopressin receptor antagonist)
How can CSW be differentiated from SIADH?
CVP, volume status and serum K (raised in CSW but not SIADH). Haematocrit is raised as patient is dry.
What is the treatment of CSW?
0.9% or 3% saline; salt can also be administered orally. Other treatments include fludrocortisone, hydrocortisone
What is a potential complication of saline administration for SIADH or CSW?
Hyperchloraemic acidosis (treat with sodium bicarbonate infusion instead)
What is the most common cause of hyperNa (Na >150)?
Diabetes insipidus
What is the mechanism behind DI?
Lack or insensitivity to ADH causing hypertonic serum osmo (patient is dry) with dilute urine (urine osmo <200 mOsmol/Kg OR urine SG <1.003). This leads to a high serum Na.
What is the difference between neurogenic and nephrogenic DI?
Neurogenic is a lack of ADH release; Nephrogenic is insensitivity to ADH in the kidneys
What are the causes of nephrogenic DI?
Lithium, demeclocycine, colchicine; also caused by chronic renal failure, sarcoidosis and sjorgren’s syndrome
What are the causes of neurogenic DI?
Iatrogenic following transsphenoidal surgery (most common after craniopharyngioma) Pituitary apoplexy Encephalitis / meningitis
What is the triphasic response?
1 - injury to posterior pituitary causes reduced SIADH release (polyuria and polydipsia) 2 - cell death causes excess SIADH release (SIADH symptoms) 3 - chronic reduction in ADH release due to loss of cells in post. pituitary
What are the diagnostic criteria for DI?
Polyuria with >250ml/h for 2 or more hours Dilute urine with urine osmo <200 or SG <1.003 High serum Na Normal adrenal function
What is the diagnosis in hypoNa if the serum and urine osmo are dilute?
Polydipsia because there is appropriately diluted urine
What is the water deprivation test?
In suspected cases of DI stop the patient from drinking / IV input. In a normal patient the urine output should decrease the urine osmo will rise to >600 mOsm/kg. In DI the urine will remain dilute.
What is the treatment for DI?
If mild - drink to thirst If severe - desmopressin
What is the action of ADH?
Water absorption from the collecting ducts through Aquaporin channels
Where does acetazolamide act in the kidneys?
Proximal convoluted tubule
Where do loop diuretics act?
Ascending loop of Henley
Where do thiazides act?
DCT - increase Na and Cl absorption
Where does Aldosterone and other K+ sparing diurects act?
DCT - inceases absorption of K+ and H+
What channel do loop diurectics (frusemide) act on?
NKCC2 - which increases absorption of Na / K and Cl
What is the action of Renin?
Released from the DCT and converts angiotensinogen to Angiotensin 1
What is the action of ACE?
Converts Angiotensin 1 to angiotensin 2 in the lungs
