Sodium and water balance Flashcards
What are the sizes of the different compartments containing fluid in the body?
ICF (intracellular fluids) are the biggest - 28L
ECF (extracellular fluid) = interstitial fluid and plasma
- Intersitial fluid = 10.5L
- Plasma = 3.5L
What is the difference in concentration of Na in the ICF and the ECF?
- Na conc high in ECF - roughly 140mmol/L
- Na conc low in ICF - roughly 4mmol/L
What is the principle relationship between sodium and water that you need to remember ?
That water follows sodium everywhere e.g. if your ECF volume is too high then the kidneys excrete more Na and thus you lose water with it.
If the ECF volume is too low, the kidneys ‘hang on’ to Na in an attempt to retain water and restore volume to normal
What are sodium levels controlled by ?
Controlled by mineralocorticoid activity - e.g. aldosterone (main one) and others e.g. cortisol
What does mineralocorticoids stimulate in terms of Na ?
The stimulate Na retention
What is water controlled by ?
ADH (anti-diuretic hormone)
What is the effect of ADH on water ?
ADH acts on renal tubules to cause:
- Water reabsorption and thus
- Antidiuretic effect (hence its name)
Increased ADH = concentrated urine
Decreased ADH = dilute urine
Describe what is meant by urine osmolality
- Concentrated urine = high urine osmolality
- Dilute urine low urine = osmolality
In general terms what can decreased sodium be due to ?
Too much water or too little Na
What are some of the causes of too much water in the body resulting in decreased Na levels ?
- Decreased excretion of water - SIADH (Syndrome of inappropriate antidiuretic hormone secretion, characterized by excessive release of ADH resulting in hyponatraemia secondary to the dilutional effects of excessive water retention)
- Increased intake of water - compulsive water drinking
- Secondary hyperaldosteronism: heart failure, liver cirrhosis
- Nephrotic syndrome
- IV dextrose
List the main causes of SIADH
Malignancy:
- Small cell lung cancer
- Also: pancreas, prostate
Neurological:
- Stroke
- Subarachnoid haemorrhage
- Subdural haemorrhage
- Meningitis/encephalitis/abscess
Infections:
- TB
- Pneumonia
Drugs:
- Sulfonylureas*
- SSRIs, tricyclics
- Carbamazepine
- Vincristine
- Cyclophosphamide
What are the signs/symptoms of SIADH ?
Symptoms:
- Mild – nausea, vomiting, headache, anorexia, lethargy
- Moderate – muscle cramps, weakness, confusion, ataxia
- Severe – drowsiness, seizures, coma
Signs:
- Decreased level of consciousness
- Cognitive impairment – short-term memory loss, disorientation, confusion
- Focal or generalised seizures
- Brain stem herniation (severe acute hyponatraemia) – coma, respiratory arrest
- Hypervolaemia – pulmonary oedema, peripheral oedema, raised jugular venous pressure, ascites
What is important to remember about the rate at which hyponatraemia develops ?
Symptoms/signs of SIADH vary depending on the rate at which hyponatraemia develops. Mild hyponatraemia may cause significant symptoms if the drop in sodium is acute, whereas chronically hyponatraemic patients may have very low serum sodium concentrations and yet be completely asymptomatic
How is suspected SIADH investigated ?
Check fluid status:
- Is the patient clinically or biochemically dehydrated? In SIADH this will not be the case – the patient is typically either euvolemic or hypervolaemic
- If dehydration is present, it may suggest an alternative cause of hyponatraemia (e.g. diuretic medication, renal failure)
Serum sodium:
- Low in SIADH – <130 mmol/L
Serum potassium:
- If serum potassium is raised in the presence of hyponatraemia Addison’s disease should be considered
Plasma osmolality:
- Reduced in SIADH (due to the low sodium concentration)
TFTs:
- Hypothyroidism is a potential cause of SIADH
Serum cortisol:
- Low serum cortisol with hyponatraemia would indicate addisions
Urine osmolality:
- Will be raised in SIADH (>500), in the context of a respectively decreased serum Na
CXR/CT-chest:
- May be done if suspect e.g. small cell lung cancer, atypical pneumonia
What must be present for a diagnosis of SIADH to be made?
- Hyponatraemia
- Low plasma osmolality
- Inappropriately elevated urine osmolality (>plasma osmolality)
- Urine [Na+] >40 mmol/L with normal salt intake
- Euvolaemia
- Normal thyroid and adrenal function
What is the treatment of these Na and water problems ?
- Too little sodium → give sodium
- Too much water → fluid restrict
- Too little water → give water
- Too much sodium → get rid of excess sodium (and water with it) e.g. diuretics to induce natriuresis, and then replace just the water
