salt and water balance Flashcards
In the average person what is required in terms of water(in L), sodium (in mmol), in potassium (mmol)
How much of water do they lose?
In health, the average person requires:
- 2 - 3L water (fluid and food)
- 100 to 200 mmol sodium
- 60 to 80 mmol potassium
and losses are:
• 1.5-2.0 L water as urine • 0.2L water as stool • 0.8 L water ‘insensible’ loss
what is the concentration of K+ and Na+ in the Intracellular fluid?
What is the concentration of Na+ and K+ and glucose and urea in the extracellular fluid?
What transporter ensures that the concentration gradient is kept.
What are the major determinants of plasma osmolality?
Water diffusses from an area of ______ osmolality to ______ osmolality
The plasma concentration of Na+ is dependent on?
What is the reference range for Na+
Sodium concentration is regulated by what system?
How is plasma water volume regulated?
Hypernatremia
- What is the commonest cause of it?
- What endocrine problems cause mildly elevated sodium
When can water loss be more than sodium?
What are artifactual causes?
Mild dementia
- Admitted with chest infection
- IV antibiotics
- Little oral intake, no iv fluids
- Day 1. Plasma sodium = 147 mmol/L
- Day 9. Plasma sodium = 183 mmol/L
- Day 10. Died
What did the 85 year old female die from? What is an important lesson to take from this?
She died from hypernatremia. Always monitor fluid intake in elderly inpatients
Hypernatraemia: Diabetes insipidus (DI)
Why does it occur?
What are the clinical symptoms?
Why does hypernatremia occur in these patients?
How is it diagnosed?
DI is due to either an inability of the pituitary to produce ADH (central DI), or of the kidney to respond to ADH (nephrogenic DI)
- Clinical symptoms: polyuria and thirst
- Thirst compensates for renal water loss
- Plasma sodium concentration normal if drinking adequate but:
- Fluid restriction -> hypernatraemia
- Diagnosed with a water deprivation test and serial sodium and osmolality measurements
Hyponatremia
Why does it occur?
What are the 3 states of water volume?
What is important to assess?
How do you assess fluid status?
Water retention in ECF IN EXCESS of sodium (NB both may be retained)
• Depending on cause, patient may be hypo-, eu-, or hypervolaemic
ESSENTIAL TO ASSESS FLUID STATUS
- Clinical: skin turgor, mucous membranes, blood pressure (lying and standing), pulse, presence of pitting oedema
- Urine output
- Serum sodium (+urea and creatinine)
- Serum osmolality = 2x Na + Urea + Glucose = 280-295 mOsm/kg (can be calculated if no osmotically active substances ingested or measured in laboratory)
- Urine osmolality – measured in laboratory. Result interpreted in context of serum osmolality
Causes of hyponatremia
What causes water retention to be in excess of sodium?
What causes sodium loss in excess of water?
Water retention in excess of sodium
Total body sodium normal or high
o oedematous states - nephrotic syndrome, cardiac / renal / liver failure
o Syndrome of inappropriate ADH (SIADH)
o Excessive drinking (psychogenic polydipsia)
Sodium loss in excess of water
Total body sodium low
o Renal – osmotic diuresis (DKA; HONK), diuretic stage of renal failure, diuretic use, hypocortisolism (Addison’s)
o Non-renal – diarrhoea, vomit, burns, fistula (also see hypernatraemia) – especially if associated with administration of hypotonic fluid
• A 61 year old female • Hypertension (on bendroflumethiazide), no other PMH • Consults the GP because she feels unwell. • Plasma Na + = 119 mmol/L • Urine osmolality = 650 mOsm/Kg
Whats the cause of hyponatremia?
What other diagnosis is possible?
Thiazide diuretics are the commonest cause of hyponatraemia in the community
Other diagnosis? -> orine osmolality high so retaining water -> SIADH
35 yr-old male, previously well, no medication
- 3/12 history of malaise, lightheadedness
- Sees GP – BP 123/76 (lying); BP 115/70 (standing)
– U/E results:
Na 131 mmol/L (133-146)
K 5.6 mmol/L (3.5-5.5)
Urea 6.1 mmol/L (1.8 - 7.1)
Creatinine 87 mmol/ (<110mmol/L)
What diagnoses do you need to consider?
differential?
Hyponatraemia with postural hypotension and no polyuria: consider adrenal failure (Addison’s)
• Would you expect the following results to be high or low?
– Plasma osmolality: low
– Urine osmolality: high
– Urine sodium
Differential diagnosis: SIADH
Syndrome of inappropriate ADH secretion (SIADH)
Why does it occur?
How does it lead to hyponatremia?
Diagnosis is made how?
Does the patient need to be clinically euvolaemic, hypovolemic or hypervolemic
What are causes of SIADH?
• ADH secretion inappropriate for ECF osmolality or volume status
renal water retention -> high urine osmolality -> hyponatraemia
Diagnosis of exclusion – ensure patient is clinically euvolaemic, exclude other causes of hyponatraemia
Many causes - CNS disease: infection/malignancy - Pulmonary disease: infection/malignancy - Porphyria - Drugs
Aldosterone retains sodium at kidney. So urine sodium is best to use.
Hypovolemic - your body will retain sodium through RAAS. When the RAAS systme is disrupted then urine sodium will be high.
Urine osmolality is high due to urea not sodium.
Clinical significance of sodium depletion
Determines brain volume
- Level <120 mmol/L associated with CNS dysfunction:
- Malaise • Confusion • Seizures • Coma
- Rapid changes in sodium concentration are more likely to be symptomatic
- If change is gradual brain adapts
