Fluid + Electrolyte Balance and Disorders Flashcards
What is the composition of extracellular fluid?
High Na+
Low K+
What is the composition of intracellular fluid?
Low Na+
High K+
What factors determine the composition of plasma and interstitial fluid?
- Hydrostatic pressure
- Oncotic pressure
- Leakiness of capillaries
- Lymphatic drainage
From what regions do the majority of water losses occur from?
- Skin
- Kidnyes
- Lungs
- ADH action
- Colon
What is the action of ADH
Insertion of water channels into the walls of the collecting duct, allowing water to be reabsorbed from the kidneys
What stimulates the secretion of ADH?
- Increase in plasma osmolality
- Decrease in plasma volume
- Pain, stress, drugs, nausea, CNS lesions
What factors decrease the secretion of ADH?
- Decrease in plasma osmolality
- Increase in plasma volume
- Ethanol
What are the physiological responses to water deficiency?
Water loss increases ECF osmolality (making it more concentrated). This leads to the following
=> Stimulation of ADH release
- Renal water retention
=> Stimulation of hypothalamic thirst centres
- Increased water intake
=> Redistribution of water from ICF
- Increase in ECF water
All these factors restore ECF osmolality to normal levels
What is hypervolemia?
Hypervolemia means excess water in plasma which can lead hyponatremia
What are the main causes of hypervolemia?
- Renal failure
- Increased sodium and water retention
- Heart failure (through a decrease in CO hence action of RAAS)
What are the clinical features of hypervolemia?
- Raised JVP
- Peripheral oedema
- Pulmonary oedema
- Pleural effusion
- Ascites
What is the management of hypervolemia?
- Oxygen if required
- Fluid restriction
- Diuretics if hypervolemia symptomatic
- Renal replacement therapy in cases of AKI/ treat the underlying cause
What is hypovolemia?
Means decreased water in plasma which can lead to hypernatremia
What are the causes of hypovolemia?
- Haemorrhage
- Burns
- Fluid loss via skin, kidneys GI tract
What are the clinical features of hypovolemia?
- Thirst (activation of hypothalamic thirst centres)
- Dizziness (through hypotension)
- Nausea
- Loss of skin elasticity (dehydration)
- Hypotension
What is hypernatremia?
Increased levels of sodium
What are the main causes of hypernatremia?
- Fluid loss without water replacement
- Diabetes Insipidus
- Osmotic Diuresis
- Primary hyperaldosteronism
- Excess IV saline
What are the clinical features of hypernatremia?
- Fever
- Confusion
- Nausea and vomiting
- Thirst
- Irritability
What are the investigations in suspected hypernatremia?
- Measure urine osmolality and plasma osmolality
=> Urine osmolality > plasma osmolality
- Water is being reabsorbed as it is the lack of water in the collecting duct that is making urine concentrated
- Therefore the water loss in extra renal
=> Urine osmolality < plasma osmolality
- Water is not being reabsorbed as the urine within the collecting duct is not concentrated
- The cause is most likely diabetes insipidus
What is the management of hypernatremia?
- Treat the underlying cause
- Give water orally if possible, if not possible, 5% IV glucose
- Rate no greater than 0.5 mmol/hour correction is appropriate
What are the causes of hyponatremia?
=> Is the patient dehydrated?
YES - Is the urine Na > 20mmol/L?
=> If YES - causes:
- Addinsons disease
- Diuretics
- HHS
=> NO - Patient not dehydrated. Are they oedematous?
=> If oedematous - causes:
- Nephrotic syndrome
- Heart Failure
=> NO - not oedematous. Is urine osmalality > 100 mmol/kg?
=> If YES - cause:
- SIADH
=> If NO - causes:
- Water overload
- Hypothyroidism
- Glucocorticoid insufficiency
What are the different categories of hyponatremia?
- Hypovolemic hyponatremia
- Euvolemic hyponatremia
- Hypervolemic hyponatremia
How is the severity of hyponatremia categorised?
=> Mild hyponatremia
Serum Na: 130-134 mmol/L
=> Moderate hyponatremia
Serum Na: 120-129 mmol/L
=> Severe hyponatemia
Serum Na: < 120 mmol/L
What is the management of hyponatremia?
=> Mild hyponatremia:
- Fluid restriction
- Loop diuretics (this drug can also cause hyponatremia through renal loss, so care should be taken during administration)
=> Moderate hyponatremia:
- Hypertonic saline until Na levels above 120 mmol/L
- Then treat with fluid restriction and loop diuretics
=> Severe hyponatremia:
- Bolus hypertonic saline until symptoms resolve
- Given with or without ADH antagonists
In what cases should care be taken in the administration of fluids?
=> Fluid intake should be less than urine output in following cases:
- Oedematous states
- SIADH
- Renal failure
- Psychogenic polydipsea
In what cases should care be taken when administrating vasopressin antagonists?
- In cases of underlying liver disease
- If patient has hypovolemic hyponatremia, as vasopressin antagonists could worsen the hypovolemia
What is the complication in cases of overcorrection of hyponatremia?
- Locked in syndrome (osmotic demyelination syndrome or central pontine myelinolysis)
- Avoided by correcting Na by only 4-6 mmol/L in 24 hour period
What drugs are associated with hyponatremia?
=> Drugs that increase ADH secretion:
- Anticonvulsants
- Antineoplastics
- Hypoglycaemics
- Narcotics
=> Drugs that potentiate ADH action:
- Tricyclics
- SSRIs
- Paracetamol
- Indomethacin
=> Diuretics
- Thiazides
- Furosemide
- K sparing diuretics
At what level does hyperkalemia become an emergency?
Plasma levels > 6.5 mmol/L
What are the causes of hyperkalemia?
- Acute Renal Failure
- Drugs
- Metabolic acidosis
- Addison’s disease
- Massive blood transfusions
- Rhabdomyolosis
What drugs cause hyperkalemia?
- K sparing diuretics
- ACE inhibitors
- NSAIDs
- Cyclosporin
- B blockers in cases of AKI
- Heparin
- Spironolactone
- ARBs
What are the clinical features of hyperkalemia?
- Weakness
- Light headedness
- Chest pain
- Fast irregular pulses
=> ECG changes:
- Small P waves / absent P waves
- Widened QRS complexes
- Tall tented T waves
What is the management of hyperkalemia?
=> Treatment focuses on:
- Stabilisation of the cardiac membrane
- Short term shift of K from extracellular to intracellular compartments
- Removal of excess K from the body
=> Stabilisation of the cardiac membrane:
- IV calcium gluconate, but this does not treat the high K levels
=> Short term shift:
- Combined IV insulin and IV dextrose
- Nebulised Salbutamol
=> Removal of excess K from the body:
- Calcium resonium
- Loop diuretics
- Dialysis
What are the causes of hypokalemia?
=> Hypokalemia with alkalosis:
- Vomiting
- Loop diuretics and Thiazides
- Cushing’s syndrome
- Conn’s Syndrome
=> Hypokalemia with acidosis:
- Diarrhoea
- Renal tubular acidosis
- Acetazolamide
- Partially treated DKA
=> Hypomagnesia
What are the clinical features of hypokalemia?
- Muscle weakness
- Hypotonia
- Predisposition to digoxin toxicity
=> ECG changes:
- Prolonged PR internal
- ST depression
- U waves
- Small or absent T waves
What is the management of hypokalemia?
- Treat underlying cause
- Oral or IV K supplementation
What is SIADH?
Syndrome of Inappropriate ADH release
What is the pathophysiology of SIADH?
- Inappropriate release of ADH
- Greater reabsorption of water leads to dilution of sodium and taking up of more space (volume)
- Causes decrease in aldosterone release
- This leads to the excess water that was absorbed being lost, meaning there is no net change in plasma volume, EUVOLEMIA
- Demococycline as treatment
- Fluid therapy is slow to avoid central pontine myelinolysis
THEREFORE SIADH CAUSES EUVOLEMIC HYPONATREMIA
