Fluid& electrolyte balance Flashcards
What proportion of our body weight is made up of total body water
- 60% of ideal body weight for men
- 50-55% of ideal body weight for women
What is TBW made up of
- 2/3= intracellular fluid
- 1/3= extracellular fluid
What is intracellular fluid made up of?
- 75% interstitial fluid
- 25% plasma
What is the blood volume for females and males respectively?
- 65ml/kg (females)
- 75ml/kg (males)
What are insensible losses?
- We can’t detect/sense how much we’re losing
- e.g skin &lungs
What generates the differences between electrolyte distribution between the ECF&ICF?
The Na+/k+ pump
What can we always be certain of in terms of ions in any fluid?
In any fluid the total cations=total anions
What is the total Na in the body made up of?
- 25% non exchangeable (in tissues such as bone)- slow turnover
- 75% exchangeable- ECF
- Normal range in plasma is 135-145mmol/L-reflects body water content
- sodium consumption variable 110-220mmol/day
- Approx 5-10mmol loss in sweat& faeces, most regulated by kidneys
How is sodium excretion regulated by?
- Renin-angiotensin-aldosterone
- Natriuretic peptides
- Intrinsic renal mechanisms
How is dehydration/ increased osmolarity monitored/ controlled
Via the ADH system
Is the collecting duct permeable to water?
-If ADH is present, the collecting duct is highly permeable to water
In the case of fluid overload, how is this detected?
- Atrial natriuretic peptide( ANP) is released when the atria are stretched
- Brain/ B-type natriuretic peptide is released when the ventricles are stretched
- Detected in the heart
What effect do the natriuretic peptides have on the afferent and efferent arterioles to the glomerulus
- Dilate the afferent arteriole to the glomerulus
- constrict the efferent arteriole
What effect do the natriuretic peptides have?
- ) cause vasodilation & this decreases blood pressure
- ) Decrease renin which decreases ang II & aldo; this subsequently increases GFR leading to natriuresis& diuresis
- ) Increase GFR leading to decreased renin & allows for natriuresis and diuresis
When and why do we prescribe IV fluids?
- ) MAINTENANCE: To maintain euvolaemia when oral intake is reduced e.g being NBM,nauseated,vomiting or diarrhoea
- ) REPLACEMENT: previous/ongoing or predictable future losses such as Diaorrhea and vomitting, drains, urine, sweat,3rd spacing,burns , surgery, polyuria
- ) RESUSCITATION: Reapidly restore intravascular compartment e.g following haemorrhage,marked dehydration, vasodilation, shock
What are the symptoms & clinical signs of someone with hypovolemia?
- Symptoms: GI losses, thirst,lethargy,postural dizziness,reduced urine volume, confusion
- Clinical signs: Pulse=fast&weak; BP postural drop >20mmHg or low BP; loss of skin turgor;sunken eyes;dry mucous membranes
What are the symptoms & clinical signs of someone with hypervolemia?
- Symptoms: breathlessness,peripheral oedema,weight gain, abdominal bloating, confusion
- Clinical signs: Pulse can be fast,bounding; BP can be high, can be low; skin turgor generally maintained; can have peripheral oedema; JVP can be elevated; can have ascites
Which fluids can we use for resuscitation?
- 0.9% NaCl
- Balanced crystalloid solutions, such as Hartmann’s solution
- Colloids
Which fluids can we use for maintenance of blood volume?
- 5% glucose
- Glucose& saline solutions, such as 0.18% saline in 4% glucose
What is hyponatraemia?
Na<135mmol/L
-Low na in the blood
How can we classify hyponatraemia?
- ) Hypovolaemia
- )Euvolaemia: urine Na>30mmol/L
- )Hypervolaemia urine Na <25mmol/L and in chronic kidney disease urine Na>30mmol/L
What is SIADH?
syndrome of inappropriate ADH secretion
What are the signs& symptoms of hyponatraemia?
Presence of symptoms related to severity
- MILD hyponatremia(130-135mmol/L) = asymptomatic
- MODERATE hyponatraemia(121-129mmol/L)=cramps,weakness,nausea
- SEVERE hyponatraemia(,120mmol/L)= lethargy, headache,confusion
- SEVERE& RAPIDLY EVOLVING hyponatraemia= seizures, coma,respiratory arrest
Osmotic gradient between ECF& ICF within the brain
- Water moves into cells
- Raised intracranial pressure due to oedema-Neurological symptoms
How can we treat hyponatraemia?
- Hypovolemia: correct volume depletion e.g 0.9% saline
- Euvolaemia: underlying cause, fluid restriction
- Hypervolaemia: underlyng cause, fluid restriction (vasopressin receptor antagonists)
- Aggressive therapy used if the symptoms are severe and there has been acute hyponatraemia for less than 24hours
What is the risk associated with rapid correction of low Na?
-Risk of central pontine myelinosis( a neurological disorder caused by severe damage of the myelin sheath of nerve cells in the pons; may be iatrogenically induced
What is hypernatraemia & what are the symptoms ?
- Na >145mmol/L
- Results from: net water loss, hypertonic Na gain
- Increase in plasma toxicity pulls water out of cells, resulting in a decrease in intracellular volume
- severe if Na>158mmol/L
- Symptoms: thirst,anorexia,weakness,stupor,seizures,coma
How can we classify hypernatraemia?
- Un-replaced water loss
- Sodium overload
How can we treat hypernatraemia?
- ) Chronic hypernatraemia:
- Treat underlying cause
- Use of hypotonic fluid e.g 5% dextrose given slowly
- Lower Na by max of 10mmol/L per day
- always reassess - ) Acute therapy/emergency
- Hypotomic fluid
- Lower Na by 1-2mmol/L/h to restore normal Na levels within 24h
- Because acute, increase in the plasma Na can lead to irreversible neurologic injury— needs expert help
What is hypokalaemia?
- K<3.5mmol/L
- K enters the body via oral intake or IV, largely stored in the cells, excreted in urine
- Results from : decreased K intake; increased entry into cells; increased losses (GI/urine)
- Manifestations are proportionate to severity…
1. ) Muscle weakness; usually when <2.5mmol/L-progresses from lower extremities
2. ) ECG changes &arrhythmias
3. ) Chronic: renal abnormalities; impaired concentrating ability, raise BP
What ECG changes may be present in hypokalaemia?
- Flat T wave
- U waves
- ST depression
- PR interval prolonged
- Prolonged QT intervals
How can we treat hypokalaemia?
- Correct Mg levels
- K replacement- oral vs IV
- If IV max 10-20mmol/hr and cardiac monitoring
What is hyperkalaemia?
k>5.5mmol/L
-Classified as 1.) increased release from cells 2.) reduced urinary excretion
What are the symptoms of hyperkalaemia?
- Paraesthesiae
- Muscle weakeness-paralysis
- arrhymias
What ECG changes may result from hyperkalaemia?
- Tall peaked T waves
- shortened QT interval
- PR interval lengthening
- QRS widening
- P waves disappear- sine waves
How can we treat hyperkalaemia?
- IV calcium gluconate- antagonise membrane action of high K
- IV insulin with glucose- drive K into cells
- Remove K from the body(consider loop diuretics, haemodialysis& haemofiltration)
- Other therapies to drive K into cells: sodium bicarbonate, Beta agonists
