Fluid and electrolyte balance Flashcards
What are the major divisions of the fluid compartments?
- intracellular
* Extracellular: plasma, interstitial, synovial, intraocular, CSF etc.
What are the barriers between the fluid compartments?
- Capillary wall between the plasma and interstitial fluid
* Plasma membrane between the extracellular fluid and intracellular fluid
Describe the exchange of fluid across the capillary membrane
- Hydrostatic pressure pushes fluid out of the capillary
* Osmotic pressure draws fluid into the capillary
How do we gain fluid?
Food and water intake
How do we lose fluid?
- Urine
- Feces
- Sweat
- Insensible loses
What are the insensible loses?
- Transepidermal diffusion - water that passes through the skin and is lost by evaporation
- Evaporative loss from the respiratory tract
What are the differences between sweat and insensible fluid
- Sweat is from specialised skin appendages - sweat glands
- There is solute loss in sweat but not in insensible loss
- Sweat is for body temperature regulation, insensible loss cannot be prevented and It is not under regulatory control
What is the central controller of body fluid?
The hypothalamus producing ADH secreted by the posterior pituitary
What happens if total sodium falls and osmolality stays the same?
Total volume falls
What happens if sodium rises and osmolality stays the same
Total volume rises
How do we gain sodium?
Food and drink
How do we lose sodium?
- Sweat
- Feces
- Urine
Describe the control of plasma Na+
- Hormones controlling sodium balance act on the kidney
- Aldosterone - retention of sodium
- Distal collecting tubule is the area of control in the nephron
- There are no detectors of Na+ conc, it is controlled indirectly via volume sensors
What happens if osmolality rises?
- Increase in thirst
- Increase in release of ADH
- Increase in water intake/retention
- Increase in volume
What happens if osmolality falls?
- Decrease in thirst
- Decrease in the release of ADH
- Decrease in water intake/retention
- Decrease in volume
What happens if there is an increase in volume?
- Increased stretch of the vascular system
- Baroreceptors detect
- Decreased renin
- Decrease in aldosterone
- Increased release of ANP
- Decreased sodium and water retention
What happens if there is a decrease in volume?
- Decrease in stretch of the vascular system
- Baroreceptors
- If pressure falls, also influences ADH and thirst centres
- Increase in renin release
- Increased levels of angiotensin II
- Increased aldosterone release
- Decreased release of ANP
- Increased sodium and water retention
How do we gain K+?
• Food/drink
How do we lose K+?
- Predominantly the urine
* Little in sweat or faeces
Describe the control of K+ secretion/absorption
- K+ is freely filtered and predominantly reabsorbed again in the PCT with controlled secretion at the DCT
- Secretion is linked to Na+ reabsorption
- Aldosterone changes the apical ion channels and changes the sodium potassium exchanger basolaterally
Describe what happens if the K+ in the plasma is increased
- Increased activity of the basolateral sodium pump
- More K+ enters the cell, increased simple diffusion across the apical membrane
- Increased secretion of aldosterone
What are the effects of aldosterone on the DCT?
- Increases the activity of the sodium pumps
- Increases the number or sodium pumps
- Increases the number of sodium and potassium channels in the apical membrane
What are the risks with intravenous fluids?
- Peripheral vascular catheter is required -> chance of infection
- Easy to give too much fluid
- Errors in prescribing
Vital signs: hypovolaemia
- Systolic BP <100mmHg
- Heart rate >90bpm
- Capillary refill >2 seconds
- Respiratory rate 20 breaths/min
- Urine output <0.5ml/kg/hour
When should you suspect fluid overload?
- history or cardiac/renal problems
- Raised JVP
- Peripheral oedema
- If pulmonary oedema: inspiratory crackles at the lung bases
- Hypertension
Which investigations may be useful for assessing volume status?
- Full blood count
- Urea and electrolytes
- Chest X ray
- Lactate
- Urine biochemistry
What are the sodium requirements?
1mmol/kg/24 hours
What are the potassium requirements?
1 mmol/kg/24 hours
What is the calorie requirement?
minimum of 400kcal per 24 hours
When do you give maintenance fluid?
When the patient doesn’t have excess losses
When do you give replacement fluid?
Replaces previous and/or current abnormal losses, this is in addition to maintenance fluid
When do you give resuscitation fluid?
When the patent is hypovolaemic and requires urgent correction of intravascular depletion
How often should an intravenous catheter be changed?
Every 72 hours
What are the crystalloid IV fluids?
- 5% dextrose
- 0.18% NaCl 4% dextrose
- 0.9% NaCl
- Plasmalyte
What are the colloid IV fluids?
- Albumin
- Blood
- hydrolysed gelatin
What are the risks of colloid IV fluids?
They are proteins so there is a risk of anaphylaxis, stays in the intravascular space
Describe the distribution of dextrose
- Effectively water
- Initially distributes through ISF and plasma, glucose is metabolised so essentially is just water
- Further distributes into cells as well as ISF and plasma
Describe the distribution of plasmalyte
Distributes through ISF and plasma, doesn’t enter the cells
Describe the distribution of 4.5% albumin
- Tends to stay in the plasma, doesn’t enter the cells
* Blood product
Describe the distribution of hydrolysed gelatin
- Initially stays in the plasma, doesn’t enter the cells
* Protein metabolised over time so then is equivalent to 0.9% NaCl
What is maintenance fluid?
• 0.18% saline 4% dextrose
Describe fluid challenge
- Consider if there is oliguria or hypotension and no signs of overload
- Therapeutic and diagnostic
- 500mls balanced salt solution given quickly then re assess
- Can repeat up to 2000mls
Cautions for fluid challenge
- Obese patients
- Elderly or frail
- Cardiac failure
- Malnourised or at risk of referring syndrome
- Chronic kidney disease
What do you do when a patient deteriorates
- CVP line to measure right atrial pressure - target of 8-12mmHg
- POC ultrasound or ECHO - look at the infer vena cava or at the ejection fraction of the heart
Diabetic ketoacidosis
- Patient presents shocked, near death and fluid depleted, ACTRAPID:
- Airway breathing circulation
- Commence fluid resuscitation
- Treat potassium
- Replace insulin
- Acidosis management
- Prevent complications
- Information for patients
- Discharge
What are the clinical features of diabetic ketoacidosis?
Hyperglycaemia: • Dehydration • Tachycardia • Hypotension • Clouding of consciousness
Acidosis • Air hunger • Acetone on breath • Abdominal pain • Vomiting
Why may someone with DKA be dehydrated?
- Hyperglycaemia
- Vomiting
- Kaussmaul respiration
- Altered consciousness (reduced intake) - may be at risk of aspiration if vomiting
Describe the clinical signs of dehydration
- Low BP
- Tachycardia
- Dry mucous membrane
- Low/insignificant urine output
Levels of what are increased in dehydration
Stress response hormones: • Cortisol • Glucagon • Growth hormone • Adrenaline
How do you treat a dehydrated patient?
• In adults start by giving 1000mls 0.9% saline over first hour
• ACTRAPID: infusion 6 units per hour
• Think about potassium
- if insulin is low, hold insulin and give K+
- if high, give K+ but check serum k+ every 2 hours
