Fluid and Electrolyte Prescribing

Question 1

Describe the body’s fluid compartments and composition.

Answer 1

• ~60% of the adult body is fluid.
• Fluid compartments are tightly controlled (volume and composition).
• Major divisions:
  
  • Intracellular (2/3)
  • Extracellular (1/3)
• Extracellular can be further divided: plasma (1/4) and interstitial (3/4) (this includes synovial, intra-ocular, CSF etc.).
• Composition is determined by movement across the plasma membrane - ions must move through channels.

Question 2

Give a summary of body fluidregulation, compartments and their separating membranes.

Answer 2

Blood volume is a mixture of extracellular and intracellular fluid.

Question 3

Describe the exchange of fluid across the capillary membrane.

Answer 3

• The main protein inside the vessel is albumin.
• If there is liver failure and albumin production is reduced, the patient will get oedema.

Question 4

What are the extracellular and intracellular concentrations of potasium, sodium and chloride?

Answer 4

• U & E is essentially measuring extracellular fluid.
• Chloride mirrors sodium.
• Bicarbonate is important in acid base balance.
• Normal K+ = 3.5-5.5mM
• K+ has to be kept in normal boundaries and if it dips it is a reflection of all the intracellular potassium that cannot be measured.

Question 5

Which barrier does fluid have to move through to move:

• Between plasma and interstitial fluid?
• Between extraellular fluid and intracellular fluid?

Answer 5

• Between plasma and interstitial fluid
  
  • Capillary wall
• Between extraellular fluid and intracellular fluid
  
  • Plasma membrane

Question 6

Describe typical gain and loss of body fluid.

Answer 6

• Gains:
  
  • Food and water intake
  • Oxidation of food
• Losses:
  
  • Urine (variable; average 1500mL)
  • Faeces (variable; average 100mL)
  • Sweat (variable; average 50mL)
  • Insensible losses (variable; average 900mL)
• Total losses (on average) = 2550mL

Question 7

What is insensible water loss?

Answer 7

• Transepidermal diffusion: water that passes through the skin and is lost by evaporation.
• Evaporative loss from the respiratory tract.
  
  • If the patient is pyrexial or they have an increased respiratory rate then factor this in when prescribing fluids - they are losing more.
• Note - Insensible losses are solute free.

Question 8

What are the risks associated with prescribing IV fluids?

Answer 8

• Risks with IV fluids:
  
  • Peripheral vascular cathater (PVC) required.
  • Easy to give too much fluid (especially in sick people).
  • Errors in prescribing.
• Note: it is easy to give fluids but it is much harder to get rid of fluid, especially in patients with renal impairment.

Question 9

What information should be elicited in a history when assessing a patient’s volume status?

Answer 9

• Limited intake?
• Abnormal losses?
  
  • How much?
  • What kind of fluid?
  • Ongoing? Can I treat the cause?
• Comorbidities?
• Current illness?
• Symptomatic?
• Fluid balance charts?

Question 10

What factor is not accounted for on a fluid balance chart?

Answer 10

Insensible losses

Question 11

Describe the examination of a patient to assess fluids?

What state would these signs be in during hypovolaemia?

Answer 11

• Trends, context and response to fluid challenge.
• Vital signs (in hypovolaemia):
  
  • Systolic BP (<100mmHg)
  • HR (>90bpm)
  • Capillary refill (>2 secs)
  • RR (20 breaths /min)
  • Urine output / colour (<0.5ml / kg / min)
  • (Hypovolaemic patient will also have dry mucous membranes, decreased skin turgor and responsiveness to passive leg raising to 45°).
• Postural hypotension is a sensitive marker of hypovolaemia.

Question 12

What are the signs associated with fluid overload?

Answer 12

• History of cardiac or renal problems
• Raised JVP
• Peripheral oedema
• Inspiratory crackles at lung bases
• Hypertension

Question 13

Assessment of volume status is a clinical decision, but investigations can be helpful.

Which investigations would be appropriate?

Answer 13

• Full blood count
• Urea & electrolytes
• CXR
• Lactate
• Urine biochemistry

Question 14

What do Kerley B lines on a CXR indicate?

Answer 14

Cardiac failure because they show interstitial pulmonary oedema.

Question 15

What are the minimum electrolyte requirements?

Answer 15

• Sodium: 1mmol / kg / 24 hours
• Potassium: 1mmol / kg / 24 hours
• Calories: minumum (to avoid catabolism) 400kcal / 24 hours.
• Note - keep an eye on magnesium, calcium and phosphate and replace as required. If you are keeping your patient fasting for 48 hours you need to start replacing calcium, magnesium and phosphate.

Question 16

What are the indications for maintenance fluids?

Answer 16

• Patient does not have excess losses.
• If no other intake they need ~30ml / kg / 24 hours.
• May only need part of this IV if there is some oral intake.

Question 17

What are the indications for replacement fluids?

Answer 17

• Replacement of previous and / or current losses.
• This fluid is in ADDITION to maintenance fluid.

Question 18

What are the indications for resuscitation fluid?

Answer 18

• The patient is hypovolaemic and requires urgent correction of intravascular depletion.
• These fluids are not only a treatment but can also be diagnostic.

Question 19

What are the different IV fluids which can be prescribed (crystalloids)?

Answer 19

• 5% dextrose (glucose)
  
  • ​Initially distributes through interstitial fluid and plasma; glucose is metabolised, so effectively adding just water.
  • Further distributes into cells as well as ISF and plasma.
  • Alternatively 0.18NaCl 4 % dextrose
• 0.9% NaCl (isotonic saline)
• Plasmalyte
  
  • ​Distributes through ISF and plasma; does not enter cells.

Question 20

What is gelofusine?

Answer 20

• Synthetic colloid
• Protein molecules come from bovine gelatin.
• High Na content and high Cl content and relatively high protein content.

Question 21

What are patients most likely to be prescribed if they are losing electrolyte-rich fluid?

Answer 21

• 0.9% NaCl
• Just lactate, not the H⁺ ion of lactic acid. Metabolised into bicarbonate.

Question 22

What is fluid challenge?

Under what circumstances would you consider this?

Answer 22

• Oliguria or hypotension and no signs of overload - consider a fluid challenge.
• Therapeutic and diagnostic.
• 500ml balanced salt solution given quickly (<15 minutes).
  
  • Needs to be quick because fluid will not stay in the intravascular space for a long time.
• Re-assess.
• Can repeat up to 2000ml.

Question 23

What are the different IV fluids which can be prescribed (colloids)?

Answer 23

• 4.5% albumin
  
  • ​Supplied in 0.9% NaCl.
  • Tends to stay in plasma; does not enter cells.
  • Blood product.
• Hydrolysed gelatin
  
  • ​Supplied in 0.9% NaCl.
  • Initially tends to stay in plasma; does not enter cells.
  • Protein metabolised over time so then equivalent to 0.9% NaCl.
• Blood
  
  • ​Stays in the vasculature and increases blood volume.

Question 24

Which patients who should be treated with caution when prescribing fluids?

Answer 24

• Obese patients (use ideal body weight)
• Elderly or frail patients
• Patients with renal impairment
• Patients in cardiac failure
• Malnourished patients or those at risk of refeeding sydrome

Question 25

What does a CVP line measure? What is the target for this measurement?

Answer 25

* The pressure in the right atrium. * Target = 8-12mmHg.

Question 26

What questions must you ask yourself when prescribing fluids for a patient?

Answer 26

* Is my patient euvolaemic, hypovolaemic or hypervolaemic? * Does my patient need IV fluid? Why? * How much? * What type of fluid?