Types of Fluids and Routes of Administration

Question 1

Define osmosis
Define osmotic pressure.

Answer 1

Diffusion of water molecules, from the region where the water molecules are in higher concentration to a region where they are in lower concentration, through a semi-permeable membrane.

The amount of pressure required to stop osmosis. The greater the concentration difference, the higher the osmotic pressure required the stop osmosis.

Question 2

Define colloid osmotic pressure.

Answer 2

A type of osmotic pressure caused by very large molecules that do not readily cross semi-permeable membranes (typically proteins e.g. albumin in blood). In the body, this helps to ‘pull’ fluid back into the capillary. Participating colloids displace water molecules, thus creating a relative water molecule deficit with water molecules moving back into the circulatory system within the lower venous pressure end of capillaries.

Question 3

What happens at the arterial end of the capillary bed?
What happens at the venous end of the capillary bed?
What happens to any fluid that is not reabsorbed by the venous end?

Answer 3

High hydrostatic pressures here are causing fluid to be pushed out of the arterial end of the capillary bed and into the tissues to nourish the cells with oxygen and nutrients.

High colloid osmotic pressure of the remaining fluid in the venules helps to ‘pull’ the fluid back in.

Unabsorbed fluid get drained away by lymphatics.

Question 4

What issues could you be aiming to address with fluid therapy?
How can we divide fluid types?

Answer 4

Changes in volume status, changes in content, changes in distribution.

Crystalloids and colloids.

Question 5

How can crystalloids be divided?

Answer 5

Hypertonic, isotonic, hypotonic.

Question 6

Give an example of a hypertonic crystalloid.
Example of isotonic “ “?
Example of Hypotonic “ “?

Answer 6

7.2% NaCl
0.9% NaCl and Hartmann’s
0.18% NaCl and 4% glucose.

Question 7

What concentration type are most colloids?
How can colloids be divided? Give examples.

Answer 7

Isotonic.
Natural e.g. plasma and synthetic. E.g. gelatin or hydroxylethyl starches.

Question 8

Difference between crystalloids and colloids?

Answer 8

Crystalloids are solutions containing SMALL molecules e.g. electrolytes dissolved in water.
They readily move out of the bloodstream through the capillary membrane.
They are cheap and widely available.
Their tonicity is variable (don’t exert any colloid osmotic pressure).

Colloids are solutions containing LARGE molecules that do not readily move out of the blood stream.
Generally more expensive and challenging to acquire.
They are isotonic but they do exert colloid osmotic pressure – varies according to size and number of large molecules within the solution.

Question 9

Define tonicity.
Define hypertonic.
Define hypotonic.
Define isotonic.

Answer 9

The capability of a solution to modify the volume of cells by altering their water content.

Having a higher concentration of solutes, water will flow out of the cell into the extracellular fluid. Cell will shrink until the 2 concentration (intra and extra) are equal.

Having a lower concentration of solutes, water will flow into the cell from the extracellular fluid. Cell will swell (may cause lysis).

Having the same concentration of solutes. There will be no net shifts of fluid.

Question 10

What tonicity of crystalloid is the most likely to be used in practice?
How do they mimic intravascular electrolyte concs?
What type of patient are they indicated for?
What can they help to replace?

Answer 10

Isotonic.
High in sodium, low in potassium.
Dehydrated and hypovolaemic patients.
Can be used to replace ongoing losses.

Question 11

How do Hartmann’s and 0.9% NaCl compare in terms of……
1. Sodium
2. Potassium
3. Chloride
4. Calcium
5. magnesium
6. dextrose
7. their buffers
8. Osmolarity
9. pH
10. Colloid osmotic pressure.

Answer 11

1. 130mmol/l vs 154mmol/l
2. 4mmol/l vs 0mmol/l
3. 109 vs 154
4. 3 vs 0
5. Same (0)
6. Same (0)
7. Lactate vs no buffer (as in pH)
8. 275 mOsm/L vs 308 mOsm/L
9. 6.5 vs 5.0
10. same (0)

Question 12

Serum values for ….
1. Sodium
2. Chloride
3. Potassium
4. Calcium
5. Osmolarity
6. pH
7. Colloid osmotic pressure

Answer 12

1. 135-150
2. 102-122
3. 3.5-4.5
4. 2.3-2.6
5. 306 mOsm/kg
6. 7.32-7.42
7. 15mmHg.

Question 13

When are hypertonic crystalloids most commonly used in small animals?
“ “ in large animals?
Examples of hypertonic crystalloids?
Recommendations for avoiding giving these fluid to patients by mistake?
In what patient types are hypertonic crystalloids indicated?

Answer 13

In small animals for the management of raised intracranial pressure.

LA – e.g. horses for rapid intravascular volume expansion.

7.2% NaCl and 7.5% NaCl.

Store hypertonic crystalloids away from isotonic crystalloids.

Large animals with hypovolaemia and small animals with high intracranial pressure. NOT dehydrated patients.

Question 14

What are hypotonic crystalloids indicated for? Why?
Examples of hypotonic crystalloids.
Consequence of using hypotonic crystalloids inappropriately?
Why is it important that the solution is isotonic at the point of administration?
Are crystalloids short-acting or long-acting? – Why?

Answer 14

Management of hyper-natraemia (rare). Lower in sodium.
0.18% saline and 4% glucose.
Acute hyponatraemia > cerebral oedema > death.
So that local irritation and tissue injury is not caused.
Short-acting – They diffuse to surrounding tissues because there are small molecules.

Question 15

Are colloids shorter or longer acting than crystalloids? – Why? – Advantage?

Answer 15

Longer acting. – Stay in the intravascular space longer. – A smaller volume of fluid can be used subsequently for volume resus.

Question 16

Potential uses of colloids?

Answer 16

Management of hypovolaemia.
Treatment of some coagulopathies (plasma).
Management of hypoproteinaemia….not very effective.
NOT for dehydration or management of electrolyte abnormalities.

Question 17

Risks and reality of colloids?

Answer 17

Coagulopathy
Allergic reactions
Anaphylaxis
Increased risk of death with human patients with sepsis
Increased AKI in people.
May not be as effective as first thought.
Expensive and rarely used.

Question 18

Routes of fluid therapy administration.

Answer 18

Per os (orally)
SC
IV
Central venous access
intra-osseous

Question 19

Per os…
How does it work?
Advantages?
Disadvantages?

Answer 19

Works the way nature intended it. Either voluntarily or via feeding tubes.

+++ Intestines can be selective about how much fluid and what electrolytes they absorb. Owners can be taught how to encourage oral fluid intake at home.

– – – Inappropriate if a loss of gag reflex (under GA, seizing, obtunded, comatose) . Absorption is slow, so this is not suitable for hypovolaemia (poor perfusion to the guts) or clinically moderate to severe dehydration.

Question 20

Subcutaneously….
How it works
Advantages
Disadvantages

Answer 20

Fluid injected by needle and syringe or via needle attached to giving set and fluid bag.

+++ Cheap and easy. O’s can be taught how to administer SC fluids at home.

– – – Absorption is slow, so this is not suitable for hypovolaemic or clinically moderate to severe dehydration. Limit to how much fluid can be administered before this becomes painful.

Question 21

Intravenous……
How it works
Advantages
Disadvantages

Answer 21

Fluid can be injected into veins, most commonly the cephalic and saphenous.

+++ Fluid can be administered slowly or rapidly and even in large volumes directly into the intravascular compartment. Appropriate for management for hypovolaemia and dehydration.

– – – Only registered RVNs and vets should be using this route.

Question 22

Central venous access…..
How it works
Advantages
Disadvantages

Answer 22

Fluid can be injected directly into the larger or ‘central’ veins, most commonly the jugular.

+++ In SA, good for if peripheral venous access proving impossible to obtain. Central lines also have ports for blood sampling, useful for critically unwell patients. Commonly used route in LAs e.g. horses, where peripheral catheters would be impractical to care for.

– – – SA, further training and special equipment required. Patients need to be very still for placement – typically anaesthesia required.

Question 23

Intraosseous…….
How it works
Advantages
Disadvantages

Answer 23

Fluid injected directly into the medullary cavity of long bones e.g. femur.

+++ SA, good when peripheral venous access proving impossible to obtain e.g. in neonates. Medullary cavity highly vascularised, allowing for rapid absorption of fluid. Hypodermic needle can be used in neonates.

– – – For adult dogs and cats, special equipment and further training required. Risk of fractures and osteomyelitis.

Question 24

Complications of fluid therapy?

Answer 24

Fluids are drugs!
there are interactions e.g. Hartmann’s and bicarbonate/blood products.
Co-morbidities need to be considered before prescribing fluid therapy.
- Cardiac disease and heart failure.
- Renal disease
- Respiratory disease.

Question 25

Volume overload – What will you look out for?

Answer 25

Pulmonary oedema – tachypnoea, dyspnoea, crackle upon lung field auscultation.
Venous engorgement e.g. jugular distension and/or pulsations.
Peripheral oedema formation e.g. swollen limbs and chemosis (swelling of eye conjunctiva).
Cavitary effusions e.g. pleural, peritoneal etc.
Serous ocular discharge.