IV fluids

Question 1

IV fluids can be categorised into what 2 types?

Answer 1

Crystalloid
- Containing small molecules such as sodium chloride or glucose
- Distribute more readily into other tissues

Colloid
- Dispersions of large organic molecules (albumin, gelatin, dextran, etherified
starches)
- Tend to remain in the intravascular space where they bind water, owing to their exertion of oncotic pressure

Question 2

How does water move between the intracellular and extracellular components?

Answer 2

Water can move freely across the membranes that separate the intracellular and extracellular components to maintain
osmotic equilibrium

Osmotically active substances — predominantly albumin — bind water
in the intravascular compartment and thereby ensure that the circulating blood volume is adequate

Question 3

How is water gained and lost from the body?

Answer 3

Gained
- Food and drink
- Small amount is generated from carbohydrate metabolism

Lost
- Urine
- Sweat
- Faeces
- Lungs
- Skin

Question 4

What are the typical fluid requirements for an adult?

Answer 4

30ml/kg/day

However, because the kidneys can concentrate urine considerably, the minimum obligatory water intake is
considered to be 1,600ml/day from any source, allowing for a urine output of 500ml

Question 5

What hormone regulates volume homeostasis?

Answer 5

Antidiuretic hormone (ADH)

Osmoreceptors in the hypothalamus and baroceptors (located in the aorta, the great
veins, right atrium and carotid artery) detect small decreases in osmolality and blood
pressure, triggering the release of ADH. This elicits a sensation of thirst and reduces renal excretion of water.

The renin-angiotensin system also plays a role and is activated by falling
renal perfusion pressure.

Question 6

What is the preferred route of fluid intake?

Answer 6

Enterally (oral)

Parenteral fluid therapy exposes
patients to risks such as fluid overload (by overriding physiological safeguards) and adverse effects associated with individual fluids.

Question 7

When is IV fluid replacement required?

Answer 7

IV fluid therapy is required when enteral intake is insufficient

Examples:
- When a patient is “nil by mouth” or has reduced absorption
- To replace large fluid losses
- When very rapid replacement is necessary (severe burns, sepsis, blood loss)

Question 8

What is ‘third spacing’?

Answer 8

Vasodilation and “leakage” of vascular epithelial walls

Results in breakdown of normal compartment integrity, which can result in loss of circulating intravascular volume

Question 9

What are the signs of dehydration?

Answer 9

• Thirst
• Reduced skin turgor
• Dry mucus membranes
• Increased capillary refill time
• Altered level of consciousness

Question 10

How does blood pressure react to a reduction in intravascular volume?

When would blood pressure fall?

Answer 10

Heart rate will increase to improve cardiac output and raise blood pressure

Blood pressure only falls after the intravascular volume has dropped by 20–30%

Question 11

How is urine impacted in volume depletion?

Answer 11

Urine becomes concentrated in cases of volume depletion — more severe cases result in a fall in urine output

Raised plasma urea (above 6mmol/L) and sodium levels (above 145mmol/L) can indicate dehydration, as can acidosis on a blood gas analysis

Question 12

How is a patient’s response to fluid therapy assessed?

Answer 12

Question 13

What invasive techniques are used to measure response to fluid therapy?

Answer 13

Question 14

What is a fluid challenge?

Answer 14

CVP = Central venous pressure

Question 15

How is fluid balance monitored?

Answer 15

Documenting overall fluid intake and output

Document losses via:
- Urine, drains, stoma or nasogastric aspirates
- Insensible losses via the respiratory tract and skin (adjusted for body temperature) should be estimated.

It is important to interpret all observations in the context of a patient’s clinical diagnosis — eg, an oedematous patient may show a positive fluid balance but still be intravascularly depleted, resulting in insufficient tissue perfusion and oxygenation.

Question 16

Children have a higher risk of electrolyte disturbances caused by fluid
therapy. True or False?

Answer 16

True

Owing to their lower ability to compensate

Question 17

What patients are at higher risk of fluid overload?

Answer 17

• Heart failure
• Renal impairment
• Respiratory failure

Question 18

What is a balanced solution?

Answer 18

Solutions with electrolyte compositions more closely matched with plasma (e.g. Hartmann’s solution)

Question 19

Crystalloid solutions contain small molecules such as NaCl or glucose.

How do they distribute in the body? When are they used?

Answer 19

NaCl distributes into the whole extracellular space (ie, into both intravascular and interstitial spaces, although not in equal proportions)
- NaCl 0.9% is isotonic with plasma
- Commonly used for fluid therapy
- HYPOtonic NaCl is used to treat hypernatraemia
- HYPERpertonic NaCl is sometimes used to correct hyponatraemia (very strong solutions are used to manage
aspects of head injury)

Glucose solutions distribute throughout the intravascular, interstitial and intracellular
compartments
- Glucose 5% solution has the same tonicity
as plasma and is used for maintenance fluid therapy
- The glucose itself is metabolised
quickly and is, in effect, providing free water
- HYPERtonic solutions of glucose (10% or 50%) are used when glucose substitution is required (eg, to treat hypoglycaemia)

Use HYPER to treat HYPO

Question 20

Colloid solutions contain large organic molecules (albumin, gelatin, dextran, etherified starches)

How do they distribute in the body? When are they used?

Answer 20

The duration of action depends on the molecular size of the starch (larger molecules tend to have a longer duration), the rate of degradation and the permeability of the vascular endothelium.

Etherified starches have relatively high molecular weights
- (tetrastarch 130,000 daltons; pentastarch 200,000 daltons; and hetastarch 450,000 daltons)
- Provide volume expansion for 6–24 hours

Modified fluid gelatin
- Derived from animal collagen and has a molecular weight of 30,000 daltons
- Its effective half-life is about four hours, but its volume-restoring effect may be
shorter in patients with capillary leakage

Dextran solutions
- Synthetic polysaccharide colloids
- Dextran 70 is the only remaining UK-licensed preparation.

Albumin
- Natural colloid, derived from whole blood
- It is commercially available in isotonic
(4.5–5% albumin) and hypertonic (20% albumin) solutions.

Question 21

Maintenance fluid

Answer 21

Glucose-based fluid + a second fluid to boost intravascular volume (usually a sodium-based fluid)

Usually a combination of NaCl 0.9% and glucose 5% infusions
- Or as “dextrosesaline” (generally glucose 4% and NaCl 0.18%). However this is not recommended for long termuse as does not provide adequate daily sodium

Usually, up to 3L will be administered over 24 hours.

In addition to providing water, maintenance fluids should also provide sodium and potassium.

Question 22

Replacement fluid

Answer 22

The objective is to restore circulating blood volume and increase cardiac output, thereby restoring tissue perfusion and oxygen delivery

Any sodium or colloid-based fluid can be used to restore intravascular volume.

Fluids that distribute throughout total body water (eg, glucose) do not restore intravascular volume and can exacerbate interstitial oedema in patients who are suffering from inflammatory conditions
(eg, sepsis).

If large volumes of fluid are required, a balanced crystalloid solution (eg, Hartmann’s solution) is preferred because of the complications associated with excessive NaCl load.

Question 23

The timing of fluid replacement and resuscitation can sometimes be as important as the volume and type of fluid administered.

When is the best time to give fluid resuscitation in critically ill patients?

Answer 23

Within six hours of deterioration

Aggressive and early fluid resuscitation achieves better outcomes than delayed fluid resuscitation

Question 24

What are some complications of IV fluid replacements?

Answer 24

Administration of too much fluid
- The heart can fail to pump the expanded circulatory volume effectively and over-distension of the left ventricle can cause heart failure and, consequently, pulmonary
oedema
- A patient with pulmonary oedema will be short of breath and have a cough,
respiratory crackles on auscultation and reduced oxygen saturation
- Often accompanied by an increased heart rate, which must not be confused with
tachycardia caused by hypovolaemia

Abdominal compartment syndrome

Acute respiratory distress syndrome

Electrolyte disturbances/biochemical irregularities
- Infusions of NaCl 0.9% can result in over-
provision of sodium and chloride and cause hyperchloraemic acidosis

Allergic reactions
- Rare but can happen with synthetic colloids such as etherified starches, gelatin and dextran

Dilutional coagulopathy
- Effect of high-volume fluid administration
- Some colloid infusions impair components of the clotting cascade (eg, dextran solutions are known to be antithrombotic)
- The higher molecular weight etherified starches (eg, hetastarch, pentastarch) have been associated with increased bleeding

Renal impairment
- Starch solutions can cause hyperoncotic acute renal failure if given with insufficient water
- The oncotic pressure of plasma is raised to
point where it effectively opposes the filtration pressure in the kidneys, thereby impairing normal glomerular filtration
- To avoid this, starch infusions should be accompanied or followed by a crystalloid infusion (e.g. saline) of the same volume.

Question 25

Compare crystalloid and colloid infusions

Answer 25

Question 26

Compare saline and balanced fluids, when would balanced fluids be preferred?

Answer 26