Peri-op Care - Fluid Management

Question 1

What are the 3 reasons for fluid prescription?

Answer 1

1. Resuscitation
2. Maintenance
3. Replacement

Question 2

What patient factors must be taken into account when prescribing fluids?

Answer 2

1. Patient size and weight
2. Co-morbidities, e.g. heart failure, CKD
3. Reasons for admission, e.g. septic or bowel obstruction patients need aggressive fluid prescribing
4. Recent electrolytes

Question 3

Describe the distribution of total body water in terms of intracellular fluid, extracellular fluid, intravascular space and interstitial space.

Answer 3

• 2/3 intracellular
• 1/3 extracellular, of this:
  ~ 1/5 intravascular
  ~ 4/5 interstitial

Question 4

Why are large volumes of IV fluids required in septic patients?

Answer 4

In septic patients, tight junctions between capillary endothelial cells break down and vascular permeability increases… fluid extravastion into tissues.

So necessary to give relatively large volumes of IV fluids to maintain intra-vascular volume, even though total body water may be high.

Question 5

What are insensible losses and how do these change in unwell patients?

Answer 5

Insensible losses = fluid output from non-urine sources, e.g. respiration, sweating, faeces.

Increase in unwell patients (may be febrile, tachypnoeic, increased bowel output).

Question 6

Suggest signs suggesting fluid depletion.

Answer 6

1. Dry mucous membranes and reduced skin turgor
2. Decreased urine output
3. Orthostatic hypotension

In worsening stages:

4. Increased capillary refill time
5. Tachycardia
6. Low BP

Question 7

Suggest 3 signs indicating fluid overload.

Answer 7

1. Raised JVP
2. Peripheral or sacral oedema
3. Pulmonary oedema

Question 8

What are the recommended water, Na+, K+ and glucose values?

Answer 8

Water: 3000 ml/day
Na+: 150 mmol/day
Cl-: 150 mmol/day
K+: 100 mmol/day
Glucose: 50 g/day

Question 9

Describe the 2 broad categories of IV fluids.

Answer 9

CRYSTALLOIDS

• more widely used than colloids (cheaper)
• e.g. 0.9% saline, 5% dextrose, Hartmann’s solution

COLLOIDS
- high colloid osmotic pressure - should theoretically raise intravascular volume faster although this is not supported by clinical trials

Question 10

Which 3 blood test results indicate volume depletion?

Answer 10

1. Increased urea:creatinine
2. ?Increased sodium (dehydration)
3. Increased haematocrit

Question 11

Why does serum urea increase relative to creatinine in dehydration?

Answer 11

Dehydration causes increased renal urea reabsorption and decreased urea excretion.

Question 12

Should 0.9% saline or 5% dextrose be given to a patient who is dehydrated? Who is hypotensive?

Answer 12

In dehydrated patient, give 5% dextrose - is distributed amongst total body water as dextrose is metabolised leaving only H2O which equilibrates between compartments.

In hypotensive patient, give normal saline - remains in ECF as contains Na+ that doesn’t enter cells (is isotonic).

Question 13

Describe a commonly used fluid maintenance regimen.

Answer 13

1L normal saline over 8hrs
1L 5% dextrose + 20mmol KCl over 8hrs
1L 5% dextrose + 20mmol KCl over 8hrs

Question 14

Explain why dehydration and increased haematocrit are features of small bowel obstruction, even before vomiting begins.

Answer 14

Fluid loss in bowel due to:

1. Accumulation of fluid
2. Increased fluid secretion
3. Decreased fluid reabsorption

Results in isotonic hypovolaemia.

Question 15

Why does severe vomiting result in hypochloremic, hypokalemic metabolic alkalosis?

Answer 15

Vomiting causes loss of hydrogen and chloride (HCl) ions, leading to hypochloremic metabolic alkalosis.

Renal compensation for alkalosis is to preserve H+ at expense of K+, leading to hypokalaemia.

Question 16

Why must dehydration and K+ imbalances be corrected prior to anaesthesia?

Answer 16

• Sympathetic nervous system is maximally activated in dehydrated patients to maintain vital organ perfusion. Anaesthetic agents dramatically reduce sympathetic tone so if dehydration not corrected, patient may become profoundly hypotensive and die at induction of anaesthesia.
• Because many anaesthetic agents affect cardiac muscle/conduction function, correction of hypo or hyperkalaemia is also very important.