Fluid Balance

Question 1

What is the approximate percentage and volume of water in a 70kg male?`

Answer 1

~60% (female: ~55%)

= ~42l

Question 2

What percentage of total body water is intracellular? What are the approximate concentrations of the different ions in the intracellular compartment?

Answer 2

2/3 (70kg male: ~28l)

[Na+] = 12mmol/l
[K+] = 150mmol/l
[Cl-] = 4mmol/l
[HCO3-] = 12mmol/l

Question 3

What percentage of total body water is extracellular?

Answer 3

1/3 (70kg male: ~14l)

Question 4

What percentage of extracellular fluid is interstitial? What are the approximate concentrations of the different ions in the interstitial compartment?

Answer 4

4/5 (70kg male: ~14l)

[Na+] = 144mmol/l 
[K+] = 4mmol/l 
[Cl-] = 114mmol/l
[HCO3-] = 30mmol/l

Question 5

What percentage of extracellular fluid is intravascularl? What are the approximate concentrations of the different ions in the intervascular compartment?

Answer 5

1/5 (70kg male: ~3l)

[Na+] = 142mmol/l
[K+] = 4mmol/l
[Cl-] = 103mmol/l
[HCO3-] = 27mmol/l
[glucose] = 5mmol/l
[protein] = 10mmol/l

Question 6

What are the approximate values for input and output of fluid?

Answer 6

INPUT: 40ml/kg/day
- drink = 1500ml
- food = 750ml
- metabolic = 350ml
= 2600ml

OUTPUT: 0.5ml/kg/hr urine
- urine = 1500ml
- faeces = 100ml
- lungs = 400ml
- skin = 600ml
= 2600ml

Question 7

Give some examples of causes of reduced urine output after surgery.

Answer 7

Normal response to fluid loss during surgery

Pre-renal = ?dehydration (insufficient intake or excessive loss before kidneys)

Renal = ?patient known to habe CKD, ?AKI due to nephrotoxic drugs

Post-renal = ?catheter blocked (anuria)

Question 8

Give some examples of cystalloid fluids and their approximate ion fluids.

Answer 8

0. 9% saline:
   - [Na+] = 154mmol/l
   - [K+] = 0mmol/l (unless added)
   - [Cl-] = 154mmol/l
   - Osm = 308 (hypotonic)

Dextrose (5%):

• [Na+] = 0mmol/l
• [K+] = 0mmol/l
• [Cl-] = 0mmol/l
• Osm = 278 (hypertonic)

Combination

Hartmann’s:

• [Na+] = 131mmol/l
• [K+] = 5mmol/l
• [Cl-] = 111mmol/l
• Osm = 275 (hypertonic)

Question 9

Give some examples of colloid fluids.

Answer 9

Gelafusine/Volplex

Starch

Albumin

Blood products

Question 10

Contrast the duration of plasma volume expansion of 5% dextrose, 0.9% saline, and Hartmann’s.

Answer 10

5% dextrose = 1/3 x 1/5 = 1/15 (therefore not good for shocked patients)
- 1l distributes through 42l of total body water, therefore only 3l goes intravascular (14l would need to be given to increase the plasma volume by 1l, 14 x 3 = 42l)

0. 9% saline = 0.2hrs (1/5 remains)
   - 1l distributes to ECF, therefore only 1/6 goes intravadcular (6l would need to be given to increase the plasma volume by 1l, 1/6 x 6 = 1l)

Hartmann’s = 0.2hrs

Question 11

What is the maximum safe rate of K+ infusion?

Answer 11

5mmol/hr

If more than 1l of 20mmol/l K+ needs to be given then it must be supervised and go through a controlled pump.

K+ is a vein irritant; if given too fast it should it should be given into a central vein.

note: K+ released by damaged tissue so usually no K+ needs to be given 48hrs post-op

Question 12

Describe the practicalities of giving fluid resuscitation (pre-existing deficit).

Answer 12

Req. clinical assessment of patient’s hydration status and CVS parameters

Fluid administered in boluses and titrated to response.

Req. freq. reassessment i.e. every 15min

Give crystalloids or colloids (blood in haemorrhage)
- note: do not give 5% dextrose (only 1/15 stays in intravascular compartment)

Question 13

Describe the practicalities of replacing ongoing fluid losses.

Answer 13

e.g. vomiting, diarrhoea, high output stoma, enterocutaneous fistula

Measure and record fluid losses

Patient assessment:
- peripheral perfusion 
,- pulse rate/BP
- JVP/CVP
- flow based measurements 
- urine output (>0.5ml/kg/hr) 

HYPOVOLAEMIA:

• consider nature of fluid loss
• resuscitate with crystalloid, colloid, or blood

EUVOLAEMIA:

• maintenance req. of 40ml/kg/day H2O, 2mmol/kg/day [Na+]. 1mmol/kg/day [K+]
• replace ongoing fluid losses

Question 14

Why is a loss of ECF much more serious than an equivalent loss from the total body volume?

Answer 14

1l lost per day from total body volume = ~17%

1l lost per day from functional ECF = 7/12 = 53% (not compatible with life)

Question 15

What fluid changes occur as a result of surgical trauma?

Answer 15

Increased ADH —> reduced fluid output

Increased aldosterone and cortisol —> Na+ retention, loss of K+ and H+

Increased catecholamines

Increased renin