Fluid Balance and Administration

Question 1

What are the three main components of bodily fluid?

Answer 1

Water
Ions
Protein

Question 2

What forces affect the net movement of fluid through the capillary membrane?

Answer 2

Osmotic pressure

Hydrostatic pressure

Question 3

What percentage of humans is water?

Answer 3

Men - 60%
Women - 50% - due to higher body fat
Children - 75%

Question 4

Definition of total body water?

Answer 4

Total volume of water in the body

Question 5

Definition and components of extracellular fluid?

Answer 5

Total volume of fluid outside of cells - intravascular (plasma), interstital (in tissues), lymphatics and transcellular

Question 6

Definition of intracellular fluid?

Answer 6

Total volume of fluid inside of cells (TBW - ECF)

Question 7

Definition of plasma

Answer 7

Blood without cells - contains protein, water and electrolytes

Question 8

Definition and examples of transcellular fluid?

Answer 8

The smallest component of ECF (mostly made up of interstital fluid and plasma). Fluid contained in epithelial lined spaces

• CSF
• intraocular fluid
• pleural
• synovial
• digestive secretions

Question 9

What are third space losses?

Answer 9

Fluid ‘lost’ into the transcellular fluid spaces as it can’t be exchanged with rest of ECF

Question 10

Breakdown of total body water in average 70kg man.

Answer 10

45L total

• 2/3 intracellular (30L)
• 1/3 extracellular (15L) - plasma 3.5L, lymph 1.5L, interstital 8.5L and transcellular 1.5L

Question 11

What are the main ions in ECF?

Answer 11

Sodium 135 - 145mmol/l

Bicarbonate - 25mmol/l

Question 12

What are the main ions in ICF?

Answer 12

Potassium - 150mmol/l
Magnesium - 40mmol/l
Phosphate - 100mmol/l
Protein - 8mmol/l

Question 13

By what cellular pump are potassium levels maintained?

Answer 13

Na+/K+ ATPase pump

Question 14

What are the 5 main ways that plasma potassium levels are controlled?

Answer 14

1) dietary intake
2) renal excretion
3) plasma pH
4) endocrine hormones
5) medications

Question 15

Describe how renal excretion can affect plasma potassium levels.

Answer 15

Filtration at the glomerular apparatus depends on the plasma concentration
In the collecting duct, sodium reabsorption depends on exchange with potassium (controlled by aldosterone)
- aldosterone is produced in response to low BP, hyponatraemia and/or hyperkalaemia

Question 16

Describe how plasma pH can affect plasma potassium levels.

Answer 16

Hydrogen ions move in and out of cells in exchange for potassium

• when hydrogen ion levels in plasma rise, they are exchanged with intra-cellular potassium, causing hyperkalaemia
• when hydrogen ion levels in plasma drop, intra-cellular hydrogen ions are are exchanged with extra-cellular potassium, causing hypokalaemia

Question 17

Describe how endocrine hormones can affect plasma potassium levels.

Answer 17

e. g. insulin, adrenaline and aldosterone stimulate cellular uptake of potassium
- hyperaldosterinism causes hypokalaemia, and can be caused by cirrhosis, renal artery stenosis, heart failure and nephrotic syndrome

Question 18

Describe how different medications can affect plasma potassium levels.

Answer 18

Hyperkalaemia - ACEI, ARBs, ciclosporin

Hypokalaemia - loop + thiazide diuretics, beta2-agonists, corticosteroids, amphotericin and theophylline

Question 19

What effects does plasma sodium have on the body

Answer 19

Excess - water retention

Deficiency - neuromuscular dysfunction

Question 20

How is sodium excreted?

Answer 20

Freely filtered through glomerular membrane
- around 65% freely reabsorbed in PCT
- remainder actively reabsorbed via sodium-potassium pumps in the ascending loop of Henle
ANP
- stimulated by fluid overload and decreases sodium reabsorption
ADH
- stimulated by increasing plasma osmolality and increases water reabsorption in DCT

Question 21

Describe plasma protein movement within the body

Answer 21

Capillary beds are impermeable to proteins, so they remain within the capillary and generate osmotic pressure
Albumin 75% of this osmotic pressure

Question 22

Describe the capillary bed and how this has a role in fluid movement.

Answer 22

This is the site of fluid and solute exchange between bloodstream and interstitum
Flow to capillary beds is controlled by pre-capillary sphincter
Capillary wall - single layer of endothelial cells surrounded by a basement membrane
- potential space between adjacent cells regulates permeability

Question 23

Definition of osmotic pressure

Answer 23

Difference in concentration between particles in solution on either side of a semi-permeable membrane (can be proteins or ions)
- water is drawn by higher osmotic pressures

Question 24

Definition of capillary osmotic pressure

Answer 24

Pressure generated by the plasma proteins in the capillaries

Question 25

Definition of tissue osmotic pressure

Answer 25

Pressure generated by the interstitial fluid - affected by interstitial protein concentration and capillary wall permeability to proteins - the more permeable to capillary wall is to proteins, the higher the tissue osmotic pressure

Question 26

Definition of hydrostatic pressure

Answer 26

The difference between capillary pressure and pressure of interstitial fluid within tissues

Question 27

What factors affect the capillary and the tissue hydrostatic pressure?

Answer 27

``` Capillary hydrostatic pressure - blood pressure - pulse pressure Tissue hydrostatic pressure - interstitial fluid volume - tissue compliance ```

Question 28

Describe the Starling hypothesis

Answer 28

Factors affecting the net flow of fluid/solutes across the capillary bed - osmotic pressure is normally constant (25mmHg) as there are few proteins in the interstitial fluid and the capillary wall is normally impermeable - tissue hydrostatic pressure is constant, so movement of fluid is determined by capillary hydrostatic pressure (so due to blood pressure; net flow of water out of capillary at arterial end, and net flow of water into capillary at venous end)

Question 29

Starling's equation

Answer 29

K = outward pressure - inward pressure K - filtration constant for capillary membrane OP - capillary hydrostatic pressure + tissue osmotic pressure IP - tissue hydrostatic pressure + capillary osmotic pressure

Question 30

Causes of oedema

Answer 30

``` Increased capillary hydrostatic pressure - fluid overload - venous obstruction Decreased capillary osmotic pressure - hypoproteinaemia (cirrhosis, nephrotic syndrome) Increased tissue osmotic pressure - increased capillary permeability (burns/inflammation) Decreased tissue hydrostatic pressure ```

Question 31

Average daily water balance

Answer 31

``` Input - 2600mls - oral fluid - 1500mls - food - 750mls - metabolism - 350mls Output - 2600mls - urine- 1500mls - faeces - 100mls - lungs 400mls ```

Question 32

Average daily electrolyte maintenance requirements (mmol/l)

Answer 32

``` Sodium - 70-140 Potassium - 70 Chloride - 70 Phosphate - 14 Calcium - 7.0 Magnesium - 7.0 ```

Question 33

How does fever affect daily water requirements

Answer 33

20% increase for each 1 degree of temperature increase | - 1L/24 hour extra

Question 34

Average urine output.

Answer 34

0.5ml/kg/hr

Question 35

Sources of excess fluid loss in surgical patients

Answer 35

Blood loss (trauma, surgery) Plasma loss (burns) Intra-abdominal inflammatory fluid loss (pancreatits) Sepsis Abnormal insensible loss (fever) GI fluid loss (vomiting, bowel obstruction, diarrhoea, ilesotomy)

Question 36

What should be taken into account when assessing fluid balance

Answer 36

``` Patient size and age Abnormal ongoing losses, pre-existing deficits/excesses, fluid shifts Renal and CV function Fluid balance charts Serum electrolytes ```

Question 37

How does the body try to prevent fluid lost post-op?

Answer 37

Surgical trauma causes ADH and aldosterone release. This promotes water conservation via sodium retention and potassium excretion

Question 38

How to assess fluid depletion

Answer 38

History - thirst, fluid loss, fluid balance chart Examination - dry mucous membranes, decreased capillary refill time, sunken eyes, low skin elasticity, decreased urine output, increased heart rate, decreased blood pressure/pulse pressure and confusion)

Question 39

Methods of fluid replacement

Answer 39

Enteral - preferred if GI tract functioning and there isn't an excessive deficiency Parenteral - GI tract non-functioning - needs rapid fluid replacement

Question 40

Types of parenteral fluid replacement

Answer 40

``` Crystalloid (isotonic) - normal saline, 5% dextrose, Hartmann's and dextrose saline (0.18% saline and 4% dextrose) Colloid - human albumin - gelatins (haemaccel, gelofusion) Dextrans (40 or 70) Hetastarch (6% in saline) Blood ```

Question 41

Describe an IV regimen to meet basal fluid requirements

Answer 41

3L fluid/24 hours - either 2 saline:1 dextrose, or 2 dextrose: 1 saline. - plus 60mmol/l potassium * doesn't account for patients requiring fluid resuscitation

Question 42

What to assess when correcting dehydration

Answer 42

Which compartment has lost fluid - e/g/ bowel losses are from ECF Extent of dehydration

Question 43

Why should no potassium be given in the 24 hours post-op

Answer 43

Potassium rises during surgery due to - cellular injury - blood transfusions - decreased renal potassiunm clearance due to transient renal impairment - 'stress hormones' encourage potassium release from cells