Surgery - Perioperative Fluid Balance

Question 1

Perioperative fluid balance

Not forgetting electrolytes

Answer 1

Fluid Compartments:

Total Body Water (TBW):

• Varies as % of body weight-variable amount of fat
• TBW 75% of total body weight in neonates
• TBW 50% of tbwt. in elderly
• Average 70 kg adult male has 60% tbwt. as water ( 42 l. )
• Adult 50 kg female has 55% tbwt. as water 30 l.
• Body water is divided between ECF and ICF space
• TBW=0.6 x body wt

Question 2

Perioperative fluid balance

Fluid spaces - ECF

Answer 2

ECF-fluid outside cells +=/3 TBW =14 litres
Subdivided:

• Intravascular fluid ( plasma ) =3 litres. Add cells = total blood volume
• Interstitial fluid=10 litres. Fluid outside the vascular system-mainly tissues touching micro-vascular circulation
• Transcellular fluids (0.5% TBW ), eg., csf, synovial, pleural

Question 3

Perioperative fluid balance

Fluid spaces - ICF and 3rd Space

Answer 3

• ICF is contained within cells=2/3 TBW-(28 l.)
• Third Space-Anywhere fluid does not normally collect or is physiologically non functional ( burns, ascites, pancreatitis, ileus, trauma )-can be anywhere and is difficult to quantify

Question 4

Electrolyte and water distribution

Answer 4

• Movement of water between ICS and ISP governed by osmotic forces
• Cell membrane selective-H2O moves freely-not solutes
• Diffusion-Movement of a substance from an area of high concentration to low concentration
• The cell membrane is charged- so cations such as Na+ and K+ cannot cross easily
• They cross via specific channels
• Na+-K+ pump maintains cell membrane potential ( +ive to the outside by moving 3 Na+ out and 2 K+ in )

Question 5

Osmosis

Answer 5

• Osmosis-the movement of solvent molecules (water ) across a semi-permeable membrane to a region of higher solute concentration tending to equalise the solute concentrations

Question 6

Body fluid electrolytes

Answer 6

Question 7

ICF composition

Answer 7

• K+ main cation
• Cell membrane selectively permeable to different ions-freely to H2O
• Osmotic equilibration continuous by movt. of water across c.membrane
• *ICF and ECF osmolalities must be equal*
• Previous slide showed concentrations of ICF ions > ECF ions
• However the number of osmotically active particles is the same ( and thus osmolalities )
• Osmolality 290 moml/kg on both sides of membrane

Question 8

Distribution of water between ICF and ECF-1

Answer 8

• Water readily crosses the cell membrane so ICF tonicity = ECF tonicity
• Assuming cell solute is constant, then ECF tonicity (which may alter ) determines that water flows till osmotic gradient is abolished
• If ECF tonicity ↑ , then water would move out of the cell and vice versa
• Rationale for the use of 20% mannitol in ↑ICP. Affects all cells but target is the brain
• Na+ and Cl- account for 90% ECF tonicity. Na+ has low permeabilty across the cell and Na+ pump excludes it from the ICF
• So distribution of TBW between ICF and ECF is really decided by the ECF Na+

Question 9

Gibbs-Donnan equilibrium

Answer 9

• Proteins cannot cross the cell membrane- more inside and are + charged. Gibbs-Donnan 1
• So water would tend to enter the cell which would swell and more solute would enter.
• The sodium pump keeps sodium out setting up Gibbs-Donnan 2 equilibrium
• Overall G-D 1 balances G-D 2
• If Na entered uncontrollably the cell would swiftly rupture

Question 10

Define osmolality and osmolarity

Answer 10

• Osmolality is determined experimentally with difficulty
• Similar to osmolality which is calculated and preferred
• Osmolarity: the concentration of osmotically active particles in an aqueous solution, expressed as osmoles per litre of solution
• Osmolality: the concentration of osmotically active particles in an aqueous solution, expressed as osmoles per kilogram of solvent
• Calculated values of osmolarity 5-10% higher

Question 11

How to calculate osmolarity

Answer 11

For a non-electrolyte (e.g. glucose): Osmolarity = Concentration (g/L)/MW x 1000mOsmol/L OR 50/180 x 1000 =277

For an electrolyte (e.g. sodium chloride): Osmolarity = Concentration (g/L)/MW x number of ions formed x 1000mOsmol/L OR 9/58.5x2x1000 =307

Water is not considered as it is the solvent

Question 12

Osmolarity and molarity

Answer 12

• N saline ( 0.9% ) and 5% dextrose are commonly used in fluid regimen.
• Normal plasma has an osmolality of 290 mOm/L
• 5% glucose has 50gm glucose/L.( C6-H12-O6 )-mol.wt =180 )
• So 180 gm glucose =1000mmol and 50gm=278mOsm/L. ( 50/180)
• 0.9% saline has 9gm NaCl/L. ( NaCl-mol. wt.58 )
• Therefore 58 gm NaCl=1 mole if undissociated
• 9 gm NaCl has a conc. of 154 mmol NaCl/L but 308 mOsm/L
• Glucose remains 1 molecule and NaCl splits into 2 atoms

Question 13

Isotonic and Iso-osmotic​

Answer 13

• An isotonic fluid is one in which a cell can be placed without transfer of fluid into or out
• An isotonic fluid exerts the same osmotic pressure as plasma
• The diffusion of water in the direction of the highest concentration of non-diffusible ions can be stopped by pressure-the osmotic pressure.
• Iso-osmotic OR having the same number of particles per unit volume
• Isotonic OR have the same osmotic pressure

Question 14

What keeps fluid in the circulation(1)

Answer 14

• Rate of exchange of fluid flow from one compartment to another is determined by hydrostatic pressure , oncotic pressure and vessel wall permeability
• Forces pushing fluid out ( all mean values in mmHg )
  
  • Capillary hydrostatic pressure 17
  • Interstitial oncotic pressure 5
  • Interstitial fluid pressure -6.3
  • Forces keeping fluid in
  • Capillary oncotic pressure 28

Question 15

What keeps fluid in the circulation(2)

Answer 15

• In most vascular beds, fluid loss at the arteriolar end of the capillary and reabsorption at the venous end
• Generally filtration>reabsorption
• Excess picked up by lymphatics
• Plasma proteins account for all osmotic force exerted by the capillary membrane ( electrolytes can move across and exert no force )
• Protein content is lower in the ISF so there is a pressure gradient –oncotic pressure gradient-tonicity is normally reserved for the cell membrane
• Capillary hydrostatic pressure and plasma oncotic pressure are main factors

Question 16

Role of capillary endothelium in water flux between plasma and interstitium

Answer 16

Capillary endothelium permeable to water and ions which flow freely in either direction

The major determinant of water flux is plasma protein.
Tight endothelial cell junctions keep protein in
Mol wt albumen 70,000
Plasma proteins make up 75% oncotic pressure

If the endothelium is damaged this leads to leakage of albumen and oedema

Question 17

Mean pressures acting across the capillary membrane-Starlings Forces

Answer 17

Question 18

Water Homeostasis

Answer 18

• Day to day fluctuations of TBW small
• Input -thirst mechanisms ( osmoreceptors in hypothalamus )-liquids (1.3L), food (0.8L), as a metabolic product (0.4L)
• Output-
• 1.Insensible losses-skin (0.5L) and lungs ( 0.4L)
• 2.Sensible losses-kidneys ( 1.5L ) and gastro-intestinal tract ( 0.1L )
• Note 5L/day into GIT-but only 100ml in faeces-may be significant loss in bowel obstuction/perforation

Question 19

Regulation of ECF volume and composition

Answer 19

• ECF volume determined by Na+ and Cl-
• Na+ most important as Cl- changes secondary to Na+
• ECF volume controlled by mechanisms regulating Na+ balance
• Control of Circulating volume is vital-volume stimuli can override the osmotic regulation of ADH secretion
• ↑ECF vol →↓vasopressin secretion
• ↓ECF vol→↑Vasopressin secretion
• ↑ECF vol →↑natriuretic hormone secretion (atrial NP→natriuresis and diuresis)

Question 20

Sodium balance

Answer 20

• Na+ ingestion 50-300 mmol
• Regulated by kidneys
• Volume and constitution of filtrate reaching collecting ducts dependent on GFR, sympathetic tone, angiotensin 11 acting via effects of ADH and aldosterone to conserve H2O and Na