Surgery - Perioperative Fluid Balance Flashcards
1
Q
Perioperative fluid balance
Not forgetting electrolytes
A
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
2
Q
Perioperative fluid balance
Fluid spaces - ECF
A
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
3
Q
Perioperative fluid balance
Fluid spaces - ICF and 3rd Space
A
- 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
4
Q
Electrolyte and water distribution
A
- 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 )
5
Q
Osmosis
A
- 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
6
Q
Body fluid electrolytes
A
7
Q
ICF composition
A
- 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
8
Q
Distribution of water between ICF and ECF-1
A
- 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+
9
Q
Gibbs-Donnan equilibrium
A
- 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
10
Q
Define osmolality and osmolarity
A
- 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
11
Q
How to calculate osmolarity
A
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
12
Q
Osmolarity and molarity
A
- 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
13
Q
Isotonic and Iso-osmotic
A
- 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
14
Q
What keeps fluid in the circulation(1)
A
- 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
15
Q
What keeps fluid in the circulation(2)
A
- 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
17
Q
Mean pressures acting across the capillary membrane-Starlings Forces
A
18
Q
Water Homeostasis
A
- 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
19
Q
Regulation of ECF volume and composition
A
- 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)
20
Q
Sodium balance
A
- 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
