Pharmacology of Fluid Therapy Agents Flashcards
Why is fluid therapy important?
- ensure adeq circulation
- considered a txt (prescribe as medication)
It is essential to ensure adequate
circulating volume
Why is fluid therapy essential
Optimizes & maintains CO thru adeq circulation, tissue perfusion, electrolyte concentration, acid-base balance
In fluid therapy, you want to avoid…
tissue oxygen debt & reduce organ failure
in fluid therapy, overinfusion can…
increase the risk of complications
Why is it essential to have good understanding of body fluid physiology for a patient receiving fluid therapy?
To ensure each animal receives
1. the correct fluid type
2. correct amount of fluid
3. at the correct rate
4. via an appropriate route
Overperfusion can cause…
- Peripheral oedema
- Pulmonary oedema
- can overperfuse the blood possible
What are the goals of fluid therapy?
- in intensive care: optimize CO
- in anaesthesia: circulatory adequacy
Total body water =
60%
ECF makes up
20% Total body wt
Interstitial fluid makes up
15% of TBW
Intravascular fluid makes up
5% of TBW
intracellular fluid makes up
40% of TBW
What is dehydration?
- loss of water from interstitum
- due to intercompartmental shifts, ICF decreases too
- severe dehydration can lead to hypovolaemia
- must give prolonged fluids over many hours
What is hypovolaemia?
- blood loss &/or water + solute loss (intra or pre-op)
- boluses given to correct
- absolute: vol loss from intravasc space only
- relative: inappropriate fluid redistribution across compartments, pathological vasodilatory space
Osmosis
is the process where solvent molecules (most usually water) move through a semipermeable membrane from a diluted solution into a more concentrated solution (which becomes more diluted).
Mvmt of water from lots of water to little water to dilute high conc solutes
Osmotic pressure
The external pressure required so there is no net mvmt of solvent across a membrane
osmolality
osmoles/kg
osmolarity
osmoles/L
osmolality/osmolarity
number of osmotically active particles generated when a compound dissociates in 1 L of water
Normal serum osmolality in dogs
300 mOsm/kg
normal osmolality in cats
310 mOsm/kg
Oncotic pressure
- Oncotic pressure = pressure exerted by larger molecules that sucks fluids into the vessel to maintain vascular volume
- vascular endothelium freely permeable to water & electrolytes but selectively for larger molecules which exert osmotic pressure (Colloid pressure/oncotic pressure)
- Sucks fluids into the vessel
Hydrostatic Pressure
- increased pressures force fluid OUT of a space
- independent of osmotic/oncotic pressure
- arterial end of capillaries higher than ISF, fluid forced out
- venous end of capillaries lower than ISF so oncotic & osmotic pressures favour mvmt of fluids back in vessels
- push out of vessel
Filtration is where the fluid
exits capillary b/c capillary hydrostatic pressure > than blood colloidal osmotic pressure
Reabsorption is where…
At the venous end of a capillary, fluid re-enters capillary since capillary hydrostatic pressure is < blood colloidal osmotic pressure
No net movement of fluid means…
capillary hydrostatic pressure = blood colloidal osmotic pressure
What is the endothelial glycocalyx layer (EGL) theory?
- re-eval of Starling forces
- web of membrane-bound glycoproteins & proteoglycans on luminal side of endothelial cells that regulates fluid mvmt
- controls vol of layer, fluid mvmt & avoidance of cell adhesion/clot formation
- EGL compromised in systemic inflammatory states, hyperglycaemia, Sx, Sepsis
tonicity is the equivalence of
effective osmolality
Tonicity is the ability of an extracellular solution to make water
move into or out of a cell by osmosis
tonicity is influenced only by…
solutes that cannot cross the membrane as these only exert effective osmotic pressure
Solutes able to freely cross the membrane do not
affect tonicity b/c they will always equilibriate w/ equal conc on both sides of the membrane w/o net solvent mvmt
Hypertonic
effective osmolarity > normal
fluid moves out of cell
Fluid moves INTO intravascular space (blood stream)
Hypotonic
effective osmolarity < normal
fluid fills cell & may burst
fluid moves OUT of intravascular space (into interstitial space)
Isotonic
shifts are similar each way
no net fluid mvmt
how do we describe fluids?
- content: crystalloid vs colloid
- tonicity: isotonic vs hypertonic vs hypotonic
- Fxn: resuscitation, replacement, maintenance; alkalinizing or acidifying
Crystalloid
aqueous sln of mineral salts or water-soluble molecules
Colloid
sln of large solutes, either naturally occurring or synthetic
If resuscitation is needed to re-establish haemodynamic stability through restoring intravascular volume, what fluids could be used?
- balanced crystalloid: Hartmann’s sln, Ringer’s acetate, Plasma-lyte 148
- Crystalloid: 0.9% NaCl
If replacement fluid therapy is required to provide daily maintenance requirements & replacement of any ongoing abnormal losses, what fluids should be used?
- Balanced crystalloid: Harmann’s sln, Ringer’s acetate, Plasma-Lyte 148
- Crystalloid: 0.9% NaCl
If fluid therapy is necessary for providing daily maintenance requirements, what fluids could be used?
- 0.18% NaCl/4% dextrose
- 0.45% NaCl
- 5% dextrose
Replacement fluids are used to…
replace lost body water & electrolytes
Replacement fluids may be used for…
short-term maintenance
What are some isotonic fluids
CSL (Hartmann’s sln)
Lactated Ringer’s
NaCl 0.9%
maintenance fluids provide
water in far excess than electrolytes
maintenance fluids replaces losses through
sensible & insensible losses
Maintenance fluids should not be…
bolused, or used until patient is stabilised
Advantages of maintenance fluids
- Commonly available incl large bags for horses/cattle
- Economical
- Replace interstitial deficits
- Often restore effective circulating vol
- Good replacement solns for V, D, diuresis losses
Disadvantages of maintenance fluids
- Rapidly redistributed in the interstitium –> About 20% retained intravascular w/I about 3 hrs or so, Large vol req.’d
- Interstitial oedema –> Cerebral, pulmonary, systemic interstitial (kidneys cannot keep up)
- Haemodilution: RBCs, Albumin, Coag factors (esp if very tiny P, easy to overload)
Compound sodium lactate, Hartmann’s (Similar to LRS)
Uses, Treats, Contraindications
- used as replacement & alkaliniser fluid
- treats dehydration, hypovolaemia, metabolic acidosis
- contains Ca2+ –> unsuitable for use w/ blood products, cannot give in same line as blood transfusion
NaCl 0.9% (saline solution)
Clinical Uses, Type of Fluid, AE/contraindications
Type: isotonic crystalloid
Clinical uses: hyponatraemia (never change serum Na lvls faster than 0.5 mmol/kg/hr), hypochloraemia, hypercalcaemia, when you need to avoid administering K+, txt of metabolic acidosis, ECF replacement esp if pre-pyloric V, w/ no D
Contraindications
* unbalanced
* slightly higher Na conc than plasma
* higher Cl concentration –> admin can lead to hyperchloraemic metabolic acidosis, dilutes bicarb, no K+ or Ca2+ ions, key difference btw this & the plasma
- ECF acidifer –> produces metabolic acidosis
Ringer’s Solution (No lactate)
Fluid type, Anions/cations, Missing?, Use
- isotonic crystalloid
- contains higher Na & Cl conc
- no lactate (buffer)
- normally used as lab sln
Plasmalyte A & Normosol-R
Contains? Contraindications
- contains acetate as buffer (Normosol-R) or acetate + gluconate (Plasmalyte A)
- NOT RECOMMENDED FOR DKA
Dextrose solutions (w/ or w/o electrolytes)
- rapid glucose metabolism
- initially isotonic, become hypotonic once dextrose is metabolised
- used when fluid lost is pure water (heatstroke, hypernatraemia, hypodipsia)
- SHOULD NOT BE YOUR MAIN SOURCE OF FLUIDS
Hypertonic Saline (7.5% NaCl)
- Plasma vol expander
- used in txt of shock
- causes intracellular dehydration, follow w/ isotonic sln
- causes intravascular expansion
- immediate vol expansion seen
- improves microcirculation & microperfusion
- reduces ICP
Disadvantages of Hypertonic Saline (7.5% NaCl)
- short duration of action
- hyperchloraemic metabolic acidosis
- rapid administration
- vein irritation
- intravascular haemolysis
- ventricular arrhythmias
Colloids are solutions w/ large solutes that do not readily
cross vascular membrane, pull fluid from the interstitium
What are the main Synthetic colloids
HES
Dextrans
Gelatins
Natural colloids
Plasma
Whole blood
Oxyglobin
Albumins
Elimination of colloids..
small molecules excreted by kidneys
large molecules by liver
In colloids, the size of the molecules determines
duration of effect
in colloids, the number of molecules determines
the degree of osmotic effect
Advantages of colloids
- 60-80% retained in intravascular space
- in intravascular space for days
- excellend blood vol support, smaller resuscitation vol
- often used in combo w/ crystalloids
Indications of Colloids
- shock, hypotension, resuscitation
- protein loss
- hypoproteinaemia, oedema
Disadvantages of colloids
- do not replace interstitial fluids, just for vascular expansion, not completely balanced so keep crystalloid too
- poor replacement fluids for GI losses
- Haemodilution: RBCs, Coag factors
- rebleeding from damaged BVs –> can cause rebleeding
- coagulopathy
Gelatins
- polydispersible sln formed by hydrolysis of bovine gelatin
- Gelofusine –> succinylated gelatin in NaCl, improves water retention w/i circulation, short term 4-6 hrs, kidney excretion, can cause anaphylaxis
- Characteristics: variable sizes, 274 mOsm/L
- T1/2 8 hrs, vol expansion lasts 3 hrs, renal excretion
- AE: anaphylaxis, haemodilution w/ large vol, increased clotting times, vol overlaod
HES (Hydroxyethyl starches)
- Synthetic polymer derived from amylopectin
- up to 24 hr intravascular retention
- renal complications –> increases w/ increasing dose & risk of mortality
- coagulopathies
- vol overload
- pro-inflammatory efects
- restricted use: not in sepsis, burns, critically ill
Dextran solutions
- polysaccharide
- plasma expander
- hyperoncotic compared to plasma –> reduces blood viscosity, improves microcirculatory flow
- Anaphylaxis possible
- interferes w/ haemostasis
- can make ligatures slip
