Exam 8 - Hemodilution & Heat Exchangers Flashcards
History of hemodilution
1 - high/normal flow with real blood (no dilution)
-70-80 mls/kg/min
2 - low flow / no dilution (azygous principle - 10% of normal)
-30-35 mls/kg/min
3 - Hypothermia and no dilution (most low flow…some high flow)
-ASDs and VSDs
- Early oxygenators required 3-5 L of prime….huge
John Gibbon
1st perfusionist
Homologous blood syndrome
- happens if you prime with blood
- decrease BP and VR
- unpredictable migration of plasma from 3rd space
- pooling in sphlanic circulation
- portal hypertension (sludging)
- metabolic acidosis
- coag and renal failure
- platelet aggregation
- decrease surfactant activity
- bleeding diastesis
Hemodilution in 1960’s - not just blood
- 5% dextrose: Isotonic...then hypotonic
Patients alert / no pulmonary congestion
- Balanced crystalloid solutions:
- serum electrolytes stable
- minimal acidosis
- decreased RBC damage
- minimal post-op pulmonary problems
- no renal problems
- fluid retention
- Balanced solutions with colloidIsotonic %
0.9%
Newtonian fluid
- uniform fluid, constant viscosity as force is applied
- crystalloid
Non-newtonian fluid
- varying viscosity as force is applied
- blood
Viscosity relationships
- Temp: inverse / 5% up with 1 degree drop
- Hct: direct / 50% decrease with 50% decrease in Hct
- Flow: inverse
Rheology
- study of deformation and flow of matter
- especially non-Newtonians
Shear stress
- force required to move liquid between two plates
- directly proportional to viscosity
Shear rate
- rate of flow during shear stress
- inverse to viscosity
Sickle cell
- more viscous then normal RBC
Poiseuille’s law
- Flow x Viscosity x SVR
- R = viscosity x SVR
So…… - Flow = dP / (viscosity)(SVR)
Changing viscosity
- as diameter decreases -> flow decreases -> shear rate down -> viscosity increases
- Produces overall increase in R to flow in capillary (sludging)
So on CPB…what if you run lower than normal flow
- decrease flow -> decrease shear rate -> increase viscosity -> increase SVR -> further decrease in flow….SLUDGING
- Even more sludging when you cool patient
Affects of hemodilution on CPB
- decrease perfusion pressure
- decrease viscosity
- change in body perception of pressure (can change flow better)
- dilution of catecholamines (helps drop pressure) - decrease sludging (counteract hypothermia)
- increase VR / flow through organs - decrease in post-op complications
- Cerberal / pulmonary / renal - decrease in O2 carrying capacity
- not good…but can make up with increased flow - decrease in colloid oncotic pressure
- edema - change drug interactions
- alter binding of proteins
Optimal Hct
- low 30’s
- lower on CPB…make up for with increase flow
- problem when Hct is 10% or lower
CPB and metabolic acidosis
Hemodilute -> more flow -> more O2 consumption -> less acid
- This is why CPB helps minimize metabolic acidosis
- If Hct increases…..chance of acidosis increases
Benefits of Hemodilution
- Decrease exposure to blood products
- Decrease viscosity
- Improved regional blood flow
- Improved O2 delivery
- Improved blood flow at lower perfusion pressures
How we hemodilute on CPB
- Patients Hct and weight (estimated blood volume)
- Infants (8-8.5) / Children and Male (7.5) / Female (7) - Circuit prime
- Pre-bypass dilution (anesthesia and cath lab)
- Pre-bypass removal (anesthesia and RAP/VAP)
- maybe Hct too high? Bad….too viscous
How much hemodilution is too much on CPB?
- not yet determined
- determining factors: O2 delivery and flow distribution
- <15% = maldistribution of coronary flow
- > 34% = risk of MI
- So… Acceptable = 16-27%???
- OPTIMAL = 23-27% …but maybe lower (giving blood is bad)
CPB and fluid balance
- edema may contribute to post-op organ dysfunction
- adults gain 1-15 pounds
Factors affecting fluid shifts
- Temp
- Pump flow rate
- Urine output
- Venous drainage
- COP (colloid oncotic pressure)
- Interstitial fluid pressure
- Hemodilution
- TIME ON BYPASS (25% contributer - biggest)
- more complicated surgery… more time on bypass
- Cardiac disease
Osmotically active prime components and fluid balance
- Albumin: less weight after CPB
- Mannitol: less weight after CPB
- Blood: MORE weight after CPB
Hemodilution reversal
Diuresis: - Mannitol - osmotic diuretic - if working kidneys - Lasix - loop diuretic - Edecrin - loop diuretic - Chlorothiazide - thiazide diuretic - Bumex - loop diuretic Ultrafiltration: - hemoconcentrate Hemodialysis: - machine / same as ultrafiltration / more complicated
Why use hypothermia
- decrease metabolic rate
- protection:
- cerebral / myocardial
How to control temperature of circuit
- Heat exchangers
- integral and external
- Room air
- Heat/Cool blanket
- Bear-hugger
How heat exchangers work
- two phases: blood and water
- warming: heat from water to blood
- cooling: heat from blood to water
Solubility and temperature
- Inverse…increase temp -> decrease solubility
- Gas more soluble in cold
- Don’t warm too fast….bubbles come out of solution
Heat exchanger material
- Biologically inert: reduce risk of clotting / coated
- Aluminum: high conductivity / poor biocomp. / Al microemboli
- Stainless steel: low conductivity / stronger / most used
- Tried in past: plastic / polypropylene / polyurethane
- much lower K (conductivity)
Heat exchanger design
- maximize heart exchange
- countercurrent flow
- increase surface area (good for exchange / bad for prime)
Constraints to heat transfer
- temp diff determines rate of exchange
- thermal boundary
- flow faster in middle of blood….warmer on outer part of flow
- warmer on outer due to slower velocity
- changing temp changes solubility
- blood/H2O gradient: 6-10 is acceptable (most use 6)
Convection
- heat transfer via motion of gas or liquid
Conduction
- heat transfer via touching of molecule to molecule
Heat capacity
- heat required to raise 1 degree
Specific heat
- heat required to raise 1 gram by 1 degree
Change in heat equation
Q = mcdT
Q = calories M = grams T = Celsius
Density of blood
1.045 gm/cm^3
Specific heat of Blood
0.9 cal/gm*C
Heat flow equation
Q/T = (c)(BF)(density)(dT)
c = cal/gm*C BF = cm^3/min or ml/min
Factors influencing rate of heat transfer
- H-E material
- thickness of conductor
- k
- heat loss
- prime volume
- blood path thickness
- blood path resistance
- water flow variation (little bit of turbulence good)
- H-E design
Coefficient of Heat Exchange
Che = (Tbo-Tbi) / (Twi-Tbi)
How to increase heat flow
- countercurrent exchange
- chevrons (turbulent flow)
- minimize thickness without integrity
- increase time in path
