Basic Concepts Of CPB Flashcards
Why perfusion?
We want a still and bloodless field
2 ( dual) stage cannula
Drains right atrium and IVC
used for AVR, CABG, Aortic Surgeries
Two single stage cannula
MVRs, tricuspid valves, ASD
Henry’s Law
P = KhC
Amount of gas that can dissolve in a volume of liquid is directly proportional to the partial pressure of the gas in that liquid
P = partial pressure of a gas, c= concentration of the gas, Kh = constant for a particular gas in a particular solution
Single stage cannula
SVC & IVC - bicaval
Can a femoral cannula be used in other arteries?
Yes
Reynolds number
(Average velocity x Diameter x Density) / Viscosity
Flow is laminar or streamlined until it reaches a critical value. Upon reaching this value it becomes turbulent with vortices. This value is a Reynolds number of 2000.
Reynolds number again…
Reynolds number is the ratio of inertial forces (vsp) to viscous forces (u/L)
It is used to identify different flow regimes - laminar or turbulent flow.
Reynolds number = inertial forces/viscous forces = (density x velocity x density) / viscosity
Laminar flow occurs at low reynolds numbers, where viscous forces are dominant, and it is smooth, constant fluid
While turbulent flow, occurs at high reynolds numbers and is dominated by INERTIAL forces - producing random Eddie’s, vortices and other flow fluctuations
Cavitation
Sudden occlusion of the inflow tubing causing the formation and collapse of gas bubbles due to low pressures by a precipitous change in mechanical forces
Roller pump ( occlusive) - directly generate flow
Pressure differential causes air to come out of solution
When a you open a soda and suds miraculously appear
Happens when pressure goes from high to low
Roller head principles
Output is determined by stroke volume each revolution
Output = RPM x volume per revolution
Ex: Larger tubing requires lower RPM to achieve same output, smaller tubing requires higher RPM - to achieve same output
Under occluded causes retrograde flow
Over occluded cause hemolysis, spallation and WBC/player activation
Centrifugal (nonocclusive)
Creates negative energy to suck blood into pump inlet, creating a vortex
Imparts KINETIC energy on blood and creates blood pressure propelling blood forward
Prone to heat generation and clot formation on rotating surfaces
Clamping tubing distal to pump causes pump to RECIRCULATE within the pump
Everything beyond the centrifugal pump is RESISTANCE
CAN CAUSE HEMOLYSIS DUE TO HEAT
Pulsation vs laminar flow
Pulsatile
- mimics cardiac cycle and physiological circulation
- increased flow through capillary networks
Increased sheer stress = increased hemolysis
Better urine output with pulsatile flow
Laminar flow
- detrimental effect of cell metabolism and organ function
Cavitation - sudden occlusion of the inflow tubing
Capillary beds don’t open and close
Venturi effect
- change of pressure and fluid flow through a narrowing in a tube
- causes damage to cells and entertainment in air (ex sucker tip)
Faster moving fluid mean LOWER pressure
V1/t1 = v2/ t2
This applies in oxygenation
Every cannula and connector has Venturi effect
V2 —> stagnation pressure
Smaller tube (3/8) —> resistance is high, flow is faster, pressure is lower
Bigger tube (1/2) —> resistance low, speed slow, pressure is higher
Tubing sizes
Venous 1/2 - larger tubing is required to gravity drain blood from the patient
Arterial 3/8 - arterial pump line, majority of the arterial tubing in the extracorporeal circuit
Cardioplegia 3/16
Vents 1/4 - suction tubing
3/4 -
Oxygenator Concepts
Volume of gas diffused = diffusion
Volume of gas diffused =
diffusion coefficient x partial pressure difference _____________________________ Distance to travel
- Provides an interface of HIGH surface area between blood on one side and gas on the other (oxygen and medical air)
–> Facilitate oxygenation and removal of carbon dioxide
Oxygenator Membrane
porous but proteins in blood coat surface preventing direct blood gas contact
- Surface tension in blood prevents plasma water from entering the gas phase of micropores during CPB and prevents gas leakage in blood phase
- most oxygenators drop efficiency after 6 hours
–> evaporation and condensation of serum leaking through pores
Driving Force of gases across Oxygenator membrane
CO2 = 42 mmHg
O2 = 720 mmHg
Fick’s Law equation (Diffusivity)
Explain what it means
J= -D(dφ/dx) ( don’t need to know)
- J = amount of substance moved per unit area, per unit time
D = diffusion coefficient
x= length
φ = substance concentration
need to understand
high concentration on one side and lower concentration on the other with a thin barrier and more surface area - it will diffuse better
- Movement of CO2 & O2 across a membrane will be in the direction of HIGHER to LOWER concentration (Partial pressure) with a magnitude proportion to the gradient and proportional to the area involved
- thinner barrier allows more diffusion than a thicker barrier
- a barrier of equal thickness with more surface area will allow more gas to diffuse during the same time interval
Ficks law
Natural process of gas exchange in the lungs involves directing the blood into small capillaries adjacent to the thin walled alveolus containing inhaled air so that oxygen may diffuse in and carbon dioxide will diffuse out.
Capillary size allows cells to move through one at a time to provide maximal exposure for gas exchange
Mass surface area of lungs provides for maximum gas exchange.
Countercurrent vs. Cocurrent
HEAT EXCHANGER
Increased movement along the entire system with counter-current, concurrent has a max 50:50 equilibrium
Cocurrent –> going in same direction, 50% transfer of heat exchange
Countercurrent –> near 100% transfer of heat exchange
the higher the flow rate the more heat exchange you have
Resovoir Concepts
Reaction time calculation
RT = (volume x 60)/Q
- NEED TO KNOW THIS EQUATION
RT = the time (in seconds) it will take to empty the reservoir [or the time (in seconds) it will take for you to “work out your problems”.
VOLUME = volume of perfusate (in mL) in the reservoir that just lost its inflow source
Q = flow rate (in mL/minute) of pump attached to the outlet of said reservoir
(1 L in the venous reservoir and coasting at 5 LPM on the master pump(arterial), the RT is 12 seconds; RT = (1000 x 60)/5000.
Spallation
release of plastic microparticles from the inner wall of the tubing
- caused by warming, cooling and pump compressions
PVC: biggest con is when the tubing gets cold it stiffens up
Roller head boot usually silicon
- it reduces hemolysis BUT shows increased spallation
Seldinger technique
anything you put over a wire
- stabilizes the outer end of the wire
How to de-air
put patient in the trendelous position ( head toward the floor
Haldane Effect
Co2 coming off hemoglobin
Reaction time calculation
RT = (Volume x60)/Q
Q = Flow rate
V= volume
t = seconds
EX: (1 L in the venous reservoir and coasting at 5 LPM on the master pump(arterial), the RT is 12 seconds; RT = (1000 x 60)/5000.
haldane effect
is the ability of deoxygenated hemoglobin (a protein composed of an amino group) to carry more carbon dioxide (CO2) than in the oxygenated state.
