Basic Concepts Of CPB

Question 1

Why perfusion?

Answer 1

We want a still and bloodless field

Question 2

2 ( dual) stage cannula

Answer 2

Drains right atrium and IVC

used for AVR, CABG, Aortic Surgeries

Question 3

Two single stage cannula

Answer 3

MVRs, tricuspid valves, ASD

Question 4

Henry’s Law

Answer 4

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

Question 5

Single stage cannula

Answer 5

SVC & IVC - bicaval

Question 6

Can a femoral cannula be used in other arteries?

Answer 6

Yes

Question 7

Reynolds number

Answer 7

(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.

Question 8

Reynolds number again…

Answer 8

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

Question 9

Cavitation

Answer 9

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

Question 10

Roller head principles

Answer 10

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

Question 11

Centrifugal (nonocclusive)

Answer 11

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

Question 12

Pulsation vs laminar flow

Answer 12

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

Question 13

Venturi effect

Answer 13

• 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

Question 14

Tubing sizes

Answer 14

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 -

Question 15

Oxygenator Concepts

Answer 15

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

Question 16

Oxygenator Membrane

Answer 16

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

Question 17

Driving Force of gases across Oxygenator membrane

Answer 17

CO2 = 42 mmHg
O2 = 720 mmHg

Question 18

Fick’s Law equation (Diffusivity)
Explain what it means

Answer 18

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

Question 19

Ficks law

Answer 19

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.

Question 20

Countercurrent vs. Cocurrent

Answer 20

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

Question 21

Resovoir Concepts

Answer 21

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.

Question 22

Spallation

Answer 22

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

Question 23

Seldinger technique

Answer 23

anything you put over a wire

• stabilizes the outer end of the wire

Question 24

How to de-air

Answer 24

put patient in the trendelous position ( head toward the floor

Question 25

Haldane Effect

Answer 25

Co2 coming off hemoglobin

Question 26

Reaction time calculation

Answer 26

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.

Question 27

haldane effect

Answer 27

is the ability of deoxygenated hemoglobin (a protein composed of an amino group) to carry more carbon dioxide (CO2) than in the oxygenated state.