Bypass Techniques Blood Pumps Flashcards
What are the ideal characteristics of a blood pump?
- Able to move large volumes of blood (up to 7L/min) against significant pressures.
- Pump should be gentle on blood(to avoid lysing)
- Smooth surfaces
- Free from dead spaces
- Avoiding stagnation(not flowing blood)
- Disposable, no contamination risk
- Pump calibration should be easy, reliable, reproducible
- **The pump should be automatically controlled for routine use but have manual backup capability (hand crank)
Inlet has what pressure?
Negative pressure
Outlet has what pressure?
Positive pressure
What are two types of blood pumps?
Positive displacement pump
- Roller pumps
- Energy transfer, fluid being sucked into a closed volume and then the fluid is pushed out.
Rotary pump
- Centrifugal pump
- Energy transfer to the fluid is due to velocity changes within impeller vanes.
Characteristics of roller pumps
*Tubing length in raceway (analogous to stroke volume) equivalent or related).
*Tubing “U” shaped
*No pulsatility (rhythmic or beating throbbing)
*Rollers mounted on ends of rotating arms (one roller is compressing at all times)
Fluid pushed ahead “positive displacement”
*When the rollers completely occlude the tubing, the pump generates both positive (right side of pump) and negative (left side of the pump) pressure.
*Can be used for suction
*210° semicircular backing plate
*Two rollers with rotating arms 180° apart
*One roller ends in occlusive phase, the other has already begun its occlusive phase
-Generates non-pulsatile flow (a mean pressure, not a systolic pressure)
-Produces hemolysis(rbc rupturing)
What is the pump output of a roller pump?
(CO)=HRxSV Flow=RPMxStroke Vol. Pump output (PO)= RPM x SV SV=PO/RPM (both are same equation)
TD SV Blood Q 3/16" 5.0 0.750 1/4" 9.65 1.448 3/8" 21.71 3.257 1/2" 38.61 5.792
Very stable, known output.
TD-Tubing diameter
SV-Stroke volume
Blood Q (l/min) @ 150 rpm
Qb= π * r2 * l *rpm
r=internal diameter of tubing (cm) l=length of contact of the roller with the tube rpm=revolutions per minute Qb=blood flow Very stable, known output
Characteristics of tubing
- Transparent
- Resilient
- Flexible
- Kink resistant
- Blood compatible (surface coating)
- Can be sterilized. (By ethylene oxide gas, can’t sterilize multiple times b/c increase residue of EO gas.
- Ionized radiation (how factory sterilizes)
Venous tubing characteristics
1/4”=0.9 lpm (only used on peds)
3/8”=4.0 lpm
1/2”=7.0 lpm
Factors affecting stroke volume
- Occlusion-(If under occluded SV goes down)
- Tubing size (internal diameter)-Major determinant of maximum flow
- Durometer (hardness)-Too hard and it will not allow compression against raceway.
- Temperature-(Cold), blood is cold and will affect tubing compliance
- RPM’s
Between patient and the venous reservoir there is a minimum __________pressure drop (we want venous tubing as big as possible)
10 mmHg pressure drop
What is flow constant?
- Amount of volume expressed in one revolution
* Will change based on diameter of tubing
This type of tubing is most popular. Why?
PVC.
*Very durable, acceptable hemolysis. Can be coated (bio compatibility)
What is occlusiveness?
- The degree to which the roller collapes in the tubing in the raceway (we want to see a miniscus)
- Counterclockwise-Tightens, clockwise loosens
- Occlusion fluid drop=1 inch per minute @ 36-39”
How to check occlusion?
*Stop the pump and watch to see how much the fluid drops. This will give us an idea of how occlusive the pump is. Shouldn’t be more than 1” per min. Only for arterial pump, every other pump is occlusive.
Which type of occlusive setting do we prefer?
Barely non-occlusive
Problem with being under-occlusive?
- Thought to be the most hemolytic (b/c red blood cells slip back and forth in raceway)
- Too little compression results in forward flow compromise
Problem being over-occluded?
Hemolysis and increased tubing wear
Two types of occlusion settings
- Static method-Pump is stopped and the outlet tube which is filled with fluid is kept 30in above the pum
- Occlusion is varied until fluid level falls at rate of approx. 1 cm/min
- Pressure method-Dynamic.
- Clamp outlet of the pump
- Turn pump slowly to pressurize the system
- Stop the pump and observe the rate at which the pressure drops
- Rollers are set to non-occlusive
- Pump is started at 5 rpm/min and the roller occlusion is adjusted to obtain a desired back pressure and resistance.
(When pressure starts to generate you are occlusive enough and stop there and observe fluid drop. Use flow probe from centrifugal pump)
Complications with a roller pump
- Tubing fracture
- Spallation-(why we use filters)-mechanical stress from repeated compression of the rollers on the tubing will cause tubing fatigue and micro particles can be released into the blood
- Runway pump (when you cannot turn off the pump)
- Capable of pumping gross air (centrifugal cannot)
- Over pressurizing the circuit-Results in tube fracture
- Cavitation-Pulling gas out of solution-microembolism.
Rotary pumps
- Axial-lowest priming volume
- expensive
- Not used for cpb
- Diagonal
- Medos pump (europe only)
*Radial (centrifugal)
-Least power required for flow
Impeller outflow is parallel to axis of rotation (fluid moves in direction of rotation)
Characteristics of centrifugal pumps
- Impeller arranged with vanes or smooth plastic cones inside plastic housing (“white fan like part”)
- Impeller magnet couples with an electric motor magnet
- Coupling ensures motor magnet rotation equals pump magnet rotation (which will be the forward movement of fluid)
- Rotation creates “constrained vortex”
- Pressure differential (negative-inflow, positive-outflow)
- Low pressure in the center-(not as hemolytic cause you don’t have occlusion)
- Area of high pressure along the sides (this will be outflow)
- Blood flow rate depends on:
Centrifugal pumps are ______ and ______ sensitive
Preload and afterload. Flow will change based on neg. pressure (preload) and positive pressure (after load)
Blood flow rate of centrifugal pump depends on________
Pressure gradient and the resistance at the outlet of the pump. Blood flow (outlet pressure(+) - Inlet pressure (-)/resistance.
Outlet resistance function of the two components:
- CPB circuit (oxygenator, filter, tubing, arterial cannula)
* Systematic vascular resistance (SVR)
What is preload?
*Volume of blood in ventricles at end of diastole(end diastolic pressure)
What is afterload?
*Resistance left ventricle must overcome to circulate blood
What is SVR?
Systemic vascular resistance.
Centrifugal pump cannot _______ gross air
*Pump.
Cent. pumps are safer than roller pumps
What is preload increased in?
- Heart failure
- Regurgitation of cardiac valves
- Hypervolemia
What is afterload increased in?
*Hypertension
*Vasoconstriction
Increased afterload =increased cardiac workload
Preload is ______ sensitive
volume
Afterload is _______ sensitive
pressure sensitive
Centrifugal pump is totally _________
Non occlusive
Prime volume of a centrifugal pump is _____ more/less than roller pump. Why?
30/60ml, less,
Because of less tubing
Why must a flow probe be used in a centrifugal pump?
To quantify arterial flow
What is the normal O2 consumption?
125ml o2 per m2
Advantages of roller pumps
- Simplistic design
- predicable pump speed
- capable of pulsatile flow
Advantages of centrifugal pumps
- afterload sensitive
* less traumatic to blood elements
Disadvantages of roller pumps
- blood damage
- spallation of tubing (can embolize parts of tubing)
- Afterload insensitive (can keep going and blow up)
Disadvantages of centrifugal pumps
- Output not indicated by pump speed
- Not capable of pulsatile flow
- Additional charges
What type of pump is preload and after load dependent?
Centrifugal pumps, an increase in downstream resistance decreases forward flow delivered to the patient.
