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by Ben Best
![[Melody Maxim]](../images/Melody_Maxim.jpg)
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In May 2009 Melody Maxim, a professional perfusionist who worked for years doing cardiac bypass perfusion, came to the Cryonics Institute to provide a centrifugal perfusion pump system (along with training in its use) to replace the roller-pump perfusion system that CI had been using. If a reservoir is inadvertently emptied, the centrifugal pump shuts-down rather than pumps the patient with air. Small air bubbles can be caught by the filter, although de-bubbling of the lines is certainly done in every case. Other safety features of the centrifugal pump (as opposed to a roller pump) is that the centrifugal pump will not over-pressurize, and that the lines can be clamped at any time and place without blowing a connection.
Mrs. Maxim also helped CI implement a two-reservoir perfusion system to allow for safer and better controlled addition of perfusate — by switching lines. The training she provided was of value not only for the new centrifugal perfusion system, but was of general value in terms of good perfusion technique.
The fluid flows in tubing from the reservoir to the centrifugal pump inlet at the apex of the pump-head. Tubing on the outlet connector of the pump-head on the side of the cone runs to the inlet end of the filter, and then from the outlet of the filter to the patient. All these tubing/device connections are made with hose-barbs, which generally don't require cable-ties to be secured. Initially, both the inlet and outlet of the filter should be clamped, leaving the filter bypass line open.
The purge line tubing runs from the top of the filter into the reservoir. There is a one-way valve at the input end of the purge line. Keep the output end of the purge line above the level of the fluid in the reservoir in order to not make bubbles as the purge line empties into the reservoir. The pressure line tubing runs from the top of the filter to the pressure display box where the circular membrane is secured onto the box by turning the white holder. The end of the pressure line should point vertically above the white holder, such that opening the stopcock (valve) at the end of the line shoots liquid upward when there is pressure in the line. Leave the valve at the end of the pressure line closed initially. The level (elevation, height) of the pressure diaphragm and the level of the pressure outlet at the top of the filter should be the same as the level of the patient's heart.
The pump-head has a magnet that is attracted to a magnet
in the socket in which the pump-head is placed on the pump.
The spring-loaded white thumb-tab on top of the pump-head
socket secures the pump-head by a ball-tipped rod that fits
into notches on the rim of the pump-head.
| This diagram is not intended to be on scale, nor are the
components positionally accurate. It is merely an informal diagram of CI's current perfusion circuit. Please see the detailed write-up for further information. |
![[ Centrifugal Perfusion System Diagram ]](./PerfusionDiagram.jpg)
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| Front view | Back view |
![[ Front view ]](./PerfusionCart.jpg)
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![[ Back view ]](./Cart_back.jpg)
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Centrifugal pumps are not self-priming, which means hat they must be filled with liquid before they will operate. Similarly, if large amounts of air enter the pump-head during operation, the pumping will cease.
During priming, once the system is filled with fluid, the cannula tip should remain below the level of the fluid in the reservoir to avoid backflow, which would otherwise occur if the pump is turned off. But before there is fluid in the lines it is best to keep the cannula tip above the level of fluid in the reservoir to avoid creating bubbles as air being displaced by fluid is coming out of the cannula. Securing the cannula in the proper position can be difficult.
![[Lift base of "Y"]](./Lift_Y.jpg)
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It is best to slowly fill the tubing with fluid to minimize bubble formation. Clamp the tubing from the reservoir to allow the pump-head to be elevated for gravity-filling. The clamp is removed and the pump-head is slowly lowered until fluid enters the pump-head. The entire line can be filled by lifting the reservoir or else the line can be clamped where there is fluid distal to the pump-head outlet (and the clamp). The pump-head is placed in the pump socket positioned such that the base of the outlet connector is aligned with the white thumb-tab on the top of the pump socket. The pump is then turned on by pushing the "Master Power" button.
Never remove the pump-head without first shutting off the pump.
Before filling the filter, remove the clamp by the pump-head outlet, allowing the fluid to flow through the filter bypass line. Run the pump at 1,500 to 2,000 RPM to fill the line. Then turn the pump speed down to 1,000 RPM once the line is filled so that the filter can be filled slowly. Ensure that the stopcock at the top of the filter allows flow between the filter and the purge line. The pressure line should be closed to prevent flow through that line. Manipulating the "Y" joints on both sides of the filter during filling can eliminate air. The base of the "Y" is lifted so that the two bifurcated tips point downward. The clamp on the input to the filter can then be removed. Slowly open the clamp at the outlet at the bottom of the filter to allow the filter to fill retrograde (reverse of normal flow) from the bottom. Then remove the clamp on the filter inlet and place that clamp on the filter bypass. After the filter is full, put the cannula tip below the level of the fluid in the reservoir.
When the purge line running to the reservoir is filled,
open the stopcock at the top of the filter to allow flow in
all lines. Then open the stopcock at the end of the pressure
line to fill the line and the membrane with fluid. (Let fluid
squirt out the top — hold a paper towel). Then turn the
stopcock at the top of the filter to stop flow through
the pressure line. The stopcock at the end of the pressure
line should still be open so that the pressure line is open
to the air (no pressure in the pressure line). Zero the
pressure display box by switching the setting to "instantaneous"
and adjust the zero knob until the pressure reading is zero.
Then switch the setting to "mean". Close the stopcock at the
end of the pressure line and open the stopcock to all lines
at the top of the filter.
| Pressure line closed to system | Pressure line open to air |
![[ 1 o'clock to de-bubble ]](./Filter_top_2.jpg)
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![[ 5 o'clock to perfuse ]](./Pressure_membrane_2.jpg)
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Make sure to keep the cannula tip below the level of the fluid in the reservoir to prevent backflow of air into the system. Try to keep the cannula tip pointing to the back of the reservior (away from the spigot). Increase the pump RPM to 2,500. Tap the tubing beginning at the reservoir leading to the pump-head to displace small bubbles adhering to the tubing. Ensure all the lines at the top of the filter are open. Remove the filter and turn it upside down while tapping it with the heel of your hand (or softly with a rubber hammer — but never with hard objects) to remove all air. Repeatedly tap the filter in an upright and inverted position to de-bubble. Turn off the pump and remove the pump head, tapping the pump head until all of the air comes out the outlet. Then replace the pump head, with the outlet pointing upward as before. Start the pump, slowly increasing the RPM to 2,500. Continue to tap the all the lines, lifting the "Y" connections as needed. Open the valve at the end near the cannula to ensure that there is no air in that valve.
Alternate de-bubbling of the filter and the pump-head
until you are confident that they are de-bubbled. The
pump-head can then be rotated clockwise so that the
outlet points to 4 or 5 o'clock (but not rotated so
far that the line could kink against the chassis of
the pump). Drop the end of the purge line below the
liquid level in the reservoir to avoid splashing, but
do not put the end so close to the bottom that bubbles
in the purge line could be sent into the reservoir outlet.
| 1 o'clock to de-bubble | 5 o'clock to perfuse |
![[ 1 o'clock to de-bubble ]](./Pumphead_debubble_2.jpg)
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![[ 5 o'clock to perfuse ]](./Pumphead_perfusion.jpg)
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Further de-bubbling can continue right up until the beginning of perfusion. Just prior to cannulation, clamp the tube between the outlet of the pump-head and the filter (but not so close to the pump-head outlet hose-barb that the tubing or hose-barb could be damaged). Another clamp may be placed near the cannula(e) to the patient. Prior to cannulation, a slow flow is used to remove all air before the cannula(e) is (are) inserted. This slow flow may be achieved by gradual opening of the clamp near the outlet of the pump-head, or the clamp near the cannula.
The procedure to observe during cannulation is:
When perfusion begins, remove the "roll-up" clamp, open all lines at the top of the filter and increase RPMs as needed.
There is a rough correlation between RPMs and flow rate. In one test, 1,000 RPM corresponded to a flow rate of 2 liter/minute, whereas 2,000 RPM resulted in 4 liters/minute.
As with gravity-filling, slowly filling the lines results in fewer bubbles, and fewer small bubbles being formed. (Smaller bubbles cling more tenaciously to the tubing.) Shutting off the pump can be a way of seeing if bubbles rise to the top of the pump-head.
With a centrifugal pump it is always safe to clamp the lines with the pump running at less than 4,000 RPM, whereas with a roller pump it is never safe to clamp the lines with the pump running. The pump should always be running with at least 1,500 RPM before removing clamps to initiate flow. When ceasing flow, lower the RPMs to 1,500 RPM, place clamps before turning the pump off or lowering the RPMs to zero. Flow rates below 1,500 RPM without clamps in place can result in backflow or forward flow due to gravity (depending on the elevation of the reservoir). Fluid could be sucked out of a patient if the RPMs are too low and clamps are not in place.
It is generally good practice to turn the purge line off and to clamp the line before turning the pump off or turning the RPMs down below 1,500, but this is not necessary if the cannula is in the reservoir with the tip below the top of the liquid level.
If there is air in the line, it can be removed by first clamping distal to the filter and then clamping the bypass line. Air can be removed — lifting the Y connections as needed — and refilling the filter from the bottom, as needed.
The filter adds to the line pressure, depending upon the flow rate, which can be over 100 mmHg for high RPMs. Test to determine these values by observing the pressures for various RPMs while the cannula tip is below the level of fluid in the reservoir.
Avoid opening the drawers of controls/indicators in the front of the pump. In the top drawer the Motor Transfer Switch must be set to Internal (Int) not External (Ext). The pump must be turned off to toggle between the two. In the bottom drawer all circuit breakers must be on.
There is a hand-crank that can be used for the pump in case of a power failure. Close all lines at the top of the filter when mounting the hand-crank.
Drastic low to high RPM changes can decouple (with a horrible clatter) the pump head. When this happens, turn the flow off, clamp the line between the pump-head and the filter, and then slowly turn the RPM up (you may have to remove and then reinsert the pump-head).
Avoid leaving clamps on the tubing for extensive periods of time.
(can empty lines by lifting cannula, filter, then pump-head)
Initial set-up:
Priming steps:
![[Handcrank for manual power]](./HandCrank.jpg)
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(reservoir goes empty while pump running)
Note: Do not leave the cap off of the hand-crank pump-head socket. The magnet attracts metal objects which can damage the socket.