Perfusion Circuits - Midterm

Positioning of central arterial cannula

Aorta/ aortic arch/ ascending aorta

Positioning of Venous cannulas

right atrium

Two-stage venous cannula (Single Venous Cannula)

• Placed in the right atrium with two collection holes one at the right atrial junction and on near the IVC
  - Greater risk of small amounts of venous return to get past the right atrium 

Bicaval Cannulation

• two venous cannulas in the SVC and IVC
  - uses caval ties to snitch around cannula and prevent venous return from getting past right atrium 

What do we need to do once venous return is collected?

1. Infuse oxygen
   2. Remove CO2
   3. Return to the body via the arterial cannula

Function of aortic cross-clamp

• used to help create a motionless field 
  - allows us to give cardioplegia to only the heart via coronary arteries
  - blocks off the aorta/blood flow to rest of the body

Identify 1-7

1. Cross-Clamp

2. Arterial Cannula

3. Antegrade Cardioplegia

4. Aortic Root Vent

5. Retrograde Cardioplegia

6. LV Vent

7. Venous Cannula

What type of things do we need to consider for circuits?

• Hospital needs/standards
  - many different sizes for dif patients
  - safety, simplicity, surgeon requirments, patient needs, component requirments

Open circuits

- hard shell venous reservoir; most popular

Closed circuits

• bag venous reservoirs, 1000-3000 mL volume capacity (less than open/hard)

• collapses as blood empties; prevents air embolism 

• venous inlet and outlet ports → mesh bubble trap

• stopcocks to manually evacuate air

Size categories for perfusion circuits

Adults: &gt;30kg<br>Pediatric: 15-30kg<br>Infant: 4-15kg<br>Neonate: &lt;4kg

Membrane Recircultation Line&nbsp;

Functions as an oxygenator purge/shunt<br>- located at the highest point of the oxygenator because air rises<br>- can be closed during bypass to keep pressure/path of less resistance<br>- can be open during bypass to act as a shunt, continued air removal

Prime volume of 1/2in tubing&nbsp;

38.61 ml/ft

prime volume of 3/8in tubing

21.71 ml/ft

prime volume of 1/4im tubing

9.69 ml/ft

Why is it important to keep balance between the circuit and prime volume?

• all components need to be primed with crystalloid solution 
  - larger intial prime volume results in greater hemodilution of the patient decreasing hgb and hct 

How to find volume of a tube?

Volume=length*pi*r^2<br>- convert diameter of tube to decimal&nbsp;<br>- convert in to cm (2.54cm/in)<br>- find radium<br>- convert length of tube to cm (12in=30.48cm)<br>- use formula

Arterial line + sizing

Returns blood to the patient via the ascending aorta or femoral artery&nbsp;<br>- up to 10kg = 3/16in<br>- 10-30kg =1/4in<br>- &gt;30kg = 3/8in

Venous Line + sizing

desautrated blood exists patient vena cava or right atrium through the venous cannula&nbsp;<br>- gravity and assisted drainage<br>sizing<br>- &lt;4g = 3/16in<br>- 4-15kg = 1/4in<br>15-70kg= 3/8 in

What must be considered when designing tubing?

• minimize blood trauma

• priming volume

Important Tubing Characteristics

• types: PVC, silicone

• wall thickness: 1/16’’, 3/32’’

• Internal diameter ID

How to minimize blood trauma with tubing design

- has a smooth inner surface, non-thrombogenic<br>- non-toxic material&nbsp;<br>- non-wettable<br>- avoid high velocities and minimize pressure gradients&nbsp;

Why does keeping tubing short help?

- decrease priming volume<br>- reduce resistance to flow, establish pressure gradients<br>- decrease blood trauma

Design of connectors

• clear polycarbonate connectors

• smooth non-wettable surfaces

• smooth junctions, no turbulence

• luted/tapered connections, 2 barbs → tight fit

• smooth curve

• minimize turbulence and pressure gradients 

Tubing manufacturing materials

-PVC vs silicon

What is spallation?

The process of tubing breaking down and releasing small microparticles as tubing undergoes continous pressure/compressions&nbsp;

What is durometer and why is it important?

• Hardness of plastic used 

• Uses the Shore hardness scale → lower # = softer material

Desirable tubing characteristics&nbsp;

• transparency: see clot formation, air, diff in blood coloration

• resilience: be able to withstand/bounce back after compression

• flexibility and kink resistance: need to be able to manipulate when setting up the pump

  • prevents the disruption of blood flow

  • tubing durometer can change when heated/cooled

• low spallation rate

• hardness, resist collapse

• non-wettable and smooth inner surface

What two main materials are used in circuit components?

1. Medical grade polyvinyl chloride (PCV) used for tubing 
   2. Polycarbonate used for connectors, reserviours, oxygenators, ect

Use of surface modifications

Used to decrease systemic inflammatory responses that can be triggered when blood is exposed to air or other foreign particles
- can be used to prevent clot formation 
- many different types: with vs without heparin 
- can mimic vascular endothelium (disguise foreign matter)

Ways to reduce Complement Activation

• Reduce component size → reduce amount of foreign material

• Reduce air interface/exposure

• Surface modifications

Venous Reservior

collects venous return&nbsp;<br>- can be closed or open&nbsp;<br>- removes air and contains filtration

Cardiotomy Reservior

collects suction blood&nbsp;<br>- remove air, filtration

Advantages of a closed system

- reduced air-blood interface
- increased safety, bag collapses → prevents pumping air into patient

Disadvantages of closed system

• limited reservoir volume; have to estimate total volume

• requires separate cardiotomy reservoir (sucker, vent, quick prime, purge) 

• can't use vacuum assist

• difficult to manage venous air → manual purge

• no electric level alarms

advantages of open system

- naturally separates air 
- electronic level alarms 
- easy to quantify volume; holds more
- vacuum assist
- integrated cardiotomy reservoir

disadvantages of an open system

• increased risk of pumping massive air to patient if low/loss of reservoir level

• increased air and defoamer contact with blood → less biocompatible

Gravity drainage variables

Influenced By

1. Cannula shape/length, placement (based on patient BSA and surgeon preference)

2. Blood viscosity: influenced by Hct and temperature

3. Venous reservoir height compared to patient, inlet resistance

Assisted venous drainage

Any method employed to enhance the venous drainage to the heart-lung machine
- improves venous drainage 20-50%
- better emptying of the heart
- reduce prime volume: smaller tubing diameter, smaller cannulas 

At minimus must a CPB circuit include?

1. tubing
   2. oxygenator
   3. arterial filter
   4. reservior
   5. roller/centrifugal pump
   6. suckers
   7. vents
   8. quick prime line
   9. manifold
   10. cardioplegia delivery system 

What does the venous sampling port do?

Used to obtain and run venous blood gases

What is placed in the arterial outlet line?

• Arterial saturation probe/ in-line continuous blood gas monitoring probe that monitors oxygenator performance

Arterial line pressure transducer

monitor and control the flow of blood and oxygen through the system&nbsp;<br>- usually placed on the arterial line downstream from the arterial cannula

A-V loop

the arterial and venous line are typically connected&nbsp;<br>- allowd for recirculation of the prime and de-airing of the circuit during setup<br>- sterile

arterial-venous crossover line

• can be used for recirculation if air gets into the circuit
  - clamped off during bypass

sucker and vent lines

used for pump suction, aortic root venting and LV or PA venting<br>- sterile wraped

Manifold System

• 3 or 4 stopcock with tubing connected to the arterial and venous sampling ports 
  - contain one-way valve to prevent accidental injection into the arterial system; NOT venous line
  - used for ACT, blood gases and drug introduction

depth filters 

• no definite pore size; formed by packing materials into a space
  - filtration depends on the thickness and tightness of the packing 
  - emboli are traped by the filler material

screen filter

• woven mesh material with a defined pore size
  - pore size influences filtration

Combination filter

• uses both a mesh screen filter and depth filter material 

Arterial Blood filters

• placed in housing that are designed to act as bubble traps
  - direct the blood to move any air toward an exit port for removal 

Cardiotomy filters

• usually combination types
  - blood may contain suction debris
  - turbulant suction causes air bubbles --> contain defoaming agents 
  - can be unfiltered (larger pore size, 70-180 micron) or filtered (smaller, 20-40 micron)

Cardioplegia filter

used to filter the crystalloid portion of the cardioplegia solution

Gas filters 

• used to remove any particulate matter that may come from the gas source

Preload of CPB Pump

• the volume in the reservoir: venous return, reservoir level, and inlet pressure

Afterload of CPB Pump

• The resistance to flow into the patient downstream

  • Arterial line, cannula size

  • Oxygenator

  • Patient SVR

Roller Pump (volume pump)

• Uses positive displacement to push a fixed volume depending on tubing size and RPM

• Not sensitive to resistance → pump keeps delivering the same flow = pressure increase (afterload independent)

• Preload independent → rollers do not require pressure gradient so volume in reservoir does not matter; can cause pumping of air if volume is low

Centrifugal pump (pressure pump)

• Pressure is controlled by RPM → how fast it spins, Kinetic energy

• Flow is generated pressure difference between the pump, hydrostatic pressure, resistance, and patient components

  • Flow = pressure - resistance

• Preload dependent and Afterload Sensitive

  • flow is depended on volume(pressure) and resistance

Components of a roller pump

• curved horseshoe shaped race way that houses tubing

• rollers mounted on ends of rotating arms → 180 degrees apart so one is compressing tubing at all times, pushing blood

How is output of a roller pump determined

1. Revolutions per minute (RMP)

2. Length of tubing

3. Diameter of tuning

Describe Positive Displacement of Roller pump

• tubing is compressed between roller and raceway backing plate behind the blood pushing it forward as the arms rotate

• Moves a fixed amount of fluid with every turn no matter the resistance or volume on the other sides

Pulsatile Flow and Roller Pumps

• Pulsatile flow occurs with roller pumps → when one roller finishes its occlusive stage it stops compressing the tubing → the nest roller takes over → can change/dip in flow at the handoff creates a pulse

• Single roller pump = higher pulsatility as flow stops between rollers

• Double roller pump = non-pulsatile flow, when one roller occlusive stage ends the other begins, no dip

Centrifugal pumps are non-pulsatile

Identify the component of the roller pump

1. Raceway

2. roller

3. tubing inserts

4. tubing guides

5. occlusion set knobs

6. Magnet for roller cover (won’t start without cover)

Conclusiveness of roller pumps

• Compression of tubing to just barely non-occlusive

• over-occlusion induces hemolysis and tubing wear

• under-occlusion compromises forward flow, increase or decrease hemolysis

Malocclusion = either over or under

Roller Pump complications

• incorrect calibration/occlusion settings - set for wrong tubing size

• spallation

• rupture of tubing → pressure buildup

• pump massive air

Cavitation

negative pressure on inlet causing gas to come out of solution (roller pumps)

Fluid Drop method

• First line of defense, first to be done

• process

  • during priming add crystalloid to arterial line, the outlet side of roller pump

  • hold fluid ~30 cm above the pump

  • count time as fluid meniscus drops

  • fluid drop 1 cm/minute

Complete occlusion

roller setting which causes the column of liquid to remain at a constant level

Static Pressure Drop Method

• When pump is off, turn roller with hand ¼ turn

• clamp outlet of the roller pump to build pressure to about ~300-400 mmHg

• make sure all shunts are closed to build pressure

• Watch pressure drop 5 mmHg/sec

Dynamic pressure drop method

• less hemolytic compared to other 2 methods

• Arterial outlet clamped + all shunts closed

• Turn pump on at 5 RPM → flow should remain at ~150-200 mmHg consistently

  • over pressurize = over-occluded

  • loss pressure = under occluded

Centrifugal Pump

• Non-occlusive pump

• Flow is dependent on pressure change created by the spinning cones of the pump

• Forward flow is by kinetic energy (pressure difference between inlet and outlet)

• Sensitive to afterload

• Tubing will not increases in resistance

Centrifugal Components

1, Housing

2. Impeller with vanes or cones inside the housing

3. Magnet that spins the cones and sits on the drive console - sets RPM

Pressure difference of centrifugal pump (+describe what influences flow rate)

• pressure differential is created with the rotation of the blades causing blood flow

• negative pressure at the inlet and positive pressure at the outlet

Flow rate = pressure (preload + RPM) - resistance (patient SVR, arterial line and cannula sizing)

Additional equipment for centrifugal pumps

• Duckbill one-way valve off the centrifugal outlet → prevents retrograde flow

• E-clamp: sense retrograde flow and will clamp down to stop it

• Flowmeter: slipped on outside of tubing, monitor transit time between signal transmission

Centrifugal Pump Complications

• Non-occlusive pump (nothing is stopping/ preventing path of least resistance) → can cause retrograde flow can create a hemodynamic siphon that can exsanguinate the patient and draw air into the line

  • Why e-clamps and additional alarms are needed

• decoupling of the magnet if there is excessive rational speed

Roller pump advantages/disadvantages

Advantages

• Flow rate remains constant despite preload or afterload

• Occlusion prevents retrograde flow

• Flow is predictable/calculated from RMP and tubing size

Disadvantages

• Pressure can climb causing tubing to fail

• hemolysis and spallation

• cavitation, air

• proper occlusion settings necessary

Centrifugal Pump: Advantages/Disadvantages

Advantages

• Non-occlusive, pressure won’t build to high

• less likely to push air (preload dependent)

• less blood trauma

• no occlusion settings

Disadvantages

• preload dependent and afterload sensitive → flow fluctuates with volume and resistance

• needs flow probe → RPM doesn’t equal flow

• Magnets can decouple

Advantages and Disadvantages of Hemodilution

Advantages

• Increased tissue perfusion by lowering viscosity - decrease resistance

• Lower line pressure, ease of fluid movement

Disadvantages

• dilution of serum proteins, oxygen carrying capacity of blood and clotting factors

• increased need for homologous blood products

Body Water Distribution

• Total body water (TBW) is about 60% of body weight in the average adult.

• About 2/3 of TBW is intracellular. About 1/3 is extracellular.

• Extracellular water splits into 1/3 interstitial and 2/3 intravascular (plasma)

Blood volume per kg

0-10 kg = 85 mL/kg

11-25 kg = 80 mL/kg

26-45 kg = 70 mL/kg

>45 kg = 65 mL/kg

Body water content and age

• Infants have the highest water content per kg of body weight. Adults have less, and the elderly have less still.

  • they is why blood volume per kg decreased with increased weight

Tonicity and Fluid movement

• tonicity decides which water moves → into or out of tissues/cells

• Isotonic = prevents fluid form shifting between interstitium and the vessel

• Hypotonic = fluid moves into vessel

• Hypertonic = fluid moves out of vessel

Factors when choosing a prime solution

• Isotonic solution → preserves the interstitial/intravascular balance

• Electrolytes should be as close to blood as possible