1. OXYGENATORS
Heart-Lung Machine
• Machine for cardiopulmonary bypass
– For open cardiac surgery
– For supporting cardiac function, pulmonary function, or
cardiopulmonary function
• In the past
– One unit
• Recently
– Separate units
• Pump system (Heart)
• Oxygenator (Lung)
History
• First open cardiotomy (Apr 5, 1951)
– Temporary mechanical takeover of both heart and lung function
– Not survive due to unexpected complex congenital defect
– 4-yr experimentation of dogs followed
• First successful OHS (Sep 2, 1952)
– Dr. F John Lewis
– ASD closure using general hypothermia and inflow occlusion
• First successful OHS using CPB (by John Gibbon May 6, 1953)
– ASD closure
– High mortality rate
• VSD closure by azygos flow concept (controlled cross-
circulation) (Dr Walton Lillehei Mar 26, 1954)
• DeWall-Lillehei helix bubble oxygenator (May 1955)
– Beginning in a large series of patients
– Method of choice worldwide for OHS
• Rotating Disk oxygenator
– Developed by Drs Fredrick Cross and Earl Kay
– Used for early OHS in USA
• Membrane oxygenator
– Developed in 1950s-1970s; but clinically not frequently used
– In the mid-1980s, microporous designs; frequently used.
• Hemodilution
– Major technologic advance in CPB
2. Cardiopulmonary Bypass
• Goals
1. Still, bloodless heart for cardiac surgery
2. Replacement of cardiac and pulmonary function
Functions of CPB
• Respiration
• Ventilation
• Oxygenation
• Circulation
• Venous drainage (by gravity, centrifugal pump, or negative
pressure)
• Arterial inflow
• Temperature regulation (hypothermia)
• Low blood flow -> decreased blood trauma
• Decreased body metabolism
Components of CPB
• Total CPB
• Partial CPB
• Integral Components of Extracorporeal Circuit
– Pumps
– Oxygenator
– Heat exchanger
– Arterial filter
– Cardioplegic delivery system
– Cannulae (aortic; arterial; vena caval)
– Suction and vent
Basic CPB circuit with oxygenator and
centrifugal pump
3. Typical CPB Circuit Pumps
• Two principle types
– Displacement pumps
• Roller pump
• Non occlusive roller pumps
– Rotatory pumps
• Radial (centrifugal) pumps
• Axial pumps (Archimedes’ screw)
• Diagonal pumps
Roller Pumps
• Most commonly used
• Volume Displacement
• Non pulsatile blood flow
• Used for
• Forward flow
• Cardioplegic delivery
• LV vent suction
Roller Pumps
• Flow determined
– Tubing diameter, roller RPM, length of tubing in contact with rollers
• Proper set occlusion for minimal hemolysis
• 100% occlusion in cardioplegia and vent pumps
– Full occlusion -> hemolysis
• Larger tubing and lesser rotations cause minimal hemolysis.
– High RPM and fully occlusive setting -> hemolysis
• Tubing spallation cause microemboli
• Easily pump air
• Resistance = resistance of tubing + oxygenator + heat exchanger
+ filter + aortic cannula + SVR
• Line pressure depends on SVR and pump flow rate
• Pressure limit = 150-350 mmHg ( >250 mmHg seldom accepted)
4. Nonocclusive Roller Pumps
• Flat compliant tubing placed over the rollers
• Positive pressure at the inlet to fill the tubing
• Unlikely microair emboli
• Require use of an in-line flowmeter
Radial (Centrifugal) Pumps
• Impeller spinning within a rigid housing
– Creates regions of lower and higher pressure
– Blood moved from inlet to outlet
• No spallation with rigid housing
• Very dependent on afterload
• Nonocclusive
– Permit back-bleeding
– Require occlusive device
• Reqiure use of in-line flow meter
Axial / Diagonal Pumps
• Axial pumps
– Low internal volume, high-velocity axial impeller
– Currently best suited for ventricular assist application
• Diagonal pumps
– Very similar to centrifugal pump in design and application
Differences of Rotatory Pumps
5. Alternate Classification of Pumps
a. Roller pumps
b. Impeller pumps (Impeller >)
c. Centrifugal pumps (Cone >)
Centrifugal pumps > Roller pumps
• Long-term CPB
• In high-risk angioplasty patients
• Ventricular assistance
• Neonatal ECMO
• Centrifugal pumps
– Biomedicus Biopump (Medtronic Inc)
– Sarns/3M centrifugal pump (Terumo)
– Levitronix CentriMag blood pump
• LVAD, RVAD, Bi VAD
• BiVAD + oxygenator in RVAD = ECMO
Pulsatile Perfusion
• Significant physiologic advantages
• Diastolic run-off
• Stimulation of the endothelium
• Problem
• Noncompliant high resistance CPB circuit
• High flow with resultant shear stress
• Hemolysis
• Possible with roller pump and diagonal pump, but not with
centrifugal pump
• Requires larger bore arterial cannulas
• Alternative method for generating pulsatile flow in high-risk
patients
• Use of IABP during CPB
• Additional cost and invasiveness
Oxygenator
• Limited reserve for gas transfer vs. natural lung
• Much smaller surface
• Limited by diffusion
• Types of oxygenator
• Disk oxygenator
• Bubble oxygenator
• Membrane oxygenator
• Maximum oxygen transfer
• Less than 25% that of normal lung
• Proportional to partial pressure difference and surface area,
inversely to diffusion distance
6. Disk or bubble oxygenator
• Direct contact oxygenators
• Bubbles in direct contact with blood
• Increasing cellular trauma
Bubble oxygenator
• Bubble oxygenator
• Larger bubbles improve removal of CO2
• Smaller bubbles are very efficient at oxygenation but poor in CO2
removal
• Larger the No. of bubbles, Greater the efficiency of the oxygenator
7. Deforming Chamber of Bubble Oxygenator
• Deforming the frothy blood
• Large surface area coated with silicone
– Increased surface tension of bubbles -> causing them to burst
Bubble Oxygenator
• Advantage
– Easy to assemble
– Relatively small priming volume
– Deforming the frothy blood
– Low cost
• Disadvantages
– Micro emboli
– Blood cell trauma
– Destruction of plasma protein
– Excessive removal of CO2
– Deforming capacity exhausted
Membrane Oxygenator
• Charateristics
– Gas exchange across a thin membrane
– No need in direct contact with blood and no need for
deformer; so more physiologic
– Minimal blood damage
• Two types
– Solid type (Silicone)
– Microporous type (polypropylene)
• 0.3-0.8-um pores
• Most popular design = hollow fibers (120-200 um)
8. Membrane Oxygenator
• Microporous / Hollow fibers
Microporous (Polypropylene) Membrane
Oxygenator
• Currently predominant design used for CPB
• Micropores
– Less than 1.0 um in diameter
– Initially porous, but plasma protein coating the membrane-gas
interface
– Surface tension of blood prevent gas leakage into the blood
phase
– Conduit for O2 and CO2 exchange
• Problems
– Plasma leakage and membrane wet at use of period > 24
hours
Silicone Membrane Oxygenator
• True membrane oxygenator
• Silicone polymer
– Improved biocompatibility -> long-term support
– The 1980s-the mid-1990s
– Still the membrane of choice for long-term procedures
• ECLS/ECMO
• Problems
– Gas exchnage inferior to that of polypropylene (microporous)
oxygenator
• Need greater surface area and larger prime volume
– Difficult in manufacturing and quality control
New Generation Membrane Oxygenator
• Silicone polymer
• A continuous sheet of silicone membrane rolled into a
coil
– Manufactured by Medtronic Cardiopulmonary Inc.
– Membrane surface area + 0.6-4.5 M2
– Most common use for ECLS/ECMO
9. Heat Exchanger
• Intergrated into oxygenator for warming and cooling of the blood
stream
• Exchange surface made of
– Stainless steel, aluminium, or polypropylene
• Counter-current mechanism
• Temperature difference between waterside and blood side
– Historic reports : maximum difference of 10 °C
– Recent recommendation : 6 °C and longer rewarming times
• To improve neurocognitive outcome
• Hyperthermic circulatory temperature
– Blood damage (protein denaturation
– Limit absolute maximum temperature (42 °C) in blood
Circuits
• Venous drainage by gravity into oxygenator
– Height difference between venae cavae and oxygenator > 20-30 cm
• Mechanical suction Not desirable
– Entrain air
– Suck the vena cava walls against the cannula orifices
• Arterial blood return to the systemic circulation under pressure
Size of cannula
Adult Children
• SVC (1/3 of total flow) 28 24
• IVC (2/3 of total flow) 36 28
• Example: 1.8 m2 patient
– Total flow 5.4 l/min
– SVC 1.8 l/min, IVC 3.6 l/min
– SVC > 30 Fr, IVC > 34 Fr : Single cannula > 38 Fr
– 36-51 Fr cannula required.
Arterial Return
• Ascending aorta just proximal to inniminate artery
• Femoral artery access in
• Dissecting aortic aneurysm (0.2-3%)
• Reoperation
• Emergency
• Problems of femoral cannulation (more than ascending aorta
cannulation)
• Sepsis
• Formation of false aneurysm
• Development of lymphatic fistula
• Arterial cannula
• The narrowest part of CPB circuit
• Should be as short as possible
• As large as the diameter of vessel permits
• < 100 mmHg in full CBP flow
10. Arterial Cannula
• Long or diffuse-tipped cannula
• Minimize risk of dislodgement of atheroma in the ascending or transverse
aorta
• Axillary –subclavian artery, innimonate artery, LV apex
• In special circumstances
• Limitations and more complications
• Dissection of aorta
• All sites of arterial cannulation
• Prompt recognition and surgical correction
• TEE helpful for diagnosis
Other circuits
• Tubing sizes and lengths and connectors
• Should minmize blood velocity and priming volume
• Search for better biomaterials
• Cardiotomy suction
• Major source of microemboli and activated blood (humeral and cellular)
• Minimize amount, substition by cell salvage
• Cell processed blood may pose hazards
• Hemoconcentrator
• During and after CPB
• Removal of plasma and raising of Hct
• More cost effective than cell salvage and washing devices
Prime Fluid
• Ideally close to ECF
• Whole blood not used
• Homologous blood syndrome
• Postperfusion bleeding diathesis
• Incompatibility reaction
• Demand on blood banks
• Advantages of hemodilution
• Lower blood viscosity
• Improve microcirculation
• Counteract the increased viscosity by hypothermia
• Risk of hemodilution
• Decreased viscosity : SVR decreased
• Low oncotic pressure
• O2 carrying
• Coagulation factor
Composition of Prime
• Average 1,500-2,000ml
• Hct 20- 25%
• Example
• Balanced salt sol. RL 1250 ml
• Osmotically active agent (Mannitol, Dextran 40, Hexastarch)
100 ml
• NaHCO3 50 ml
• KCL 10 ml
• Heparin 1 ml
11. • CPB for cardiac surgery
• ECMO for ECLS
• ECMO for supporting cardio/pulmonary function
• VAD for supporting cardiac function
– RVAD; LVAD; Bi VAD
– BiVAD + oxygenator in RVAD = ECMO
