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- 1. OXYGENATORS Heart-Lung Machine • Machinefor 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 CircuitPumps • 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 ofPumps 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 bubbleoxygenator • 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 ofBubble 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 • Intergratedinto 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 • Longor 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 forcardiac surgery • ECMO for ECLS • ECMO for supporting cardio/pulmonary function • VAD for supporting cardiac function – RVAD; LVAD; Bi VAD – BiVAD + oxygenator in RVAD = ECMO