Cardiopulmonary bypass can be defined as a technique in which a machine temporarily takes over the function of the heart and lungs during surgery, maintaining the circulation of blood and the oxygen content of the patient's body.
Cardiopulmonary bypass machines are operated by perfusionists to bypass the heart and lungs during open heart surgery. Blood returning to the heart is diverted through a heart-lung machine (a pump-oxygenator) before returning it to the arterial circulation. The machine does the work both of the heart (pump blood) and the lungs (supply oxygen to red blood cells).
The oxygenator was first conceptualised in the 17th century by Robert Hooke and developed into practical extracorporeal oxygenators by French and German experimental physiologists in the 19th century. Bubble oxygenators have no intervening barrier between blood and oxygen; these are called 'direct contact' oxygenators. Dr. Clarence Dennis led the team that conducted the first known operation involving open cardiotomy with temporary mechanical takeover of both heart and lung functions on April 5, 1951 at the University of Minnesota Hospital. The patient did not survive due to an unexpected complex congenital heart defect. The first successful open heart procedure on a human utilizing the heart lung machine was performed by John Gibbon on May 6, 1953 at Thomas Jefferson University Hospital in Philadelphia. He repaired an atrial septal defect in an 18-year-old woman. Advances in cardiac surgery have been possible due to the development of cardiopulmonary bypass (CPB).
Quote from Dr Christiaan N. Barnard, the first surgeon to carry out a successful heart transplant.
For a dying person, a transplant is not a difficult decision. If a lion chases you to a river filled with crocodiles, you will leap into the water convinced you have a chance to swim to the other side. But you would never accept such odds if there were no lion.
Left: A Heart-Lung Machine for coronary artery bypass surgery.
CPB is a form of extracorporeal circulation whose function is circulatory and respiratory support along with temperature management to facilitate surgery on the heart and great vessels. The CPB pump itself is often referred to as a heart-lung machine or "the pump". Cardiopulmonary bypass pumps are operated by perfusionists in association with surgeons who connect the pump to the patient's body. CPB is a form of extracorporeal circulation. Cardiopulmonary bypass is commonly used in heart surgery because of the difficulty of operating on the beating heart. Operations requiring the opening of the chambers of the heart require the use of CPB to support the circulation during that period. The safe conduct of CPB requires a team effort between the surgeon, perfusionist, and anaesthesiologist.
CPB can be used for the induction of total body hypothermia, a state in which the body can be maintained for up to 45 minutes without perfusion (blood flow). If blood flow is stopped at normal body temperature, permanent brain damage normally occurs in three to four minutes — death may follow shortly afterward.
Extracorporeal membrane oxygenation (ECMO) is a simplified form of CPB, sometimes used as life-support for newborns with serious birth defects, or to oxygenate and maintain recipients for organ transplantation until new organs can be found.
CPB mechanically circulates and oxygenates blood for the body while bypassing the heart and lungs. It uses a heart-lung machine to maintain perfusion to other body organs and tissues while the surgeon works in a bloodless surgical field. The surgeon places a cannula in right atrium, vena cava, or femoral vein to withdraw blood from the body. The cannula is connected to tubing filled with isotonic crystalloid solution. Venous blood that is removed from the body by the cannula is filtered, cooled or warmed, oxygenated, and then returned to the body. The cannula used to return oxygenated blood is usually inserted in the ascending aorta, but it may be inserted in the femoral artery. The patient is administered heparin to prevent clotting, and protamine sulfate is given after to reverse effects of heparin. During the procedure, hypothermia is maintained; body temperature is usually kept at 28ºC to 32ºC (82.4-89.6ºF). The blood is cooled during CPB and returned to the body. The cooled blood slows the body’s basal metabolic rate, decreasing its demand for oxygen. Cooled blood usually has a higher viscosity, but the crystalloid solution used to prime the bypass tubing dilutes the blood.
Cardiopulmonary bypass consists of two main components, the pump and the oxygenator which remove oxygen-deprived blood from a patient's body and replace it with oxygen-rich blood through a series of hoses. The components of the CPB circuit are interconnected by a series of tubes made of silicone rubber or PVC. The pump console usually comprises several rotating motor-driven pumps that peristaltically "massage" tubing . This action gently propels the blood through the tubing. This is commonly referred to as a roller pump, or peristaltic pump. Many CPB circuits now employ a centrifugal pump for the maintenance and control of blood flow during CPB. By altering the speed of revolution (RPM) of the pump head, blood flow is produced by centrifugal force. This type of pumping action is considered to be superior to the action of the roller pump by many because it is thought to produce less blood damage (Haemolysis, etc.).
Oxygenator: The oxygenator is designed to transfer oxygen to infused blood and remove carbon dioxide from the venous blood. Cardiac surgery was made possible by CPB using bubble oxygenators, but membrane oxygenators have supplanted bubble oxygenators since the 1980s. Membrane oxygenators consist of hollow microporous polypropylene fibres (100–200 μm internal diameter). Blood flows outside the fibre while gases pass inside the fibre, thus separating the blood and gas phases. They have lesser propensity for air embolism and give greater accuracy in blood gas control. Newer designs have an integrated filter to manage emboli, thus making additional arterial filters unnecessary. Another type of oxygenator gaining favour recently is the heparin-coated blood oxygenator which is believed to produce less systemic inflammation and decrease the propensity for blood to clot in the CPB circuit. A heat exchanger is integrated with the oxygenator and placed proximal to it to reduce the release of gaseous emboli due to alterations in the temperature of saturated blood.
During CPB, venous blood is drained through gravity into a reservoir. The pump moves blood from the reservoir to the oxygenator through a heat exchanger, before returning it to the arterial circulation. Additional components include suckers (to remove blood from surgical field), vents (to decompress the heart), haemofilters (for ultrafiltration) and cardioplegia system. See ‘Cardiopulmonary Bypass Machine’ below.
Multiple cannulae are sewn into the patient's body in a variety of locations, depending on the type of surgery. A venous cannula removes oxygen deprived blood from a patient's body. An arterial cannula is sewn into a patient's body and is used to infuse oxygen-rich blood. A cardioplegia cannula is sewn into the heart to deliver a cardioplegia solution to cause the heart to stop beating.
A CPB circuit consists of a systemic circuit for oxygenating blood and re-infusing blood into a patient's body (bypassing the heart); and a separate circuit for infusing a solution into the heart itself to produce cardioplegia (i.e. to stop the heart from beating), and to provide myocardial protection (i.e. to prevent death of heart tissue). A CPB circuit must be primed with fluid and all air expunged before connection to the patient. The circuit is primed with a crystalloid solution and sometimes blood products are also added. The patient must be fully anticoagulated with an anticoagulant such as heparin to prevent massive clotting of blood in the circuit.
CPB is only used during the several hours a cardiac surgery may take. Most oxygenators come with a manufacturer's recommendation that they are only used for a maximum of 6 hours, although they are sometimes used for up to 10 hours, with care being taken to ensure they do not clot off and stop working. The roller pump has two rollers positioned on a rotating arm, which compress a length of tubing to produce forward flow. This action can produce haemolysis and tubing debris, the incidence of which increases with time. Hence, the use of roller pumps for longer procedures is discouraged. Centrifugal pump consists of impellers/stacked cones within housing. When rotated rapidly, negative pressure is created at one inlet, and positive pressure at the other, thus propelling the blood forward. They are afterload dependent, so if the patient's systemic vascular resistance (SVR) increases, the cardiac output generated will drop unless the flow through the pump is increased. Centrifugal pumps may improve platelet preservation, renal function and neurological outcomes in longer cases. For longer periods than this, an ECMO (extra-corporeal membrane oxygenation) or VAD (ventricular assist device) circuit is used, which can be in operation for up to 31 days.