Cardiopulmonary bypass

Cardiopulmonary bypass (CPB) is a technique that temporarily takes over the function of the heart and lungs during surgery, maintaining the circulation of blood and the oxygen content of the body. The CPB pump itself is often referred to as a Heart-Lung Machine or the Pumper. Cardiopulmonary bypass pumps are operated by allied health professionals known as Perfusionists in association with surgeons who connect the pump to the patient's body. CPB is a form of extracorporeal circulation.

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Uses of cardiopulmonary bypass

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.

CPB can be used for the induction of total body hypothermia, a state in which the body can be maintained for an hour or more 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.

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.

Surgical procedures in which cardiopulmonary bypass is used

• Coronary artery bypass surgery
• Cardiac valve repair and/or replacement (aortic valve, mitral valve, tricuspid valve, pulmonic valve)
• Repair of large septal defects (atrial septal defect, ventricular septal defect, atrioventricular septal defect)
• Repair and/or palliation of congenital heart defects (Tetralogy of Fallot, transposition of the great vessels)
• Transplantation (heart transplantation, lung transplantation, heart-lung transplantation)
• Repair of some large aneurysms (aortic aneurysms, cerebral aneurysms)
• Pulmonary thromboendarterectomy
• Pulmonary thrombectomy

History

Dr. Clarence Dennis led the team that conducted the first known operation involving open cardiotomy with temporarary 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. This followed four years of laboratory experimentation with dogs.^[1]

John Gibbon is credited with developing the first truly practical heart-lung bypass machine; he performed the first successful surgery with it on May 6, 1953 in Philadelphia, an atrial septal defect repair. But although he is accredited with the invention, many suspect that he was not awarded the Nobel Prize for it because of his failure to mention the other members of the team working with him. Other surgeons such as Bernard J. Miller, a young doctor at the time, created many of the pieces that make the machine function.

Components of cardiopulmonary bypass

Cardiopulmonary bypass consists of two main functional units, 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.

Tubing

The components of the CPB circuit are interconnected by a series of tubes made of silicone rubber, or PVC. The tubing in the CPB circuit is similar to transparent garden hose.

Pumps

Roller pump

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.

Centrifugal 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 (Hemolysis, 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.

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. Membrane oxygenators introduce a gas-permeable membrane between blood and oxygen that decreases the blood trauma of direct-contact oxygenators. Much work since the 1960s focused on overcoming the gas exchange handicap of the membrane barrier, leading to the development of high-performance microporous hollow-fibre oxygenators that eventually replaced direct-contact oxygenators in cardiac theatres.^[2]

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.

Cannulae

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 patients 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.

Cardioplegia

Main article: Cardioplegia

A CPB circuit consists of a systemic circuit for oxygenating blood and reinfusing 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).

Operation

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.

Complications

CPB is not benign and there are a number of associated problems:

• Postperfusion syndrome (also known as Pumphead)
• Hemolysis
• Capillary Leak Syndrome
• Clotting of blood in the circuit - can block the circuit (particularly the oxygenator) or send a clot into the patient.
• Air embolism
• Leakage - a patient can rapidly exsanguinate (lose blood perfusion of tissues) if a line becomes disconnected.

References