Cardiopulmonary Bypass

In simple terms, CPB is a process in which blood is diverted from the heart and the lungs into the heart-lung machine, therefore, making it possible to operate on a still and bloodless heart. The heart-lung machine (Fig. 2) adds oxygen to the blood, removes carbon dioxide, and controls the temperature of the blood. It is also possible to add agents like buffers to the blood for pH control. CPB is a mandatory aspect of cardiac surgery in most of the adult and the paediatric cases. This technique has been used extensively in cardiac surgery for the last 50 years, since its first clinical use by John Gibbon in 1953.

Fig. 2 Cardiopulmonary bypass machine.

Role of Oxygenators in Haemostasis

The last 50 years have seen major changes in the materials used and in the technology of the individual components of the heart-lung machine. As all these components are foreign to the host tissues they elicit strong inflammatory response leading to the activation of platelets, leucocytes, and the complement system and haemolysis. Oxygenators used in the early 1950s were either of rotating disc or of stationary screen types. They were large and cumbersome units and were not available in disposable form. They also gave rise to haemolysis, platelets' and leucocytes' activation, and large scale inflammatory response. In the 1960s and 1970s, bubble oxygenators were in widespread use. They worked on the principle of gas exchange taking place across the walls of microbubbles. The size of the individual bubble would determine the amount of diffusion of gases across the gas-blood interface. Bubble oxygenators required a pump downstream in the circuit, posing a potential hazard of major air embolism. In the 1980s and onwards, most of the cardiac centres transferred to the next generation of oxygenators called membrane oxygenators (Fig. 3). In these oxygenators, the gas exchange takes place across a membrane made up of either semi-permeable Teflon or silicone rubber. Membrane oxygenators come in either spiral coil form or in hollow fibre membrane form. A pump is used upstream in the circuit of the membrane oxygenator and, therefore, the risk of major air embolism is significantly reduced. These newer oxygenators are "kinder" to the blood and its formed elements. Therefore, theysignif-icantly reduce haemolysis, platelets' and leucocytes' activation, and complement system. This reduces the overall inflammatory response of the CPB. The use of filters, heparin coated circuits, and centrifugal pumps has further reduced the inflammatory response of the CPB.

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