Extracorporeal Circulation: Prevention and Management of Complications

The development of extracorporeal circulation (ECC) has indubitably been extremely important in the progress of cardiac surgery. The possibility of bypassing blood outside the heart and lungs, pumping oxygenated blood from the venous system directly to the arterial system, has permitted all types of cardiac surgery. Extracorporeal circulation has also allowed surgical approaches to complex diseases, such as correction of thoracic aortic aneurysms, resection of renal cell carcinoma, treatment of neurobasilar aneurysms, lung transplantation, and thrombo-endarterectomy of pulmonary arteries. Others uses of ECC include veno-venous bypass in liver transplantation, in acute respiratory failure as part of a “lung protective strategy”, in acute pulmonary hypertension of newborn, in high-risk angioplasty patients and ventricular assistance. Ideally, to replace the functions of the heart and lungs cardiopulmonary bypass (CPB) should achieve several aims, with minimal risks and interferences to normal physiology. The main purpose of CPB is to maintain systemic perfusion with adequate oxygen transport and carbon dioxide elimination, preserving homeostasis while the heart and lungs are not providing these functions. All extracorporeal circuits that return blood to the patient require the incorporation of some kind of pumping device. This pumping mechanism may occur by various basic principles of fluid movement which include positive displacement, centrifugal vortexing and pneumatic and electrical pulsation. Positive displacement pumps have been largely used in ECC since it was proposed by Gibbon [11. The second type of pump device used in ECC is the centrifugal pump or resistance-dependent pump. This system works by the addition of kinetic energy to the fluid through the forced rotation of an impeller or cone. A centrifugal pump was proposed for clinical use in 1976, and since then its acceptance in CPB has increased progressively. The other components of cardiopulmonary circuitry are the oxygenators, heat exchangers, cannulae and tubing, and the cardioplegia delivery system. The oxygenators have the main function of gas transfer, and, at the same time, they should have a low rate of bioreactivity. Oxygenators utilized in CPB are currently divided into two classes based on the method of gas exchange: bubble and membrane oxygenators. Although more complex in design, membrane oxygenators are much more used nowadays in routine CPB, mainly because this is thought to be a much safer system than bubble oxygenators, which cause gaseous microemboli and induce hematologic damage to the formed elements of blood [2, 3]. As described below, it has been known for several years that CPB triggers an inflammatory response that can lead to the development of postoperative organ dysfunction. This response may occur with both oxygenators, bubble and membrane [4], but the use of bubble oxygenators has been shown to be able to induce a greater inflammatory response when compared to membrane oxygenators. Bubble oxygenators seem to be responsible for greater complement activation and lung leukocyte sequestration when compared to membrane [5]. Another study demonstrated higher levels of intercellular adhesion molecule-1 (sICAM-1) in the blood during use of bubble oxygenators when compared to membrane [6].