Cardio-pulmonary bypass is a technique which mechanically provides systemic perfusion during cardiac surgery. It differs from other forms of mechanical circulatory support in that it provides extracorporeal oxygenation, and is only used during cardiac surgery. It is used for surgical techniques which require asystole and/or a bloodless field, for example almost all valve surgery and most coronary artery bypass grafting.
Cardio-pulmonary bypass circuit
The cardio-pulmonary bypass circuit drains blood by gravity (suction may sometimes be applied) from the right atrium or vena cavae into a venous reservoir. Additional suction lines are also used for intracardiac blood venting, and to drain shed blood from the operative field. Suction is generated by a roller pump. Blood from these suction pumps also drains into the venous reservoir. The blood is then pumped through a membrane oxygenator, and carbon dioxide and oxygen are exchanged. A heat exchanger is also used to regulate blood temperature. From the oxygenator, blood is pumped by an arterial pump, through a filter and back to the ascending aorta. The circulation is usually largely pulseless during cardio-pulmonary bypass. Systemic heparinisation is used during cardiopulmonary bypass to prevent thrombosis at a dose of 3-4mg/kg.
After initiation of cardio-pulmonary bypass, the heart is stopped, and an aortic crossclamp is applied distal to the coronary arteries, known as the crossclamp. Cardiac arrest is usually achieved by the administration of cardioplegia, which contains a high concentration of potassium and results in diastolic cardiac arrest. This provides a still, bloodless field enabling complex cardiac surgery. When the crosslamp is on there is no flow to the coronary arteries. The oxygen demand of the heart is reduced by almost 90% during normothermic asystole. It is therefore essential to maintain asystole by readministration of cardioplegia every 15-20 minutes. Various cardioplegia solutions have been used, but the commonest are cold crystalloid cardioplegia and cold blood cardioplegia. Cardioplegia may be administered antegradely by infusing solution into the clamped aortic root or coronary artery ostia, or retrogradely by infusion into the coronary sinus. Often both antegrade and retrograde cardioplegia are given to maximise myocardial protection from ischaemia. In some circumstances, for example aortic calcification, it is not possible to crossclamp. In this circumstance, hypothermic fibrillatory arrest may be used to obtain a still surgical field for performing distal anastamoses.