Cardiopulmonary bypass

The primary function of the cardiopulmonary bypass (CPB) machine is to provide oxygenated blood flow to the systemic circulation while providing the surgeon with a motionless, bloodless surgical field. The CPB circuit consists of a reservoir, blood pump, oxygenator, heat exchanger, arterial filter, cardioplegia delivery device and cannulae, interconnected by various sized tubing. The venous cannula directs blood away from the heart and lungs via the CBP circuit and the arterial cannula returns the oxygenated blood to the systemic circulation. A blood pump propels the blood volume forward through a membrane oxygenator and allows rapid transfusion of oxygenated blood back into the systemic circulation. The CPB flow needs to be enough to maintain an adequate cardiac output, therefore a flow of 1.8-2.2 litres/minute/m2 is recommended when at normothermia, although these flows can be reduced if the temperature is less than 28°C. The mortality and neurological complications after cardiac surgery are similar using either normothermic or hypothermic CPB. Maintenance of anaesthesia on CPB is often achieved with a propofol infusion (sometimes with the addition of remifentanil), but the use of volatile anaesthetic is also possible through the CPB machine. A vaporizer can be attached to the CPB circuit and volatile anaesthetic delivered into the sweep gas passing through the oxygenator. A safety checklist before separation from bypass is essential, and it may include: optimal temperature, heart rhythm, de-airing, acid-base status, ventilation, electrolytes and patient position. If heparin was used to maintain anticoagulation, it should be reversed with protamine after the patient is stable off-CPB. Some patients require inotropic or mechanical support to facilitate 'weaning' from CPB.