Cardiopulmonary bypass

Cardiopulmonary bypass (CPB) is a medical technique used during complex heart surgeries. It is also commonly called a heart-lung machine or a CPB pump. This extracorporeal device, meaning it functions outside the body, temporarily takes over the work of the heart and the lungs. By doing this, the machine maintains the circulation of blood and oxygen throughout the patient's entire body. This allows surgeons to operate on a heart that is not beating. It also provides a bloodless surgical field, which improves the surgeon's visibility.

The mechanism of CPB involves a precise sequence of steps to mimic human biology. First, a venous cannula removes oxygen-depleted blood from a specific site, such as the right atrium. This blood travels through tubes made of silicone rubber or PVC. A pump then moves the blood through the circuit. Some systems use a centrifugal pump, which uses spinning force to create flow. Other systems use a roller pump, which uses a peristaltic action to massage the tubing.

Once the blood is pumped, it enters the oxygenator. This component performs the role of the lungs by allowing red blood cells to pick up oxygen. It also allows carbon dioxide levels to decrease. After oxygenation, the blood must be returned to the body. An arterial cannula infuses this fresh, oxygen-rich blood back into the arterial system. To keep the blood from clotting inside the machine, all components are internally coated with heparin or another anticoagulant.

There are several distinct types of components used in these circuits. The pumps are the primary drivers of circulation. Centrifugal pumps are often considered superior because they cause less damage to blood products, a process called hemolysis. Roller pumps are more affordable but require constant supervision to avoid air embolisms. Heat exchangers are another vital part of the system. These allow the medical team to control the patient's temperature by passing blood through warm or ice water baths.

CPB is essential for many different surgical scenarios. It is used during coronary artery bypass grafting (CABG), where the heart is often arrested. It is also required for repairing or replacing heart valves, such as the mitral or aortic valves. Surgeons use it to fix large septal defects or congenital heart defects like Tetralogy of Fallot. Furthermore, CPB is necessary during heart, lung, or liver transplantations. It can even be used for treating large aneurysms or performing pulmonary thromboendarterectomies.

Temperature management is a significant aspect of the procedure. CPB can be used to induce total body hypothermia. In this state, the body's basal metabolic rate slows down. This decreases the body's demand for oxygen. While blood flow stopped at normal temperatures can cause brain damage in three to four minutes, hypothermia allows for safer periods of surgery. The machine can also rewarm patients, provided their core temperature is above 16 °C.

Despite its benefits, CPB carries specific risks and complications. The process can activate the coagulation cascade, leading to hemolysis or coagulopathies. A common complication is a protamine reaction during the reversal of anticoagulation. This reaction can cause life-threatening hypotension or anaphylaxis. There is also a risk of gas embolism, which occurs if air enters the circulation. In some cases, the debris from the surgery can cause mini strokes or immediate cognitive decline. Because of these risks, most oxygenators are recommended for use for a maximum of six hours, though some can function longer.

Extracorporeal membrane oxygenation (ECMO) is a simplified version of this technology. ECMO uses a centrifugal pump and an oxygenator to support heart or lung function. It is often used for patients with acute pulmonary failure or lung trauma. While CPB is used during active surgery, ECMO provides a temporary solution to give organs time to repair and recover. It is a vital tool in modern intensive care, though it is not suitable for patients with terminal conditions like uncontrolled sepsis or severe nervous system damage.