Cardiopulmonary bypass
Cardiopulmonary bypass or heart-lung machine, also called the pump or CPB pump, is a machine, operated by a cardiac perfusionist, that temporarily takes over the function of the heart and lungs during open-heart surgery by maintaining the circulation of blood and oxygen throughout the body. As such it is an extracorporeal device.
The machine mechanically circulates and oxygenates blood throughout the patient's body while bypassing the heart and lungs allowing the surgeon to work in a bloodless surgical field.
Uses
CPB is commonly used in operations or surgical procedures involving the heart. The technique allows the surgical team to oxygenate and circulate the patient's blood, thus allowing the surgeon to operate safely on the heart. In many operations, such as coronary artery bypass grafting, the heart is arrested, due to the degree of the difficulty of operating on a beating heart.Operations requiring the opening of the chambers of the heart, for example mitral valve repair or replacement, requires the use of CPB. This is to avoid engulfing air systemically, and to provide a bloodless field to increase visibility for the surgeon. The machine pumps the blood and, using an oxygenator, allows red blood cells to pick up oxygen, as well as allowing carbon dioxide levels to decrease. This mimics the function of the heart and the lungs, respectively.
Hypothermia
CPB can be used for the induction of total body hypothermia, a state in which the body can be maintained for up to 45 minutes without perfusion. If blood flow is stopped at normal body temperature, permanent brain damage can occur in three to four minutes — death may follow. Similarly, CPB can be used to rewarm individuals who have hypothermia. This rewarming method of using CPB is successful if the core temperature of the patient is above 16 °C.Cooled blood
The blood is cooled during CPB and is returned to the body. The cooled blood slows the body's basal metabolic rate, decreasing its demand for oxygen. Cooled blood usually has a higher viscosity, but the various crystalloid or colloidal solutions that are used to prime the bypass tubing serve to dilute the blood. Maintaining appropriate blood pressure for organs is a challenge, but it is monitored carefully during the procedure. Hypothermia is also maintained, and the body temperature is usually kept at.Extracorporeal membrane oxygenation
is a simplified version of the heart-lung machine that includes a centrifugal pump and an oxygenator to temporarily take over the function of heart and/or the lungs. ECMO is useful for post-cardiac surgery patients with cardiac or pulmonary dysfunction, patients with acute pulmonary failure, massive pulmonary embolisms, lung trauma from infections, and a range of other problems that impair cardiac or pulmonary function.ECMO gives the heart and/or lungs time to repair and recover, but is only a temporary solution. Patients with terminal conditions, cancer, severe nervous system damage, uncontrolled sepsis, and other conditions may not be candidates for ECMO.
Usage scenarios
CPB is used in scenarios such as:- Coronary artery bypass surgery
- Cardiac valve repair and/or replacement
- Repair of large septal defects
- Repair and/or palliation of congenital heart defects
- Transplantation
- Repair of some large aneurysms
- Pulmonary thromboendarterectomy
- Pulmonary thrombectomy
- Isolated limb perfusion
Contraindications and special considerations
Heparin-induced thrombocytopenia and heparin-induced thrombocytopenia and thrombosis are potentially life-threatening conditions associated with the administration of heparin. In both of these conditions, antibodies against heparin are formed which causes platelet activation and the formation of blood clots. Because heparin is typically used in CPB, patients who are known to have the antibodies responsible for heparin-induced thrombocytopenia and heparin-induced thrombocytopenia and thrombosis require alternative forms of anticoagulation. Bivalirudin is the most studied heparin-alternative in people with heparin-induced thrombocytopenia and heparin-induced thrombocytopenia and thrombosis requiring CPB.
A small percentage of patients, such as those with an antithrombin III deficiency, may exhibit resistance to heparin. In these patients, patients may need additional heparin, fresh frozen plasma, or other blood products such as recombinant anti-thrombin III to achieve adequate anticoagulation.
A persistent left superior vena cava is thoracic system variation in which the left-sided vena cava fails to involute during normal development. It is the most common variation of the thoracic venous system, occurring in approximately 0.3% of the population. The abnormality is often detected on pre-operative imaging studies, but may also be discovered intra-operatively. A persistent left superior vena cava may make it difficult to achieve proper venous drainage or delivery of retrograde cardioplegia. Management of a persistent left superior vena cava during CPB depends on factors such as the size and drainage site of the vena cava variation.
Cerebral perfusion, brain blood circulation, always has to be under consideration when using CPB. Due to the nature of CPB and its impact on circulation, the body's own cerebral autoregulation is affected. The occurrence and attempts of preventing this issue has been addressed many times, but still without complete understanding.
Risks and complications
CPB is not without risk, and there are a number of associated problems. As a consequence, CPB is only used during the several hours a cardiac surgery may take. CPB is known to activate the coagulation cascade and stimulate inflammatory mediators, leading to hemolysis and coagulopathies. This problem worsens as complement proteins build on the membrane oxygenators. For this reason, most oxygenators come with a manufacturer's recommendation that they are only used for a maximum of six hours, although they are sometimes used for up to ten hours, with care being taken to ensure they do not clot off and stop working. For longer periods than this, a membrane oxygenator is used, which can be in operation for up to 31 days — such as in a Taiwanese case, for 16 days, after which the patient received a heart transplant.The most common complication associated with CPB is a protamine reaction during anticoagulation reversal. There are three types of protamine reactions, and each may cause life-threatening hypotension, anaphylaxis, or pulmonary hypertension. Patients with prior exposure to protamine, such as those who have had a previous vasectomy or diabetics, are at an increased risk of type II protamine reactions due to cross-sensitivity. Because protamine is a fast-acting drug, it is typically given slowly to allow for monitoring of possible reactions. The first step in management of a protamine reaction is to immediately stop the protamine infusion. Corticosteroids are used for all types of protamine reactions. Chlorphenamine is used for type II reactions. For type III reactions, heparin is redosed and the patient may need to go back on bypass.
CPB may contribute to immediate cognitive decline. The heart-lung blood circulation system and the connection surgery itself release a variety of debris into the bloodstream, including bits of blood cells, tubing, and plaque. For example, when surgeons clamp and connect the aorta to tubing, resulting emboli may block blood flow and cause mini strokes. Other heart surgery factors related to mental damage may be events of hypoxia, high or low body temperature, abnormal blood pressure, irregular heart rhythms, and fever after surgery.