cardiopulmonary bypass cardiopulmonary bypass cardiopulmonary bypass (cpb) is a technique in which a machine temporarily takes over the function of the heart and lungs during surgery, maintaining the circulation of blood and oxygen to the body. the cpb pump itself is often referred to as a heart–lung machine or "the pump". cardiopulmonary bypass pumps are operated by perfusionists. cpb is a form of extracorporeal circulation. extracorporeal membrane oxygenation is generally used for longer-term treatment. cardiopulmonary bypassa heart–lung machine (upper right) in a coronary artery bypass surgery.icd-10-pcs12icd-9-cm39.61meshd002318ops-301 code14other codes22570829 cpb mechanically circulates and oxygenates blood for the body while bypassing the heart and lungs. it uses a heart–lung machine to maintain perfusion to other body organs and tissues while the surgeon works in a bloodless surgical field. the surgeon places a cannula in the right atrium, vena cava, or femoral vein to withdraw blood from the body. venous blood is removed from the body by the cannula and then filtered, cooled or warmed, and oxygenated before it is returned to the body by a mechanical pump. the cannula used to return oxygenated blood is usually inserted in the ascending aorta, but it may be inserted in the femoral artery, axillary artery, or brachiocephalic artery (among others). the patient is administered heparin to prevent clotting, and protamine sulfate is given after to reverse effects of heparin. during the procedure, hypothermia may be maintained; body temperature is usually kept at 28 °c to 32 °c (82.4–89.6 °f). the blood is cooled during cpb and 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 crystalloid solution used to prime the bypass tubing dilutes the blood. uses illustration of one typical way that a heart-lung machine may be connected to the veins and arteries near the heart. the three implements on the left represent (from top to bottom) the pump, the oxygenator, and the reservoir. cardiopulmonary bypass is commonly used in operations involving the heart. the technique allows the surgical team to oxygenate and circulate the patient's blood, thus allowing the surgeon to operate on the heart. in many operations, such as coronary artery bypass grafting (cabg), the heart is arrested (i.e., stopped) because of the difficulty of operating on the beating heart. operations requiring the opening of the chambers of the heart, for example, mitral valve repair or replacement, requires the use of cpb 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. 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 (blood flow). if blood flow is stopped at normal body temperature, permanent brain damage normally occurs in three to four minutes – death may follow shortly afterward. 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.: 117 extracorporeal membrane oxygenation (ecmo) 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 in post cardiac surgery patients with cardiac or pulmonary dysfunction, in 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 or 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.: 141 surgical procedures in which cardiopulmonary bypass is used coronary artery bypass surgery cardiac valve repair and/or replacement (aortic valve, mitral valve, tricuspid valve, pulmonic valve) repair of large septal defects (atrial septal defect, ventricular septal defect, atrioventricular septal defect) repair and/or palliation of congenital heart defects (tetralogy of fallot, transposition of the great vessels) transplantation (heart transplantation, lung transplantation, heart–lung transplantation, liver transplantation) repair of some large aneurysms (aortic aneurysms, cerebral aneurysms) pulmonary thromboendarterectomy pulmonary thrombectomy isolated limb perfusion: 117 contraindications and special considerations there are no absolute contraindications to cardiopulmonary bypass. however, there are several factors that need to be considered by the care team when planning an operation. heparin-induced thrombocytopenia (hit) and heparin-induced thrombocytopenia and thrombosis (hitt) are potentially life-threatening conditions associated with the administration of heparin. in hit or hitt, 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 hit and hitt require alternative forms of anticoagulation. bivalirudin is the most studied heparin-alternative in patients with hit or hitt 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 (plsvc) 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 plsvc may make it difficult to achieve proper venous drainage or deliver of retrograde cardioplegia. management of a plsvc during cpb depends on factors such as the size and drainage site of the pslvc. risks and complications potential complications of cardiopulmonary bypass complication incidence (events/1000) death or serious injury (%) protamine reaction 1.3 10.5 thrombosis 0.3–0.4 2.6–5.2 aortic dissection 0.4–0.8 14.3–33.1 gas embolism 0.2–1.3 0.2–8.7 massive systemic gas embolism 0.03–0.07 50–52 dislodging of cannula (causing massive bleeding) 0.2–1.6 4.2–7.1 acute respiratory distress syndrome – – arrythmias – – capillary leak syndrome – – hemolysis – – postperfusion syndrome ("pumphead") – – cpb is not benign 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, an ecmo (extracorporeal membrane oxygenation) 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 (type i), anaphylaxis (type ii), or pulmonary hypertension (type iii). patients with prior exposure to protamine, such as those who have had a previous vasectomy (protamine is contained in sperm) or diabetics (protamine is contained in neutral protamine hagedorn (nph) insulin formulations), 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 (anaphylactic) 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. components cardiopulmonary bypass consists of two main functional units, the pump and the oxygenator which removes relatively oxygen-depleted blood from a patient's body and replaces it with oxygen-rich blood through a series of tubes (hoses). a heat exchanger is used to control body temperature by heating or cooling the blood in the circuit. it is important that all components of the circuit are coated internally by heparin or another anticoagulant to prevent clotting within the circuit. perfusionist operating a modern heart lung machine tubing the components of the cpb circuit are interconnected by a series of tubes made of silicone rubber or pvc. centrifugal pump many cpb circuits now employ a centrifugal pump for the maintenance and control of blood flow during cpb. by altering the speed of revolution (rpm) of the pump head, blood flow is produced by centrifugal force. this type of pumping action is considered to be superior to the action of the roller pump by many because it is thought to prevent overpressurization, clamping or kinking of lines, and produce less damage to blood products (hemolysis, etc.). roller pump the pump console usually comprises several rotating motor-driven pumps that peristaltically "massage" tubing. this action gently propels the blood through the tubing. this is commonly referred to as a roller pump, or peristaltic pump. the pumps are more affordable than their centrifugal counterparts, but are susceptible to overpressurization if the lines become clamped or kinked. they are also more likely to cause a massive air embolism and require constant, close supervision by the perfusionist. oxygenator the oxygenator is designed to add oxygen to infused blood and remove some of the carbon dioxide from the venous blood. cardiac surgery was made possible by cpb using bubble oxygenators, but membrane oxygenators have supplanted bubble oxygenators since the 1980s. the main reasons for this are that membrane oxygenators tend to generate far fewer micro-bubbles, referred to as gaseous microemboli, which is generally considered harmful to the patient and reduce damage to blood cells, compared to bubble oxygenators. more recently, the use of hollow-fiber oxygenators has become more widespread. these derivatives of membrane oxygenators further reduce the occurrence of microemboli by reducing the direct air-blood interface while simultaneously providing adequate gas exchange. another type of oxygenator gaining favour recently is the heparin-coated blood oxygenator which is believed to produce less systemic inflammation and decrease the propensity for blood to clot in the cpb circuit. heat exchangers because hypothermia is frequently used in cpb to reduce metabolic demands (including that of the heart), heat exchangers are implemented to warm and cool blood within the circuit. the heating and cooling is accomplished by passing the line through a warm or ice water bath. a separate heat exchanger is required for the cardioplegia line. cannulae multiple cannulae are sewn into the patient's body in a variety of locations, depending on the type of surgery. a venous cannula removes oxygen depleted venous blood from a patient's body. an arterial cannula infuses oxygen-rich blood into the arterial system. the main determinants of cannula size selection is determined by the patient's size and weight, anticipated flow rate, and the size of the vessel being cannulated. a cardioplegia cannula delivers a cardioplegia solution to cause the heart to stop beating. some commonly used cannulation sites: venous arterial cardioplegia right atrium proximal aorta, distal to the cross-clamp proximal aorta, proximal to the cross-clamp vena cavae femoral artery coronary sinus (retrograde delivery) femoral vein axillary artery coronary ostia distal aorta bypass grafts (during cabg) apex of the heart cardioplegia main article: cardioplegia cardioplegia is a fluid solution used to protect the heart during cpb. it is delivered via a cannula to the opening of the coronary arteries (usually by way of the aortic root) and/or to the cardiac veins (by way of the