Extracorporal circulation is a technology of using a special device to temporarily substitutethe work of human heart and lungs for blood circulation and gas exchange. This device can be subcategorized into artificial heart and artificial lung, alsocalled artificial heart and lung, artificial cardiopulmonary device orextracorporal circulation device.
Inextracorporal circulation, the venous blood is injected into the artificiallung for oxygenation to discharge the carbon dioxide, and the oxygenated blood maintains a certain pressure and is pumped into the in vivo arterial system viathe artificial heart. In this way, quiet and clear surgical fields areguaranteed during the surgery and the blood supply in other important organsother than heart is guaranteed. It is an important guarantee measure in the development of heart and great vessel surgery. In 1953, Gibbon first clinically applied.
The basic devices inextracorporal circulation: including blood pump, oxygenator, heat exchanger,blood storing room and strainer.
Blood pump: namely artificial heart. It is a device that substitutes heart to discharge blood andsupply the generalized blood circulation. It is classified into non-pulsating pump and pulsating pump according to the modes of blood discharge. Currently,the non-pulsating pump is relatively wide, the injected blood is in the form ofcontinuous nonpulsatile flow with rolling type pump as the main type, and thepump head is mainly regulated to rotate and squeeze the pumping pipe todischarge the blood. The pulsating pump discharging the blood in the pulsatingform, and it can be classified into two types: synchronous type and asynchronous type.
Oxygenator:namely artificial lung. It substitutes the lungs to combine the venous bloodand oxygen and discharge the carbon dioxide. Currently, there are three types available: ①Blood film type: the blood is disseminated on the plane to form the blood film,which contacts with the oxygen for gas exchange, the dish spinning type is the typical representative, it can be used repeatedly, but it is time and labor consuming,and currently, it is rarely applied domestically. ② Blistering type: gas exchange is performed during the process in which blood is blow byoxygen (or the mixed gas between oxygen and carbon dioxide), and the bubbles formed in the blood are removed using the silicon defoaming agents; currently,there are two types: cylinder type and bag type; currently, the most widely applies to the Xijing-87 type oxygenator developed and manufactured by Xijing Hospital of the Fourth Military Medical University, and it is well-received indomestic hospitals. ③ Membrane type: it is made using themacromolecular osmotic membrane, as for blood and gas exchange is performedusing the semipermeable membrane, blood and gas has no direct mutual contact,the visible component of blood is little destroyed, and there are plain membrane type and hollow fiber type according to the appearance.
Heat exchanger: it isthe device to regulate the blood temperature in extracorporal circulation, itcan exist as an independent component, but it is integrated with oxygenators inmost cases. The difference between the water temperature and blood temperature of heat exchanger is less than 10 to 15℃, and the highest water temperature shouldnot exceed 42℃.
Blood storing room: itis a container, which contains filtering net and defoaming device and is usedto reserve the priming solution and return the blood in the heart.
Strainer: during thefiltering extracorporal circulation, it can produce bubbles, platelet clot,cellulose, fat granules, silicone oil suppository, and miliary tissue blocks detached within patient’s body, and the different meshes of the strainers should be used in different sites.
When the open heart surgery under the extracorporal circulation is performed, first adopt the longitudinal split chest bone approach is generally adopted, perform longitudinal incision of percardium to expose the heart, inject heparin at thedose of 2 to 3 mg/kg from the heart, sequentially insert the ascending aorticdelivery tube and delivery tube after the blood does not become clotted as detected, and after the superior vena cava is connected with the pre-filledheart lung machine, then the extrinsic cycle by-pass can be started. The hemodilution method is conventionally used currently for the extracorporeal circulation prefilling, and osmotic pressure, electrolyte content and blood dilute strength should be considered as for the priming solution. Different people have different materials on the degree of hemodilution, and that ofhemoglobin ranges from 5% to 10g%, and the hematocrit ranges from 10% to 30%.The crystalloid solution for prefilling includes Lactated Ringer’s solution, normal saline and 50% glucose solution etc., ACD blood, plasma and albumin can beselected as colloidal solution, and potassium, magnesium, sodium bicarbonateand antibiotic drugs need to be added.
According to the surgical needs,the extracorporal circulation methods can be classified into the following types. ① Common temperature extracorporal circulation, the cardiological operation is easy, and the duration isshort. It is required that the performance of extracorporal circulation oxygenation be good and satisfy the needs of high flow perfusion. ② Superficial low-temperature extracorporal circulation: the extracorporal circulation blood flow is used forcooling, and the nasopharyngeal temperature is maintained at about 28℃ during thecardiac operation. After the cardiacoperation is about to end, start rewarming the blood; end the rewarming whenthe nasopharyngeal temperature is at 35 to 36℃. ③ Deep low-temperature low-flowextracorporal circulation: in most cases, it is used to the patients with poorheart functions, complicated heart deformities and rich collateralcirculation. When the nasopharyngeal temperature drops to about 20℃, the key steps in cardiac operation canlower the perfusion flow, and the minimum can be up to 5 to 10 ml/kg/min. It can not only main the clear surgical fields but also prevent the airembolism induced by the air entry into the systemic circulation. Microperfusion is actually the circulationarrest on the organism and the time should be shortened as much as possible; ④deep low-temperature circulation arrest is mainly used to the infantile openheart surgery and adult aortic aneurysm surgery. During the operation, the body temperature isreduced to less than 20℃ and the blood circulation is discontinued;in this way, good surgical fields can be provided, but the doctors should have good conditions and proficient perfusion skills.
During thecardiac surgery, in order to facilitate the accurate operation and obtain the blood-free surgical fields, the ascending aorta must be incarcerated to block the blood circulation of coronary artery so that the myocardium is at the status of ischemia and anoxia. One of the main reasons for the high mortalityrate in the early surgery is myocardial ischemic necrosis. During the recent years, many scholars have been devoted to the study on cardiac protection to obtain the blood-freesurgical fields and proper protection of myocardium. After the operation, the good functions are recovered. Currently, the most-widely applied one is the systemic moderate low temperature, the heart has local deep low temperature,cold-arrest and cardioplegia solution method is adopted for aortic perfusion,and the generalized temperature is maintained at about 28℃. The myocardialt emperatures is maintained from 15 to 20℃; the method is as follows: after the ascending aorta is obstructed, theroot of aorta is perfused with the prepared 4℃ cold-arrest cardioplegia solution to make the myocardium rapidly stopactivities and reduce energy consumption. One perfusion is performed every 20minutes, the pericardium is wrapped with ice slush, or perfused circulatorily with 4℃ normal saline. As the endocardium temperature is slightlyhigh, cooling must be performed inside the cardiac chamber if necessary.
After thecardiac operation ends, the heart is resuscitated, and the extracorporal circulation is stopped. After the cycle is stabilized the cardiac cannula ispulled off, and the nucleoprotamine is used to neutralize the heparin.
