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"Internal" Workings of the Cardiopulmonary Bypass Machine
Welcome to the first article in ChE in Medicine Resource Page. This article is related to the cardiopulmonary bypass machine (CBM), also called the pump oxygenator. This article will be of special interest to those of you who enjoy eating at fast food restaurants on a regular basis. Chances are that if you
Returning blood flow to the heart is important to help the heart pump the 4 to 5 liters that it normally pumps per minute. The heart beats over 4 million times per year and requires 5 watts of power per hour to continuously pump blood. These facts help us to see how important it is to have the heart pumping as efficiently and as unobstructed as possible. A CABG as mentioned above is performed to help the heart beat in an efficient manner. When a CABG is performed, a CBM is used to perform the myocardial revascularization (restoration on blood flow to the heart) This same machine can also be used when heart valve surgeries are performed. To perform a CABG, in most cases, a median sternotomy incision is made. The sternum (breast bone) is split with a sternal saw from the manubrium (upper portion of the breast bone) to below the xiphoid process (lower portion of the breast bone). The reason that I stated earlier that this is the incision used in most cases, is to make all of you aware that many teaching and research facilities are performing CABG surgeries with many innovative new techniques, which are beyond the scope of this article. For this article, we will consider the traditional CABG surgery which is the main technique used in hospitals today.
After the sternum is split, the ribs are spread to expose the anterior mediastinum and pericardium. When the pericardium is opened and the heart and aorta are exposed, the patient is then placed on the CBM, while the CABG surgery is performed. The CBM oygenates blood and circulates is throughout the body. The first step in initiating the CBM is to prime the tubing of the CBM with a balanced electrolyte solution. The patient's blood is then brought to the pump either through one cannula being placed in the right atrial appendage or by two cannulas one being placed in the inferior vena cava and the other in the superior vena cava. In addition, another cannula is placed in the ascending aorta to return oxygenated blood back to the patient's systemic circulation. ( Please see the photo of this basic anatomy of the heart, so as to better visualize how the CBM works.)
Patients are also given an anticoagulant (blood thinner) called Heparin, which helps to prevent massive extravascular coagulation (clotting of the blood while out of the body in the CBM) while the blood is circulating through the mechanical parts of the bypass system. Once the CBM is established, blood is pumped through a circuit by a series of roller-type pumps. Venous blood from the patient flows through a venous cannula to a cardiotomy reservoir and then to the oxygenator where exchange of oxygen and carbon dioxide occurs. Blood goes through the heat exchanger in the CBM, it is cooled initially and then later it is rewarmed. During the CABG surgery, the core body temperature is decreased to 28 to 32 degrees Celsius (82.4 to 89.6 degrees Fahrenheit). This cold temperature of the body, which decreases metabolism, helps to protect major organ systems from ischemic injury (lack of blood flow to an organ of the body that results in damage to that organ). For every 1 degree Celsius drop of body temperature, the metabolic demands of the body are decreased by 7%.
Oxygenated blood is then filtered and returned to the patient's ascending aorta by way of the arterial cannula. Once extracorpeal (outsi