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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 (outside the body) circulation and hypothermia are acheived, the aorta is cross-clamped just above the coronary arteries. Crystalloids (intravenous fluids) or blood cardioplegia solution is then infused in the aortic root. Once the aorta is cross-clamped, no blood goes through the coronary arteries, so the myocardium is ischemic. Cold cardioplegic solution (4 degrees Celsius or 39.2 degrees Fahrenheit), which is high in potassium, is infused in the aortic root under pressure. The high potassium level causes the heart to stop (asystole). The heart is then relaxed, the combination of asystole and hypothermia protect the heart against myocardial ischemia. Iced normal saline is also placed directly on the heart to make sure all parts of the heart are hypothermic.
Some surgeons also do CABG surgeries keeping the blood at normal body temperature 37 degrees Celsius (98.6 degrees Fahrenheit). This technique is used in simple CABG surgeries, most complicated CABG surgeries use the cold technique. CABG surgeries done a normal body temperature have been shown to have more complications. Because of the increased complications, most CABG surgeries are done using the cold technique.
If the cold technique is used at the end of the CABG surgery, the blood is rewarmed to 37 degrees Celsius (98.6 degrees Fahrenheit) using the heat exchanger in the CBM. The aortic cross clamp is removed so that the blood perfuses coronary arteries, which warms the myocardium. After the heart rhythm is maintained, then the cardiopulmonary bypass is reduced to partial bypass. If the patient tolerates this then the heart assumes total responsibility for cardiac stability and the CBM is discontinued. The cannulas placed in the heart are removed and the patient's chest is closed.
The patient is then taken to a Surgical Intensive Care Unit where postop care is provided. I would also like to add that this CBM's is maintained in the operating room by a perfusionist. A perfusionist is a highly trained specialist who operates the CBM so that the surgeon can focus more on the operative procedure. The article has described what the CBM is used for today. In order to understand how engineering has helped to shape this modern technology, it is necessary to take a look at the history of the development of the CBM.
The first heart-lung machine was built by John Gibbon in 1937. This machine used two roller pumps and the machine had sufficient capacity to replace the heart and lungs of experimental cats. The machine was too small to serve for dogs or humans. Most animals died from the haemolysis( clotting of blood), air embolism introduced into the body, and blood-foreign surface interaction. The problem of haemolysis was alleviated somewhat in 1939 when the anticoagulant Heparin was discovered.
John Gibbon, who was a physician, met an engineer by the name of Thomas Watson in 1946. This engineer was also the chairman of IBM (International Business Machines). Watson was able to provide both financial and technical support to Gibbon to help in the development of heart-lung machine. Watson along with 5 other engineers helped Gibbon to invent a machine that minimized haemolysis and prevented air bubbles from entering the circulation. This heart-lung machine was contained within a cabinet that kept the body at normal temperature and blood flow was carefully controlled to maintain a constant blood volume. With the invention of this new machine, there was only a 10% mortality in dogs, whereas the mortality rate had been 80%.
About the same time as Gibbon was working on his heart-lung machine, Bill Bigelow was also doing open heart operations on dogs but he was using a different approach. Bigelow was using cooling blankets and ice bags to cool the dogs to 20 degrees Celsius (68 degrees Fahrenheit). The oxygen consumption of the heart fell to 15% of normal, thereby allowing the heart to be isolated from circulation for 15 minutes to allow for an operation with a better survival rate than previously found.
Clarence Dennis in 1945 set out to build a mechanical heart-lung appartus that would permit complete bypass of the heart and lungs during surgical operations of the heart. Dennis built a modified Gibbon pump. Dennis needed an oxygenator that would do the following: use a minimum amount of blood, do as little damage to the blood cells as possible and to acheive a critical level of oxygenation without excess foaming. This new pump consisted of a nest of vertically revolving stainless steeel cylinders. These were mounted over a revolving funnel in which the blood, oxygenated on the walls of the cylinders, was collected. New roller pumps were used to cause less haemolysis. This apparatus was cumbersome and difficult to clean. Most dogs died during the testing of this machine either during perfusion or shortly afterwardss. Dennis discovered that the oxygenator was contaminated and many of the animals died from infections. The method of sterilization along with the design of the oxygenator was changed.
In Sweden, a physician by the name of Viking Olov Bjork had invented an oxygenator with multiple screen discs that rotated slowly in a shaft, over which a film of blood was injected. Oxygen was passed over the rotating discs and provided sufficient oxygenation for an adult human. Bjork along with help of a few chemical engineers, one of which who was his wife, prepared a blood filter and an artificial intima of silicon under the trade name UHB 300. This was applied to all parts of the perfusion machine, particularly, the rough red rubber tubes, to delay clotting and save platelets.
The first heart lung bypass machine was first used on a human in 1953. In 1960, it was considered safe to use the CBM along with hypothermia to perform CABG surgery. Heat exchangers were used to cool and rewarm the blood. Using these heat exchangers helped to eliminate the tedious process of surface cooling.
The CBM has not changed much in the past 30 years. The CBM is a large machine that takes up a lot of room in an operating room. Engineers are working on designing a smaller unit that will not take up as much room. This is just one example of how engineering and medicine have worked together to invent machines to advance medical technologies.
If you would like more information on CBM that is available on the internet you could visit the following site: Baxter Healthcare