Cardiopulmonary Bypass The first cardiac surgery was performed using cardiopulmonary bypass (CPB) in 1953. Since then, CPB has been used during thousands of surgeries and the technology has continued to evolve. Some cardiac surgeries are performed without CPB (off-pump), but the majority are performed using CPB. Even with current techniques, patients undergoing surgery using CPB face a number of potential complications. It is important to keep in mind the effect of CPB on the body and watch for these complications. Objectives In this chapter, you will learn: 1. The purpose of cardiopulmonary bypass during cardiac surgery 2. The effects of cardiopulmonary bypass on the patient 3. Potential complications related to cardiopulmonary bypass CBP MACHINE A CPB machine consists of a series of chambers connected by synthetic tubing, similar to tubing used for intravenous fluids. This tubing is typically made with biocompatible material and may be coated with substances, such as heparin, to make it more biocompatible. Blood drains by means of gravity into a reservoir, then moves through a chamber where it is oxygenated. A roller pump or centrifugal pump is used to propel blood through the machine. Just before being returned to the body, the blood passes through a filter to remove any clots or particles. Blood may also be heated or cooled as it moves through the CPB machine, assisting in obtaining mild hypothermia or rewarming (Figure 4.1). CPB is set up and managed by a perfusionist, who is specially trained to manage this therapy. Before placing a patient on CPB, the perfusionist primes the machine with a balanced electrolyte solution. It requires up to 2,000 mL of fluid to prime the machine, which results in hemodilution when the patient’s blood mixes with the fluid. Often, the addition of albumin to the electrolyte solution is used to minimize the effects of hemodilution, but this will still result in a decrease in hematocrit. In some instances, the patient’s own blood is used to displace the priming solution (called retrograde autologous priming) and virtually eliminate hemodilution. This can decrease the need for blood transfusions after surgery, but can cause problems with hypotension during surgery (Moorjani, Ohri, & Wechsler, 2014). FIGURE 4.1 Cardiopulmonary bypass circuit. Ao, aorta; RA, right atrium; V, ventricle. Patients placed on CPB generally experience hemodilution and a corresponding fall in hematocrit. This can be clinically significant, especially if the patient was anemic prior to surgery. INITIATION OF CBP The patient is brought into the operating room. An arterial line and a pulmonary artery catheter are placed for hemodynamic monitoring. Anesthesia is induced and the patient is intubated. An incision must be made and the heart must be accessible for placing catheters prior to placing the patient on the CPB machine. The patient must be anticoagulated to prevent clot formation in the CPB circuit. Heparin is used most commonly for anticoagulation unless the patient has recently experienced heparin-induced thrombocytopenia (HIT), a type of heparin allergy. Heparin is administered until the activated clotting time (ACT) is 480 seconds or greater. The exact ACT required depends on the complexity of the surgery and preference of the surgeon. Catheter Placement Once the patient is ready to be placed on CPB, a large catheter is inserted into the right atrium through an opening made in the right atrial appendage or wall of the right atrium. This is the venous catheter, which will drain blood via gravity into the CPB machine. Another catheter is placed into the ascending aorta. If the ascending aorta is calcified or for some other reason cannot be used, this catheter will be placed in the femoral or axillary artery. Blood is returned to the patient from the CPB machine through this arterial catheter. Once the catheters are placed, the CPB machine slowly begins to remove blood from the patient. The patient’s response is monitored and catheter placement checked. With the catheters in place and the CPB machine oxygenating the blood, the aorta is clamped to stop blood flow. At this point, the heart is stopped and the lungs deflated to perform the surgery. Cardioplegia A concentrated electrolyte solution is infused into the heart to cause cardiac arrest (cardioplegia). This solution is infused either through a catheter placed in the aortic root (cardioplegia moves forward into the coronary arteries—called antegrade) or through a catheter in the coronary sinus (cardioplegia moves backward into the coronary veins—called retrograde). This infusion may be repeated multiple times during the surgery to ensure that the heart does not beat during the surgery. Cardioplegia serves several functions. It keeps the heart muscle still to allow surgery to be more easily completed. It also greatly reduces the metabolic needs and oxygen requirements of the heart muscle, reducing the amount of ischemia during surgery. DURING BYPASS While the patient’s blood is being oxygenated via CPB, ventilations are stopped and the lungs are usually deflated. This expands the surgical field and allows the surgeon more access to the heart. During this time, oxygenation is controlled by the perfusionist. Adequate anticoagulation must be maintained throughout the period the patient is on CPB. Blood Pressure During CPB CPB maintains a steady blood pressure; there is no systolic–diastolic pulsatile flow. The arterial line, used to measure blood pressure, registers a mean arterial pressure (MAP), not a systolic or diastolic pressure. During CPB, it is important to keep the MAP high enough to perfuse the brain, kidneys, and other organs. The perfusionist works with the anesthesiologist to keep this pressure between 60 and 80 mmHg. If the patient had preexisting renal insufficiency, the MAP during CPB will be kept closer to 80 mmHg. The kidneys are especially sensitive to low blood pressures and do not tolerate the nonpulsatile flow of CPB as well as other organs. Similarly, if the patient has known atherosclerosis of the carotid or cerebral arteries, the MAP will need to be kept higher to perfuse the brain during surgery. Coagulopathy CPB can cause bleeding or clotting problems during and after surgery. Despite advances in making the tubing used for CPB more biocompatible, the blood still reacts to contact with this foreign substance. Platelets are activated, placing the patient at high risk for clotting. This is the reason anticoagulation is essential during CPB. As the blood mixes with the fluid used to prime the CPB machine, platelets and clotting factors are diluted. In addition, platelets may be destroyed as they move through the pumping mechanism of the CPB machine. This becomes important after surgery, when the patient is at high risk for bleeding. Systemic Inflammatory Response to CPB When a patient’s blood comes in contact with the components of the CPB machine, the complement system and several other proinflammatory pathways are activated. This produces a systemic inflammatory response, which typically lasts for several days after surgery. Mechanical trauma caused by the oxygenator can contribute to the inflammatory response. The endothelial lining of the arteries senses the inflammatory markers released by the complement system, leading to endothelial dysfunction. In addition, factors related to the operative field and the CPB oxygenator can produce particulate emboli, consisting of red cell debris, platelet aggregates, fibrin, fat, and foreign material. The majority of large emboli (greater than 40 micrometers) are removed by the filter in the CPB circuit, but smaller emboli may obstruct capillaries and result in ischemic cell death. The combination of systemic inflammatory response and cell death results in increased capillary permeability (leaking capillaries), vasodilation (which can lower blood pressure and systemic vascular resistance), interstitial and peripheral edema, and organ dysfunction. In some patients, organ dysfunction is subclinical, but in patients with a larger inflammatory response or when there is less functional reserve, organ dysfunction may be clinically significant (Moorjani et al., 2014). The risk of damage to organs increases the longer the patient remains on CPB. FAST FACTS in a NUTSHELL 
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