The development of cardiopulmonary bypass (CPB) has enabled operative treatment for countless numbers of patients with a multitude of acquired and congenital cardiac and vascular anomalies. The CPB circuit is a unique device designed to divert a patient’s circulation around the area that requires surgical repair so that surgeons can operate in a virtually bloodless field. The typical CPB circuit consists of a series of blood pumps that provide cardiopulmonary support through disposable components consisting primarily of tubing, a blood collection reservoir, and an oxygenator (Fig. 61-1). The deoxygenated venous blood is pumped through the oxygenator and is returned to the arterial system of the patient. The first successful open heart surgical procedure using an extracorporeal CPB machine was performed in 1953 by John Gibbon.1 Since then, there have been dramatic improvements to the device itself, to the components required for its application and to the techniques and strategies used to apply it, and to the monitoring, while safety systems have been designed to enable safe and consistent results, with steadily improving patient outcomes. This chapter will provide a general overview of the CPB circuit components [extracorporeal circuit (ECC)] and a brief discussion of strategies used to apply this technology in the field of pediatric cardiac surgery.
Deoxygenated blood is diverted from the systemic atrium to the collection reservoir via a venous cannula. Although CPB can be simply established using a single venous cannula placed in the right atrium, most pediatric operations require direct or indirect (via the right atrium) systemic (or bicaval) venous cannulation. As the situation warrants, venous drainage may be achieved through peripheral cannulation of one or more of the large peripheral veins. This is particularly true for larger children, in which femoral vessels might be sizeable. For smaller children and infants presternotomy cannulation of the internal jugular vein in the case of a truly hostile mediastinum might be an option for venous drainage. The diameter of the venous cannula/e depends on the size of the patient, the number of cannulae, and the desired CPB blood flow rates for the particular child and operation. Venous drainage is normally facilitated by gravity, but may be assisted by adding low-level vacuum, allowing in turn for the use of smaller venous cannulae. The vacuum is applied to the venous reservoir in the circuit, and should not exceed a negative pressure of 40 mm Hg. The potential drawback for vacuum assisted venous drainage is air entrainment and hemolysis through excess negative pressure; when applied appropriately, this risk of such complications is nevertheless minimal.2,3
The venous cannulae are connected to tubing that provides a path for blood volume to be diverted to a reservoir. The tubing is made of polyvinyl chloride, causing foreign surface activation of blood components that requires systemic anticoagulation with heparin to avoid clotting of the CPB circuit. Coated tubing, lined with heparin or a biologically active surface, is currently available and is used by many centers in an attempt to mimic the vascular endothelium and minimize the activation of clotting and inflammatory pathways. Tubing is available in many sizes, ranging from ⅛″ to ½″ inner diameter. Selecting the proper size is based on the requirements for calculated CPB blood flow rates.
There are two common types of blood pumps available: roller pumps and centrifugal pumps. Roller pumps consist of two roller heads placed 180 degrees apart inside a raceway. Tubing is placed between the roller heads and the raceway and, as the rollers spin, blood is propelled forward in the same direction as the rotation of the roller head. Roller pumps require partial occlusion of the tubing to ensure blood flow in the proper direction. Pump output is measured by the speed of rotation and the diameter of the tubing. Centrifugal pumps have a rotating impeller in the blood compartment that propels blood forward. Centrifugal pumps are nonocclusive, and are therefore dependent on the amount of volume available on the inflow side and the resistance on the outflow side. Pump output is measured by a flow probe placed distal to the outlet of the pump. Both pump systems can be monitored and regulated with integrated safety devices to avoid air emboli and overpressurization of the ECC.
The oxygenator is the component of the circuit that allows for blood to upload oxygen and unload carbon dioxide. In modern circuits, the blood chamber or surface is separated from gas surface by a polypropylene or silicone membrane (see also Chapter 87). The gas flow is regulated by a blender that enables manipulation of oxygen content, and the speed at which the gas passes through the membrane is known as “sweep speed.” This creates a pressure gradient for both oxygen and carbon dioxide that facilitates gas exchange via diffusion across the membrane, similar to the alveolar capillary interface within the lung. Inhalational anesthetic can be delivered to the patient by including a vaporizer, inserted inline with the ventilation gas.
Most oxygenators have an integral heat exchanger that enables manipulation of the patient’s core temperature. This is particularly important in neonates and small children, in which hypothermia is still widely utilized. Water lines are attached from a heating and cooling device that allow for precise and efficient temperature control. Maintaining proper temperature gradients during cooling and warming is important to avoid bubble formation. Overheating blood beyond 40°C carries the risks of severe deleterious effects, such as enzyme and protein inactivation.
Most venous reservoirs contain a depth filter to remove gross particles and facilitate air bubble purging. Once the blood has passed through the oxygenator, the inclusion of an arterial line filter is common. The arterial line filter is a screen with a defined pore size to remove particulate and gaseous emboli before the blood is returned to the patient via the arterial cannula. The arterial cannula is typically placed in the ascending aorta, but can be placed in a multitude of sites depending on the pathology and type of repair planned. Other common sites include the axillary or subclavian or right brachiocephalic artery (sometimes through a Goretex© graft), or the femoral artery (also occasionally through a graft). In particular cases (interrupted aortic arch or certain variants of hypoplastic left heart syndrome), the aortic return line may be split, to allow simultaneous proximal and duct-dependent perfusion or selective antegrade cerebral perfusion during aortic arch reconstruction. Proper selection of the arterial cannula size is based on patient size, calculated CPB blood flow rates, and number of cannulae required.
To allow surgical repair, it is usually necessary to induce diastolic chemical arrest by using a high potassium solution (cardioplegia). This solution can be crystalloid-based, blood-based, or a combination of the two, for instance, given at 4:1 blood to crystalloid ratio. Although institutional variance is great, most pediatric cardiac centers utilize cold cardioplegia, which maximizes suppression of myocardial metabolism. Typical cardioplegia delivery systems consist of a bubble trap, a heat exchanger, and a pressure monitor. When the heart is isolated from the systemic perfusion by placement of the aortic cross clamp, cardioplegia can be delivered directly within the aortic root with a separate catheter (antegrade). Alternatively, in cases with aortic valve regurgitation, cardioplegia can be infused directly within the coronary ostia. Experience with retrograde (via the coronary sinus) delivery of cardioplegia in pediatric patients is not ubiquitous, but reported. There is also great variability between centers in the timing of redosing of cardioplegia during cardioplegic arrest. This interval ranges from redosing every 30 min (with a lower volume) to the performance of long and complex repairs with a single upfront dose of cardioplegia. This is particularly applicable with crystalloid or very diluted (1:4 blood to crystalloid ratio) solutions.4,5
The left ventricle (LV) receives blood throughout CPB from the bronchial circulation via the pulmonary veins and vents placed in the left side of the heart can keep the heart empty and the surgical field more “bloodless.” This is particularly true for patients with profound and persistent cyanosis, in whom development of aortopulmonary collaterals is prominent. In order to keep the heart empty, vents are occasionally placed into the left atrium or LV, most commonly through the right superior pulmonary vein (SPV). In very small neonates and infants, insertion of a vent in the right SPV might result in stenosis, or might be unfeasible because of the size of the vessel. Venting can be accomplished through the left atrial appendage, in a retrograde fashion via the pulmonary artery or through a patent foramen ovale, or atrial septal defect (ASD). The surgeon must individualize the venting strategy according to the specific lesion and clinical situation. For example, in the correction of transposition of the great arteries (see Chapter 74) venting through the SPV during cooling or cardioplegic arrest might be unnecessary if an ASD is present or if the neonate has undergone a preoperative balloon atrial septostomy. When the heart is reperfused, venting becomes necessary when the atrial communication has been closed. Occasionally, venting is accomplished through the pulmonary artery or the LV apex.
Vents are connected to the CPB circuit using one of the cardiotomy suction tubes. The cardiotomy suction tubes are small lengths of tubing that are designed to return blood, via a roller pump, back to the CPB circuit. Once the patient is heparinized, blood loss in the surgical field can be removed and returned to the venous reservoir. This diminishes blood loss by returning “shed blood” to the circuit where it can recirculate. Some centers will also return shed blood using a “cell-saver” device, which does not return the blood to the venous reservoir, but functions instead as a suction device. Blood is collected in a chamber from which it can be processed to eliminate the serous component. Blood from the cell-saver often has a hematocrit in excess of 70 percent or greater and is reinfused to minimize the need for blood transfusion. The cell saver can be used regardless of whether the patient is anticoagulated or not, and is often used prior to heparinization and then after administration of protamine. Minimizing the length of tubing and miniaturization of the CPB circuit, as well as priming the circuit with patient’s own blood prior to instituting CPB have enabled many centers to avoid blood transfusion altogether, even in small patients.
