Cardiopulmonary bypass (CPB), which originated in the mid-twentieth century, was designed to allow for the repair of congenital heart defects. Its history has since been characterized by perpetual technological advancements that have been instrumental in sustaining the momentum of clinical progress of this field.
Because of the morbidity associated with the “time on pump,” many early surgeries were performed at profoundly hypothermic temperatures by using circulatory arrest.
The current philosophy underpinning the use of pediatric CPB is to meet the metabolic demands of the patient throughout the repair while minimizing the impact of associated nonphysiologic effects.
All aspects of CPB have experienced major technological improvements. Circuits are miniaturized and cause less blood trauma, blood component therapy is highly directed, and on-pump patient monitoring techniques have advanced.
The progress of pediatric CPB has played a major role in the steady reduction of morbidity and mortality associated with cardiac surgery in children. Pediatric mortality rates are now comparable to those in adult patients.
Surgery for congenital heart disease has evolved into a relatively safe intervention considering its brief history and countless hurdles. This historical journey is, of course, filled with triumphs and tragic failures, telling a story of progressive intuition and challenges steadily surmounted. This has culminated in the generally successful model that is used today ( Table 35.1 ). The early years of cardiac surgery spawned many novel techniques for operations that did not rely on cardiopulmonary bypass (CPB) as used today. Surgeons initiated their efforts in cardiovascular surgery with attempts to repair extracardiac vascular anomalies such as patent ductus arteriosus and coarctation of the aorta. On August 26, 1938, at the Boston Children’s Hospital, Dr. Robert Gross performed the world’s first successful patent ductus arteriosus closure on a 7-year-old girl. Soon, exposing the heart and attempting to correct life-threatening cardiac defects became a reality. In the early 1950s, surgeons began to explore several different approaches to repairing intracardiac defects. One technique, popularized by Dr. F. John Lewis, used total body hypothermia and vena cava inflow occlusion to achieve direct visualization of atrial septal defects. Although this technique proved to be fairly safe for simple atrial septal defects, failure was often the result when more complex defects were attempted. , Surgeons needed a way to safely perfuse the patient’s circulatory system and extend the “safe” surgical time. In the late 1930s, Dr. John Gibbon and his wife Mary, a nurse and research assistant, began developing a heart-lung machine to do just this. By the early 1950s, Dr. Gibbon, in an interesting collaboration with International Business Machines Corporation (IBM), reported promising success in the laboratory using a heart-lung machine on cats and dogs. After a previous fatal attempt to repair an atrial septal defect (ASD) in a 15-month-old child in February 1952, Dr. Gibbon successfully closed an ASD in an 18-year-old patient using his heart-lung machine on May 6, 1953. Unfortunately, Dr. Gibbon was not able to repeat the same success with the heart-lung machine on subsequent cases, and his next four patients died. Other surgical teams devised their own versions of CPB but were unable to replicate laboratory successes, and no other human survivors were reported. It was theorized that perhaps these hearts were too sick to be repaired and that it was unrealistic to expect that these hearts could recover. CPB became a widespread disappointment, and most investigators abandoned the technique. While others were reporting their attempts using the heart-lung machine, however, Dr. C. Walton Lillehei and his colleagues at the University of Minnesota introduced a new approach for supporting patients during surgery: controlled cross-circulation. During cross-circulation, the patient’s parent was used as the “heart-lung machine” and supported the patient during the operation ( Fig. 35.1 ). Considering the potential for a 200% operative mortality, this was a highly controversial technique. However, using this method, Dr. Lillehei was able to effectively close an ASD on March 26, 1954. Dr. Lillehei and his colleagues continued a remarkable series of successes using cross-circulation by performing 45 operations for anomalies that included ventricular septal defect, atrioventricular canal, and tetralogy of Fallot, with an operative mortality of only 38%. This progress with more complex lesions prompted investigators to rethink their options for supporting, repairing, and recovering these patients. Two surgical camps ignited the resurgence of the artificial heart-lung machine: Dr. Lillehei and his colleagues at the University of Minnesota and Dr. John Kirklin and his colleagues at the nearby Mayo Clinic. Dr. Kirklin and colleagues reported a 50% mortality among eight patients using a modification of the Gibbon-IBM pump oxygenator in the spring of 1955. Months later, Lillehei and colleagues reported a 29% mortality among seven patients using their own heart-lung machine and the groundbreaking DeWall Bubble Oxygenator. These two groups demonstrated that surgical repair of complex congenital defects could be performed in a more controlled environment than cross-circulation or inflow occlusion, with promising results. What followed were many groups initiating open-heart programs primarily addressing congenital heart disease. Despite significant improvements in survival rates, congenital cardiac repairs remained a daunting undertaking with significant risk. Bypass circuits were enormous when compared with the patient blood volume, the systemic response was an extreme shock, and the understanding of the physiologic response to this “nonphysiologic” extracorporeal circulation was quite limited. Investigators sought to use CPB but limit the actual cumulative time that nonphysiologic blood flow is provided to the patient—with its attendant risk. The bypass circuit could be used to cool the patient down to profound hypothermia after a lengthy period of topical cooling. The circulation of the patient could then be safely terminated for lengthy periods of time, allowing for complex cardiac repairs. At the conclusion of the repair, the heart-lung machine could be used to fully warm the patient. These hypothermic circulatory arrest techniques with limited periods of extracorporeal circulation were popularized in the early 1970s by Dr. Barratt-Boyes and proved to dramatically extend the “safe” period of support. Surgeons began to perform increasingly complex congenital heart repairs. Pediatric cardiac surgical care was further refined over the subsequent several decades. The development of smaller, more efficient, and customizable heart-lung machine hardware and components, as well as improvements in myocardial protection, have allowed surgical teams to move away from the concept of limited CPB and toward a more “full-flow” philosophy wherein the metabolic demands of the body are continuously met while the patient is on the heart-lung machine. This chapter explores the concepts that form the basis of this philosophy and the techniques that surgical teams currently use to support pediatric patients during cardiovascular surgery.
|1938||Patent ductus arteriosus ligation||Gross|
|1944||Blalock-Taussig shunt||Blalock, Taussig|
|1947||Closed pulmonary valvotomy||Sellors|
|1948||Atrial septectomy||Blalock, Hanlon|
|1951||Pulmonary artery band||Muller, Dammann|
|1952||Atrial septal defect closure using atrial well||Gross|
|1952||Atrial septal defect closure using hypothermia||Lewis|
|1953||Atrial septal defect closure using cardiopulmonary bypass||Gibbon|
|1954||Ventricular septal defect closure using cross-circulation||Lillehei|
|1958||Superior cavopulmonary shunt (Glenn shunt)||Glenn|
|1958||Senning operation for transposition of great arteries||Senning|
|1963||Mustard operation for transposition of great arteries||Mustard|
|1968||Fontan procedure for tricuspid atresia||Fontan|
|1975||Arterial switch for transposition of great arteries||Jantene|
|1981||Norwood procedure for hypoplastic left heart syndrome||Norwood|
|1985||Neonatal heart transplantation||Bailey|
The surgical team consists of highly trained specialists, each of whom plays a vital role in the safety and success of the surgical procedure. This specialized team is led by the cardiac surgeon and typically includes an assistant surgeon or physician assistant, anesthesiologist, perfusionist, and several nurses, surgical scrub technologists, anesthesia assistants, and perioperative surgical assistants.
A perfusionist is a healthcare professional who specializes in all aspects of extracorporeal circulation. The primary focus of a perfusionist is to support the cardiac surgical patient during CPB. Because of this, the perfusionist’s clinical expertise is a critical component of operative success. Perhaps the first perfusionist was Mary Gibbon, Dr. Gibbon’s wife. In addition to helping design the Gibbon-IBM heart-lung machine, she assembled and operated it as well. The term perfusionist did not emerge until the early 1970s; in the early days of cardiac surgery, surgical groups would typically use any locally available combination of physiologists, biochemists, cardiologists, or surgical residents to help operate the heart-lung machine. Now, cardiovascular perfusionists are highly trained, nationally certified (Certified Clinical Perfusionist), state-licensed allied health professionals. The common scope of practice for a perfusionist consists of CPB, extracorporeal membrane oxygenation (ECMO), isolated limb/organ chemoperfusion, ventricular assist devices, autotransfusion, and intraaortic balloon counterpulsation.
Equipment and preparation for cardiopulmonary bypass
Heart-lung machine console and pumps
The CPB machine, commonly referred to as the heart-lung machine , is the mechanical hardware that a perfusionist uses to support the patient during surgery. Until the late 1950s, the CPB hardware and circuitry were typically handmade, and many of the components had to be handwashed and sterilized for reuse. The hardware components were designed at that time with two objectives: to pump blood through the patient’s cardiovascular system and to successfully perform respiratory gas exchange, hence, the term heart-lung machine . Unfortunately, this heart-lung apparatus was large, difficult to move, had no safety features, and was not available to other institutions eager to operate. Surgeons interested in these handcrafted devices would often visit the surgical groups at the University of Minnesota and Mayo Clinic, but few could replicate their expensive and intricate systems. Eventually, industry developers began to commercially release heart-lung machines with hardware components consolidated onto a wheel-mounted console. Interestingly, although cardiac surgery began with the pediatric patient population, heart-lung machines were developed as one-size-fits-all units and were not customizable for smaller patients.
Modern heart-lung machine consoles are mobile, offer many pump configuration options, are loaded with safety features, and seamlessly send intraoperative CPB data to the electronic medical record. These design improvements allow for better configuration options for the pediatric surgical population. An ideal heart-lung machine for pediatric CPB is customizable for circuit miniaturization and offers safety devices and hardware that accommodate both smaller tubing sizes and circuitry. Customizations such as mast mounting pumps in various configurations and incorporating mini-roller pumps with shorter raceway lengths are two popular heart-lung machine configurations. ,
Several different types of mechanical pumps have been used to substitute the function of the heart; interestingly, the roller pump has remained a standard pump mechanism since the beginning of CPB. A roller pump functions by positive fluid displacement. Tubing is placed in a curved raceway; as occlusive rollers rotate over the compressible tubing, blood is pushed forward, creating a continuous nonpulsatile flow. The flow output is controlled by changing the revolutions per minute (RPMs) of the pump. Roller pumps are the most commonly used arterial (heart) pump in pediatrics ( Fig. 35.2 ). While roller pumps are used as the arterial pump, the heart-lung machine console also holds several other roller pumps used for cardiotomy field suction, venting the heart, and cardioplegia delivery.
The centrifugal pump is another type of arterial pump that has gained significant popularity since the mid-1970s. A centrifugal pump uses an impeller cone and rotational kinetic energy to propel the blood. Because it is nonocclusive, it is thought to be safer and cause less hemolysis than roller pumps. Centrifugal blood flow is controlled by the impeller cone RPMs and is also dependent on preload and sensitive to resistance distal to the pump. Because the pump is not occlusive, any resistance or occlusion will result in a reduction or cessation of flow. These pumps require the use of a flow probe to measure actual flow; the nonocclusive property is considered a safety feature in the event of cannula obstruction or accidental arterial line occlusion. The use of centrifugal pumps during ECMO has become increasingly popular owing to the suggested hemolytic and safety benefits; however, these benefits have often been refuted. Roller pumps remain the main arterial pump type in pediatric CPB because they are simple, inexpensive, and, importantly, require a much smaller prime volume than centrifugal pumps.
Cardiopulmonary bypass circuit
The handmade circuits used on children in the mid-1950s were elaborate, and the large blood volume required to prime them was a burden on the blood bank. Perfusionists would have to spend the evening of surgery assembling the circuit and then tackle the tedious task of dismantling, rewashing, and sterilizing the same circuitry after surgery. Fortunately, manufacturers now offer a wide variety of disposable circuit components that are fairly simple to assemble. The modern CPB circuit is a series of components consisting of cannulas, tubing, venous reservoir, filters, oxygenator, heat exchanger, hemoconcentrator, suction, and cardioplegia delivery system.
Deoxygenated blood from the superior vena cava (SVC) and inferior vena cava (IVC) travels down a venous line, usually pulled by simple gravitational siphon effect, and into a venous reservoir. The deoxygenated blood in the reservoir is pumped through the oxygenator and then back to the patient’s aorta (or other major artery) via the arterial line ( Fig. 35.3 ). This blood pathway diverts blood away from the heart and lungs, creating a bloodless operative field. In the adult patient population, where the circuit prime volume is typically no greater than 25% of the patient’s blood volume, a single circuit size can be used for almost all patient sizes. The small circuit prime-to-patient blood volume ratio helps to minimize patient hemodilution during CPB, ultimately reducing the likelihood of donor blood exposure. The same adult circuit, with a prime volume of about 1 L, would be approximately 500% of the circulating blood volume in a neonate. This discrepancy would seem outrageous considering current circuit options, but the prime-to-blood volume ratio was even higher before manufacturers began to release pediatric oxygenators in the mid-1980s. Since the oxygenator is one of the largest volume components of the CPB circuit, any significant reduction in size would result in large prime volume reductions. A circuit miniaturization movement began—the new clinical challenge in pediatric CPB was to reduce both circuit prime volume and surface area. The goal of circuit prime and surface area reduction is to minimize hemodilution and the deleterious effects of foreign surface blood contact activation. Strategies such as using smaller diameter and shorter tubing lengths and incorporating neonatal and pediatric CPB components have allowed clinicians to reach this goal. At Children’s Health Dallas, the perfusionists have made many circuit modifications to achieve a static prime volume of approximately 165 mL in our neonatal circuit. This prime volume lowers the circuit size to approximately 45% of the blood volume of a 3-kg patient ( Fig. 35.4 ).
An oxygenator, the artificial lung of the CPB circuit, might be considered the most important component of the circuit. It is responsible for oxygen (O 2 ) and carbon dioxide (CO 2 ) gas exchange, as well as volatile anesthetic administration. A heat exchanger, used for cooling and warming the perfusate—and, hence, the patient—is housed inside the oxygenator. Certain newer models now integrate the arterial filter, to reduce particulate matter, into the oxygenator. A venous reservoir, which includes both venous line and cardiotomy suction filters and various ports for drug and fluid administration, is typically packaged with an oxygenator. Currently, hollow fiber membrane oxygenators, which fully separate the blood flow from gas flow by a thin polymer membrane, are used during CPB. A brief history of oxygenator development reveals much about some of the important engineering solutions that have allowed for cardiac surgery to be performed more safely in progressively smaller patients.
The first oxygenators used in the early days of cardiac surgery were hardware units that either used rotating discs or large mesh screens. These oxygenators worked by creating a large surface area film of blood, either over rotating discs in a pool of venous blood or trickling over large mesh screens and exposing the film of blood to an oxygenated atmosphere. , Though these units were successful at oxygenating blood, they required extremely large priming volumes; were not disposable; were difficult to assemble, operate, and clean; and lost significant efficiency during hemodilution. In addition to these disadvantages, these oxygenators were not commercially available to clinicians looking to operate beyond the University of Minnesota and Mayo Clinic.
The University of Minnesota team dramatically changed this landscape in the late 1950s by releasing the simple, disposable, inexpensive, and commercially available DeWall-Lillehei bubble oxygenator. Though the safety of actively adding bubbles to the blood was debated, the commercial availability of this device contributed to a rapid global expansion of cardiac surgery. The bubble oxygenator was a distinct improvement over the previous unwieldy direct blood contact oxygenators, yet it was still limited in that the direct blood-air interface could produce significant blood trauma. This trauma accrues over time; thus, the safety margin for longer pump runs was diminished for longer, complex cases.
The next generation of oxygenators, membrane oxygenators, better mimicked the function of the lungs. These microporous, gas-permeable membranes eliminated direct contact between gas and blood, thus, reducing blood trauma. The concept of a microporous membrane separating the gas and blood was sound, but it took decades of research to find a suitable membrane material before these oxygenators could replace bubble oxygenators commercially. Initial success with silicone membranes was observed with long-term support during ECMO. However, in the operating room, these membranes proved to be less efficient and prone to plasma leakage and thrombus formation. , The development and release of polypropylene microporous membranes allowed for efficient gas exchange over a wide range of temperatures and pump flow rates, replacing the bubble oxygenator during CPB in the mid-1980s. In these oxygenators, CO 2 and O 2 flow meters and a gas blender control gas and volatile anesthetic flow through the inside of the hollow polypropylene fibers. The gas within the hollow fibers passively diffuses into the blood flowing on the outside of the fibers.
In 1985, Cobe released the popular Variable Prime Cobe Membrane Lung (VPCML) designed for the pediatric market. This oxygenator was divided into separate compartments and gave clinicians three maximum blood flow options, 1.3, 2.6, and 4.0 L per minute (LPMs) depending on which compartments were opened. The VPCML also tried a new concept with the heat exchanger. Once a separate external CPB component, the stainless-steel heat exchanger was placed inside the venous reservoir. The stainless-steel coil wrapped around the inside of the reservoir was not efficient unless a large amount of volume was held in the reservoir. This was counter to the efforts to reduce the overall circuit prime volume. Despite this shortcoming in the VPCML model, the move toward integration and consolidation of functionalities continued. These heat exchangers are now integrated within the oxygenator housing. Considering that pediatric cardiac surgery is more likely than adult surgery to use moderate to deep hypothermia, these heat exchangers need to be extremely efficient with a small surface area.
As technology relentlessly improved, membrane hollow fibers were wrapped into tighter configurations. This eventually allowed for a priming volume low enough to release a dedicated neonatal oxygenator. In 2006, Dideco released the first neonatal oxygenator with a prime volume of 31 mL and maximum rated flow of 700 mL/min. The new generation of neonatal and pediatric oxygenators achieves much higher maximum flow rates while keeping prime volumes appropriate for neonates. This has allowed clinical teams to achieve consistent physiologic outcomes after pump runs in neonates and small infants. A modern pediatric device such as the Terumo Baby FX oxygenator with integrated arterial filter (Terumo Cardiovascular Group) offers a low total prime volume and a high maximum blood flow ( Fig. 35.5 ). With arterial filter integration, this oxygenator has a total prime volume of 43 mL and a maximum rated blood flow of 1.5 LPMs. This low prime oxygenator is suitable for neonates but also accommodates patients up to approximately 15 kg. This wide range of blood flow and low prime improves the likelihood of bloodless surgery—wherein an asanguineous prime is used—for the larger patients in range for this device. The Maquet Quadrox-i Neonatal oxygenator (Maquet Holding) is another oxygenator with an integrated arterial filter that has a 40-mL total prime volume and 1.5-LPMs maximum flow. When considering the additional volume of an external arterial line filter, this high-efficiency oxygenator with an integrated filter offers the lowest total prime volume unit on the market today. Current trends in oxygenator design and development include integration of the arterial line filter, biocompatible surface coatings for circuit tubing, decreasing flow resistance, and more efficient heat exchange.
The tubing used to connect the various components of the CPB circuit to the patient is made of a medical-grade polyvinyl chloride. Tubing length and diameter are the two main factors to consider when designing a circuit. Shorter tubing with the smallest internal diameter will reduce prime volume, but the tubing must also be large enough to safely manage required blood flows and line pressures for a given patient. In the past, ¼-, 3⁄8-, and ½-inch tubing were the only tubing options, which made circuit miniaturization a difficult task. Currently, a wide range and selection of pediatric tubing and connector sizes are available. Tubing sizes such as 1⁄8, 3⁄16, and ¼ inch have become the new standards in pediatrics. Changing the internal diameter of tubing affects blood flow resistance and must not impede venous drainage or arterial blood flow. At our institution, we select arteriovenous line sizes that accommodate gravity venous drainage and do not exceed an arterial line pressure of 350 mm Hg ( Table 35.2 ). Large reductions in tubing length have been made possible by positioning the smaller new-generation pump consoles close to the patient and using mast mounted pumps to bring components closer together. Also, smaller-diameter venous line tubing may be used to further reduce the prime volume, but vacuum-assisted venous drainage (VAVD) must be used to augment the gravity siphon. The bioreactivity of blood coming into contact with artificial surfaces, such as tubing, is known to exacerbate the systemic inflammatory response and disrupt hemostasis. A major advancement has been the development of surface coatings that attempt to mimic the endothelial surface of blood vessels. These coatings have been shown to attenuate the increase of cytokines and inflammatory markers and preserve platelets. , When selecting tubing for the pediatric circuit, the goal is to safely achieve maximum blood flows, decrease prime volume, and attenuate blood trauma.
|TUBING SPECIFICATIONS||CHILDREN’S HEALTH DALLAS PROTOCOL|
|Internal Diameter (in)||mL/ft||mL/rev||Maximum Arterial Flow (mL/min)||Maximum Gravity Drainage (mL/min)|
A hemoconcentrator is an ultrafiltration device that consists of semipermeable membrane fibers that remove plasma water and solutes. They function similarly to hemodialysis units but are simpler in that they do not require a dialysate solution. Blood flows through microporous membrane fibers, and since the hydrostatic pressure is higher inside the membrane fibers, effluent fluid permeates the membrane and can be removed. The membrane pore sizes are typically less than 55,000 Da, which preserve plasma proteins such as albumin (65,000 Da) and maintain the colloid oncotic pressure. The ultrafiltration rate of a hemoconcentrator is dependent on the hydrostatic pressure gradient across the membrane, blood flow rate through the membrane fibers, membrane pore size, and the hematocrit. Ultrafiltration is useful for increasing hematocrit, reducing high potassium levels after cardioplegia delivery, and removing harmful inflammatory mediators. Hemodilution during pediatric CPB is difficult to avoid; a 2004 survey of pediatric cardiac surgery centers reports that 98% of perfusionists routinely use a hemoconcentrator during CPB.
The CPB circuit is primed with a crystalloid replacement fluid. Common solutions include Plasma-Lyte A, lactated Ringer, and Normosol-R. Lactated Ringer is a replacement fluid that contains 29 mEq/L of lactate but lacks magnesium. Plasma-Lyte A and Normosol-R both closely mimic human physiologic plasma electrolyte concentrations, osmolality, and pH. However, these two solutions do not contain calcium. At Children’s Health Dallas, the perfusionists use Plasma-Lyte A because it does not contain lactate or calcium. This allows the perfusionists to lower CPB perfusate calcium levels, which is desirable, as is discussed later.
Once the CPB circuit is primed with a crystalloid solution and cleared of any air, the total prime volume of the circuit is estimated. The perfusionist must then choose between initiating CPB with or without adding heterologous blood. Unlike the adult patient population, blood products are often added to the neonatal and pediatric CPB circuits due to the small patient blood volume-to-circuit prime volume ratio. The dilutional effect of the crystalloid prime is determined by calculating the patient resultant hematocrit (HCTr). The HCTr formula, HCTr = (Patient blood volume × HCT)/(Patient blood volume + Circuit prime volume), is calculated once the patient hematocrit value is measured in the operating room before surgery. The institutional protocol at Children’s Health Dallas is to maintain a CPB HCTr above 30%. If that value cannot be reached, then packed red blood cells (PRBCs) are added to the circuit. The institutional protocol also directs that a half unit of fresh-frozen plasma, approximately 100 mL, will be added to the circuit prime for all patients less than 6 kg.
The pre-CPB circuit prime drug additives at our institution include heparin (1000 U/mL), 8.4% sodium bicarbonate, 20% mannitol, furosemide (10 mg/mL), methylprednisolone, tranexamic acid, and 25% albumin ( Table 35.3 ). The ideal prime solution should be “physiologic” and attempt to attenuate the adverse response to artificially supporting a patient with an extracorporeal circuit.
|Heparin||Anticoagulant||Calculated by Medtronic HMS and varies per patient|
|Sodium bicarbonate||Buffer||Achieve pH 7.40|
|Mannitol||Osmotic diuretic; oxygen radical scavenger||0.5 mg/kg; 12.5 g maximum dose|
|Furosemide||Loop diuretic||0.25 mg/kg; 20 mg maximum dose|
|Methylprednisolone||Corticosteroid||30 mg/kg; 1 g maximum dose|
|25% Albumin||Plasma protein||10% circuit prime volume|
|Tranexamic acid||Antifibrinolytic||20 mg/kg; 20 g maximum dose|
Due to the foreign surface contact and resultant intrinsic activation of the coagulation cascade, the patient must be anticoagulated before CPB. Heparin is the most widely used anticoagulant during CPB. It acts by super-activating antithrombin III (ATIII), which then inactivates thrombin and other proteases involved in coagulation. Heparin is used because it is fast-acting, and anticoagulation reversal can easily be achieved by administering protamine. Anticoagulation helps prevent circuit thrombus formation and avoid the devastating effects of potential arterial thromboembolism. Heparin was the anticoagulant used during Dr. Gibbon’s first successful cardiac surgery in 1953; its use during CPB has continued for more than 60 years. Before heparin administration and dosing protocols were available, anticoagulation methods were cumbersome and unsafe. The dosing was empiric, and the only methods for testing heparinization were lengthy laboratory heparin concentration tests. Fortunately, the activated clotting time (ACT) test was introduced in 1966—this bedside whole blood test became the foundation of how heparinization is monitored in the cardiac operating room today. Traditional laboratory tests such as partial thromboplastin time (PTT) and prothrombin time (PT) are sensitive to low doses of heparin and therefore are not useful during CPB. The ACT test is a point-of-care test that measures the time (in seconds) needed for activated whole blood to form thrombin. In 1975, Bull et al. reported a heparin management approach using the ACT test, and the technique quickly became universally accepted. The report describes the heparin dose-response curve technique and suggests an optimal ACT range of 480 seconds during CPB. In this technique, ACT tests are run on various whole blood samples containing different heparin concentrations and results are plotted versus the heparin concentration. The heparin dose-response curve, commonly referred to as the Bull curve, demonstrates the individualized ACT response to different levels of heparinization and is a useful tool in estimating the concentration of heparin necessary to achieve an ACT of 480 seconds ( Fig. 35.6 ). Maintaining ACT results of at least 480 seconds during CPB remains the standard of care today. Though most clinicians will agree that 480 seconds is acceptable during CPB, there is debate regarding whether that value should be universally applied considering that not all ACT analyzers operate and activate blood in the same manner.
Pediatric patients undergoing cardiac repair suffer disproportionate postoperative bleeding complications after CPB, likely because of their size and immature coagulation system. Contributing factors to postoperative bleeding are dilution of coagulation factors during CPB, induction of the systemic inflammatory response, hematologic changes in cyanotic patients, hypothermia, and numerous coagulation factor deficiencies. All these situations can inhibit adequate anticoagulation with heparin and ultimately lead to the generation of thrombin. It has been shown that prolonged ACT results of pediatric patients poorly correlate with the plasma levels of heparin during CPB. Reports have shown that pediatric patients require higher plasma heparin concentrations than adults because they metabolize heparin faster, have a larger blood volume-to-body weight ratio, and have lower ATIII levels. , Therefore, weight-based heparin doses and ACT monitoring used with adult patients are not recommended for use in pediatric patients.
The potential variability of a pediatric patient’s response to heparin necessitates an individual dosing regimen and the use of different coagulation tests. A useful bedside hemostasis management tool in pediatric cardiac surgery is the Hepcon HMS PLUS (Medtronic Inc.). The Hepcon HMS PLUS is fully automated and is used to run the following tests: ACT, heparin dose response (HDR) to identify individual heparin needs, and heparin-protamine titration (HPT) to verify heparin concentration. A baseline sample is collected from the arterial line before heparinization and is used to test the HDR. The HDR test determines the baseline ACT and patient response to increasing amounts of heparin. Results are used to identify heparin-resistant or heparin-sensitive patients and determine the patient heparin concentration needed to achieve appropriate anticoagulation. To test the blood heparin concentration with the HPT test, blood is added to tubes containing different mg/mL concentrations of protamine. Heparin and protamine bind in a 1:1 ratio; thus, the tube that produces a clot can be used to determine the unit/mL heparin concentration. The HPT test is used frequently during CPB to maintain heparin concentrations suggested by the HDR and is run post-CPB to verify proper heparin reversal after protamine administration. Without heparinization, hemodilution and the degree of hypothermia alone could extend the ACT beyond 480 seconds; this effect is amplified in the pediatric patient. Administering heparin to maintain a patient heparin concentration calculated by the HDR, despite an ACT greater than 480 seconds, will help to reduce consumptive coagulopathy, thrombin generation, fibrinolysis, neutrophil activation, and the need for transfusions. ,
Once the patient is ready to be cannulated for CPB, a heparin bolus is administered to the patient by the anesthesiologist into an intravenous line or by the surgeon directly into the right atrium. In general, the patient receives a 400 U/kg dose of heparin minutes before arterial cannulation. Once the heparin has circulated within the patient for approximately 5 minutes, a blood sample from an arterial or intravenous line is used to run an ACT and HPT. If the ACT reaches 480 seconds or the HPT confirms an adequate heparin concentration, cardiotomy pump suction may be used, and CPB may be initiated when the surgeon inserts the arterial and venous cannulas. ACT and HPT tests are run every 30 minutes during CPB, and heparin is administered if the ACT falls below 480 seconds or the heparin concentration falls below the maintenance value calculated by the HDR. If a parameter is low, the Hepcon HMS PLUS uses a formula based on the HDR, blood volume of the patient, and circuit prime volume to calculate the amount of heparin needed to adequately raise the ACT or HPT.
Cannulation refers to the process in which the surgeon attaches the venous limb of the CPB circuit to the systemic venous circulation of the patient while attaching the arterial limb to the systemic arterial system of the patient. This is most commonly accomplished by placing an arterial cannula in the distal ascending aorta and venous cannulas in the SVC and IVC, respectively. The cannulas are inserted through appropriately sized purse-string sutures and secured with tourniquets. This bicaval configuration allows for the achievement of “total” CPB; the vast majority of cardiac repairs can be accomplished using this technique. In pediatric cardiac programs, patients ranging in weight from approximately 1000 g up to adulthood are placed on CPB. Therefore, a wide range of cannula sizes must be kept in stock. Arterial cannulas range from as small as 8 Fr (2.67 mm) in diameter up to over 20 Fr. Venous cannulas for CPB are available in straight and angled varieties and range down to as small as 10 Fr. Inserting these cannulas into the diminutive aorta and venae cavae of neonates is a taxing technical exercise that must be accomplished without complication in order to appropriately support the patient during the repair and leave the patient with undamaged vessels at the cannulation sites postoperatively.
Exceptions to standard bicaval cannulation are frequently seen in pediatric practice. First, patients can have anomalies of systemic venous return, such as bilateral SVCs, ipsilateral hepatic veins, or an interrupted IVC with azygous continuation to the SVC. All these anomalies have to be assessed, and an appropriate venous cannulation strategy must be devised. Occasionally, if these anomalies are prohibitive for selective cannulation or the overall patient size is so small that the venae cavae are too small to cannulate, the right atrial appendage is cannulated in isolation and periods of circulatory arrest, wherein venous return is not required, are used to accomplish intracardiac portions of the repair.
Alternatives to standard ascending aortic cannulation are also used. In order to accomplish aortic arch reconstructions without resorting to circulatory arrest, a small prosthetic vascular tube graft is anastomosed to the innominate artery and the arterial cannula is inserted into this “chimney” graft. Alternatively, if the patient is large enough, the innominate artery can be cannulated directly. These innominate artery cannulation techniques allow the brain to be perfused up the right carotid artery while the aortic arch is being repaired.
Reoperations are common in congenital heart surgery. A number of these patients have pulmonary outflow conduits that are densely adherent to the sternum. Patients with transposition of the great arteries have an abnormally anteriorly located ascending aorta that can also be adherent to the chest wall in the midline. Peripheral cannulation via a femoral artery is sometimes necessary in these instances. Peripheral arterial cannulation in children should be performed only when absolutely necessary and converted to the ascending aorta as soon as possible. The obturation of the femoral artery by the cannula almost always causes hypoperfusion of the lower extremity. With longer cannulation times, the leg can be at significant risk for ischemic complications.
Pediatric vs. adult considerations
Although many of the management techniques governing pediatric and adult CPB are similar, several differences do exist ( Table 35.4 ). The small size of the pediatric patient and nature of the surgical repair often expose these patients to moderate or deep hypothermic temperatures, wide ranges of perfusion flow rates, and hemodilution. These management techniques represent extreme shifts from normal physiologic parameters, and the harmful effects are potentially more pronounced in these small patients. Low-flow perfusion or circulatory arrest at deep hypothermia (15–20°C) is often required because of the complexity of the repair, significant aortopulmonary collateral blood flow returning to the operative field from the pulmonary veins, or simply because position of the perfusion cannulas interferes with access to the surgical site.