Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Anesthesia for Children Undergoing Heart Surgery
Preoperative Evaluation
Congenital heart disease (CHD) accounts for nearly one-third of major congenital abnormalities, with an estimated worldwide birth prevalence of 9.1 cases per 1000 live births. In the United States, this represents the birth of 40,000 babies with CHD each year. Although CHD can occur in isolation, it is often associated with other cardiovascular and extracardiac malformations. The incidence of CHD is increased in the presence of other congenital abnormalities, in children with chromosomal disorders such as trisomy 21, as well as in siblings of other children with CHD. As diagnostic techniques have improved, many children with CHD are diagnosed antenatally or early postnatally. In association with improvements in diagnostic ability, surgical techniques, and perioperative care, most centers have shifted their practices to definitively repair the defects earlier, with many undergoing corrective surgery as neonates. Overall, about one-half of all children with CHD undergo cardiac surgery in the first year of life, and about 25% undergo surgery in the first month of life.
The perioperative management of children with complex cardiac defects requires a dedicated team of surgeons, cardiologists, anesthesiologists, intensivists, perfusionists, and nurses. Professionals caring for these children are challenged by some of the greatest physiologic aberrations encountered in clinical medicine. The anesthesiologists responsible for the care of these children require a comprehensive understanding of cardiac anatomy, physiology, and pathophysiology and must be able to adapt to each nuance of rapidly changing pathophysiology as it is encountered.
In addition to treating children with CHD, the pediatric cardiac anesthesiologist may also be responsible for the care of adults with CHD, whose underlying cardiac problems differ substantially from those in children. The success of pediatric cardiac surgery has resulted in an ever-increasing population with “grown-up CHD” (GUCHD), as most children with CHD are now expected to survive into adulthood. The ideal approach for this group of patients is to care for them in specialist units. Although these GUCHD centers are increasing in number and capacity, they are currently unable to provide universal coverage. In the meantime, the care of these patients falls to the most qualified physicians, including the pediatric cardiac anesthesiologist. Along with their underlying CHD, these patients often present with comorbidities of old age, thus presenting additional challenges.
When assessing children with complex cardiac defects, we rely to a large extent on echocardiography and magnetic resonance imaging (MRI) to acquire diagnostic data. Although fewer children are subjected to diagnostic angiography today, more interventional cardiac catheterization procedures are being performed. Many conditions such as patent ductus arteriosus (PDA), atrial septal defects (ASDs), and ventricular septal defects (VSDs) that would previously have been treated surgically are now treated in the angiography suite by interventional cardiologists. Other interventions include dilating arteries with balloon catheters with and without stents and coiling of aberrant or excessive collateral vessels. The pulmonary artery is commonly balloon dilated and stented, and coarctation of the aorta is treated similarly by balloon dilation. Stenotic valves are also commonly dilated. These procedures have led to the risk of patients being transferred emergently from the angiography suite to the operating room. For the individual child, there has been a dramatic decrease in morbidity as increasing numbers of conditions are treated in the angiography suite, but the risks of complications that occur in the angiography suite have increased as more complex procedures are performed (see Chapter 22 ).
The Preoperative Visit and Evaluation
The preoperative visit is an important part of the overall management of anesthesia for children with CHD. The preoperative visit has several aims:
- ▪
Medical assessment
- ▪
Prescribing premedication
- ▪
Providing information
- ▪
Creating a relationship with the child and family
- ▪
Formulating an anesthetic plan
Medical Assessment
The anesthesiologist must have a clear and detailed understanding of the cardiac anatomy and pathophysiology, the surgery to be undertaken, as well as any associated congenital abnormalities or medical conditions. The medical assessment includes collation of information from the history, physical examination, and review of imaging and laboratory data. Most diagnostic information is obtained from the medical record. Particular attention should be paid to the echocardiographic, angiographic, MRI, and other imaging data; the chest radiograph; and the electrocardiogram. Many centers have joint cardiac conferences where decisions about treatment are discussed in a multidisciplinary forum. Reports from these meetings are valuable in the preoperative assessment.
In addition to gathering this specific diagnostic information, a directed history and physical examination should be performed to assess the overall condition of the child. Attention should focus upon assessing the presence and degree of cardiac failure, cyanosis, or risk of pulmonary hypertension. Information about previous surgical procedures should also be sought, as it may alter access to the central circulation and placement of invasive monitors. The general nutritional state of the child should be assessed; poor growth and development may be a sign of severe CHD. Other information should be sought that may have a bearing on the anesthetic plan. For example, repeat surgery and redo sternotomy may indicate the need to establish peripheral cardiopulmonary bypass (CPB) before surgically accessing the heart and great vessels for central CPB cannulation. This has a bearing on line placement because either jugulo-carotid or femoro-femoral bypass may be required; the appropriate area should be preserved for CPB cannulation and avoided for line placement. If the child received aprotinin within the preceding 12 months, another dose should not be given because the risk of anaphylaxis is increased within this period (Trasylol package insert: Bayer Pharmaceuticals Corporation, West Haven, CT. December 2006).
The type of surgery to be performed is important. For example, if a Blalock-Taussig shunt is placed on the left, the arterial line should not be placed in the left arm because the trace will be lost or distorted during subclavian cross-clamping. If a superior cavopulmonary anastomosis (Glenn shunt) is planned, a short internal jugular catheter can be useful to monitor pulmonary artery pressure, but it should be removed early in the postoperative period so as not to risk the formation of thrombosis in the superior vena cava (SVC), with its potential disastrous consequences.
Good veins should be sought and marked for the application of local anesthetic cream. This is useful in sick children even if an inhalational induction is planned because it allows placement of a venous cannula during a very light plane of anesthesia and avoids myocardial depression from large concentrations of inhalation anesthetics.
Prescribing Premedication
The use of sedative premedication can be useful, but this practice varies widely. Numerous medications and routes of administration may be used, and ample recommendations exist, but the use of premedication is often dictated by local preferences and not always evidence-based. There is heightened awareness and increasing concerns about the possibility of postoperative behavioral problems resulting from inadequate preparation and handling the uncooperative child preoperatively. It is important for of the pediatric anesthesiologist to reduce perioperative anxiety in children by both nonpharmacologic and pharmacologic methods. While prescribing premedication is best assessed on an individual basis, some general considerations apply to most children who present for pediatric heart surgery. Premedication for infants younger than 6 months of age is usually unnecessary. Premedication for older, healthy children who show little anxiety and with whom good preoperative rapport can be established is also often unnecessary. However, older children, particularly those who have undergone previous surgery, have fears about anesthesia and surgery. Although it is important to address their fears, sedative premedication may play a pivotal role in achieving adequate anxiolysis for parental separation and a smooth induction. Premedication in children with severe congestive heart failure is probably best used judiciously, if at all, as the effects of the usually prescribed doses may be unpredictable. On the contrary, children with dynamic obstruction to the left or right ventricular outflow tracts often benefit from sedative premedication because crying and struggling during induction may worsen obstruction. Cyanotic children (e.g., those with tetralogy of Fallot [TOF]), may develop increasing cyanosis if agitated during induction. However, it is important to monitor cyanotic children after premedication and provide supplemental oxygen as needed because they exhibit a blunted ventilatory response to hypoxia. In the United States, supplemental premedication is sometimes administered under the direct supervision of the anesthesiologist in the preoperative facility, providing for a calm child and gentle separation from the parents. In the United Kingdom, where induction of anesthesia takes place in a dedicated anesthesia room, parents are present until after the induction, often making additional premedication unnecessary.
The most common premedication is oral midazolam (0.5-1.0 mg/kg). However, the effect of midazolam may be unpredictable as it may cause paradoxical reactions, with agitation and dysphoria instead of anxiolysis and sedation. Numerous other medications including ketamine, clonidine, temazepam, chloral hydrate, and dexmedetomidine have been effective premedications in children with CHD.
Giving Information
Providing information to the parents and to the child in a manner that is nonthreatening and appropriate to the child's age and developmental stage is a key element of the preoperative visit. This information includes the use of sedative premedication, fasting times, the type of induction, the type and likely position of invasive lines, the need for a stay in an intensive care unit (ICU) postoperatively, and the expected length of that stay. The use of other monitors such as transesophageal echocardiography (TEE) should be outlined and any contraindications identified, along with the probability that a blood transfusion may be necessary. Questions about the risk of anesthesia and surgery should be addressed to the satisfaction of the parents (see Chapter 4 ).
Creating Rapport With the Child and Family
By creating a good relationship with the family, the anesthesiologist can reduce the anxiety of the child and the parents. The family develops a sense of trust, which can improve their hospital experience. A good rapport with the child may also facilitate a smoother anesthetic induction, and the use of specific nonpharmacologic techniques of reducing perioperative anxiety can be tailored to the child's individual preferences and possible previous experiences.
Formulating an Anesthetic Plan
After assessing the child, it is possible to formulate a detailed anesthetic plan. The anesthesiologist should have acquired a complete understanding of the child's heart defect and its hemodynamic consequences, as well as any comorbidities. The detailed anesthetic plan consists of a choice of anesthetic agents, techniques, ventilatory management and inotropic/vasoactive support to attain a set of appropriate hemodynamic goals for the individual patient.
Upper Respiratory Tract Infection and Cardiac Surgery
Otherwise healthy children undergoing elective noncardiac surgery in the presence of an upper respiratory tract infection (URI) are more likely to suffer respiratory complications ( Table 17.1 ). These complications typically are minor, are easily managed, and usually result in minimal morbidity ; the decision to proceed with noncardiac surgery in a child with a URI is made on an individual basis (see Chapter 4 ).
TABLE 17.1
Diagnosis of Upper Respiratory Tract Infection
Data from Schreiner MS, O'Hara I, Markakis DA, Politis GD. Do children who experience laryngospasm have an increased risk of upper respiratory tract infection? Anesthesiology 1996;85:475–480.
| At least two of the following signs plus confirmation by a parent: |
|
The decision to proceed with cardiac surgery in children with a URI may be difficult. Although children with cardiac failure are prone to multiple URIs, they may also have signs that can mimic URIs. Surgery may be relatively urgent, and postponing surgery could increase the risk to the child. Cardiac surgery in children with URIs is likely to increase the duration of stay in the ICU and prolong the duration of mechanical ventilation, although overall hospital stay may not be prolonged. Proceeding with surgery increases the incidence of atelectasis and postoperative bacterial infections. However, neither the mortality rates (4.2% with URIs vs. 1.6% without URIs) nor long-term sequelae in children with URIs who undergo cardiac surgery are significantly increased. The children with URIs were significantly younger and smaller, which may account in part for the greater but statistically insignificant increased mortality rate; this should be taken into consideration when contemplating whether to proceed with surgery. Children who are scheduled for a Glenn shunt or completion of the Fontan circulation may be at particular risk because an increase in pulmonary vascular resistance (PVR) can adversely affect surgical outcome. It is prudent to postpone surgery in a child with a URI who is scheduled for elective cardiac surgery. If the surgery is urgent, discussion with the surgical team is required to correctly assess the risks and benefits to the child.
Perioperative Challenges in Pediatric Cardiac Anesthesia
Cyanosis
Cyanotic children compensate for chronic hypoxia with increased erythropoiesis, increased circulating blood volume, vasodilation, and metabolic adjustments of factors such as the circulating concentration of 2,3-diphosphoglycerate (2,3-DPG). These changes facilitate greater delivery of oxygen to tissues. The increase in blood viscosity with polycythemia increases vascular resistance and sludging, which may result in renal, pulmonary, and cerebral thromboses, especially in dehydrated children. Long periods without oral intake preoperatively and postoperatively should be avoided in children with polycythemia, unless adequate intravenous (IV) hydration is provided.
PVR increases more than systemic vascular resistance (SVR) when the hematocrit increases, further decreasing pulmonary blood flow in children who already have a compromised pulmonary circulation. Coagulopathies are common in children with cyanotic CHD and may adversely affect surgical hemostasis. Furthermore, chronic hypoxemia can cause important changes in vascular function and structure, some of which are maladaptive and probably contribute to impaired cardiovascular performance. When the hematocrit exceeds 65%, excessive viscosity impairs microvascular perfusion and outweighs the advantages of increased oxygen-carrying capacity. Reduction of red blood cell volume can correct the coagulopathy and improve hemodynamics when increases in hematocrit are extreme. However, treatment of hyperviscosity in patients with cyanotic heart disease is controversial ; guidelines for managing adults with CHD suggest the judicious use of phlebotomy and address the issue of potential complications.
Intracardiac Shunting
In CHD, much of the pathophysiology involves communications between chambers or vessels that are normally separate, resulting in shunting of blood between ventricles, atria, the great arteries, or a combination of these, depending on the nature of the lesion. Management of shunting during anesthesia is a major concern that requires an understanding of the factors that control shunting.
Restrictive and Unrestrictive Shunts
When communications are small, the size of the defect limits shunting and considerations of relative PVR and SVR become correspondingly less important in determining the degree of shunting. When there is a large pressure differential at the same level of the circulation on either side of a communication, the communication is restrictive. Flow is limited across the defect, and other factors that determine shunt flow become less important. This is usually the situation in children with mild heart disease that is asymptomatic or minimally symptomatic, such as small ASDs and VSDs or a small PDA.
Dependent Shunting During Anesthesia
In children with dependent shunts, the direction and degree of intracardiac shunting are determined by the circulatory dynamics. Control of circulatory dynamics to minimize the shunt is a major goal of anesthesia management. Because shunting depends on the relationship between SVR and PVR, anesthesia management often revolves around control of relative vascular resistances.
In children with dependent right-to-left shunts, the shunt increases when SVR decreases or PVR increases. In children with dependent left-to-right shunts, the shunt increases when SVR increases and PVR decreases. In children with bidirectional or balanced shunting, changes in vascular resistance increase the net shunt away from the side with increased vascular resistance.
For practical purposes, acute increases in left-to-right shunts during anesthesia are of clinical importance in several situations. A substantial steal of systemic blood flow by the pulmonary circulation can occur in conditions with unrestrictive, significant, left-to-right shunting such as atrioventricular (AV) canal, truncus arteriosus, and hypoplastic left heart syndrome. Left-to-right shunting is well tolerated, except when pulmonary steal leads to systemic hypotension, increasing acidosis from poor systemic end-organ perfusion or insufficient coronary perfusion. Shunting from right to left, because it is accompanied by at least some degree of arterial oxygen desaturation, is more frequently a problem during anesthesia.
Impaired Hemostasis
Hemostasis is impaired after bypass in infants and children to a greater extent compared with adults. The initiation of CPB triggers contact activation of the hemostatic systems, with ongoing coagulation and fibrinolysis, as well as the initiation of a systemic inflammatory response, both contributing to the coagulopathy. In infants and children, these effects are further compounded by a larger size of the CPB circuit relative to patient size. In this patient population, impaired hemostasis after bypass results from a combination of immature coagulation factor synthesis, hemodilution after bypass, and a complex interaction involving consumption of clotting factors and platelets. At birth, the levels of vitamin K–dependent coagulation factors in healthy, full-term neonates are only 40% to 66% of adult values. During the first month of life, these levels increase to 53% to 90% of adult values (see also Chapter 2 ). However, in children with CHD, especially those with cyanosis or systemic hypoperfusion, coagulation factors often continue to be depressed owing to impaired hepatic protein synthesis. Although antithrombin III levels are also low, true heparin resistance is rare in infants because of parallel decreases in coagulation factors.
At the onset of CPB, the introduction of the prime volume, which is two to three times greater than the child's blood volume, dilutes the clotting factors, particularly fibrinogen to 50% and platelets to 30% of their prebypass values. This degree of dilution occurs even when the pump circuit is primed with whole blood. Greater dilution may occur when packed red blood cells (PRBCs) are used in the priming volume. At the conclusion of neonatal bypass, the activity of clotting factors is often extremely low, the fibrinogen concentration is frequently less than 100 mg/dL, and the platelet count is only 50,000 to 80,000/mm 3 . In addition to these quantitative changes, functional changes in the platelets occur during bypass. Extracorporeal circulation causes a loss of platelet adhesion receptors, activation of platelets, and formation of leukocyte-platelet conjugates. Platelet adhesion receptors in cyanotic children are depressed to a greater extent than in those with acyanotic cardiac defects. Heparin also impairs platelet function independent of CPB.
Cardiac surgery is associated with significant activation of the fibrinolytic system. Inadequate heparin concentrations during CPB may also contribute to postoperative bleeding because inadequate anticoagulation may allow continued activation of the hemostatic pathways. Ongoing activation of the coagulation cascade causes the consumption of platelets and clotting factors. The standard measurement of anticoagulation for bypass, the activated clotting time (ACT), shows a poor correlation with heparin concentrations (usually measured using the surrogate, anti-Xa) in children undergoing CPB. In one study, the use of individualized heparin monitoring and heparin titration was associated with larger doses of heparin but smaller doses of protamine for antagonism. Activation of the clotting cascade using that heparin-protamine regimen is also reduced, thus potentially decreasing bleeding in the postoperative period. As a result of this multifactorial coagulopathy, blood loss is a greater problem in children than in adults and is a particular problem in neonates and small infants (see Chapter 19 ).
Strategies to Reduce Bleeding After Bypass
In an effort to normalize factors and platelets to effective concentrations, some medical centers use fresh whole blood in the cardiopulmonary circuit prime. In adult patients and in an in vitro aggregation study, transfusion of fresh whole blood provided equal or greater hemostatic and functional benefit when compared with transfusion of platelet concentrates. In children, transfusion with fresh whole blood less than 48 hours from harvest reduced the blood loss compared with transfusion of reconstituted whole blood (e.g., packed erythrocytes, fresh frozen plasma [FFP], and platelets). Other studies have shown that the use of fresh whole blood in the prime in neonatal and pediatric cardiac surgery reduced transfusion requirements and improved outcomes. However, the benefits of using fresh whole blood to prime the CPB circuit have been questioned in at least one study, which showed no advantage from its use and an increased length of stay in the ICU, as well as increased perioperative fluid overload in the group treated with fresh whole blood. It is possible that different pediatric patient populations (e.g., age, cyanotic vs. noncyanotic CHD) derive different benefits from the use of fresh whole blood. Until this matter is clarified, it is difficult to make clear recommendations about its use in pediatric cardiac surgery. Moreover, fresh whole blood is often difficult to obtain. The units must be refrigerated for 24 to 48 hours while donor screening is performed, and storage causes significant platelet injury. Insistence on fresh whole blood places tremendous pressures on the transfusion service and donor center to coordinate the matching of donor types with recipient needs. Furthermore, in the presence of suitable, simpler alternatives to this blood component management strategy, it is likely that a considerable number of centers will continue to use individual blood component administration to treat bleeding and coagulopathy in children undergoing heart surgery.
Therefore individual component therapy remains the standard of practice in most institutions. In neonates and small infants with dilutional coagulopathy, platelets should be given in combination with cryoprecipitate to correct the defect in clotting. An initial dose of platelets (10 mL/kg) may need to be repeated. Platelets are usually administered if bleeding persists and the platelet count is less than 100,000/mm 3 . Cryoprecipitate contains high concentrations of fibrinogen, factor VIII, von Willebrand factor, and factor XIII. Fibrinogen and von Willebrand factor are required for platelet adhesion and aggregation to occur. Platelet adhesion and aggregation are the fundamental first steps in primary hemostasis (see Chapters 10 and 12 ). The subsequent step of platelet degranulation switches on the entire coagulation cascade and cannot take place without adhesion and aggregation. Administration of FFP, which is not evidence-based for this type of coagulopathy, may excessively dilute the red cell mass and platelets.
Transfusion guidelines have been described for adults and have been shown to reduce postoperative bleeding and transfusion requirements. Although similar guidelines have not been as forthcoming for children in whom the practice frequently seems to be more empirical, there is a growing body of evidence that point-of-care (POC) monitoring of hemostasis is useful to guide specific blood component therapy in children undergoing heart surgery. In a surgical context, the time it takes to return routine coagulation tests is often too long for clinical decision making. Consequently, POC platelet count and viscoelastic monitoring of coagulation such as thromboelastography and thromboelastometry are being increasingly used to make timely informed decisions about blood product administration. The use of transfusion algorithms in pediatric cardiac surgery has reduced blood product requirements and bleeding ; however, further work is needed to produce well-validated guidelines for monitoring and treating bleeding in children undergoing heart surgery.
Antifibrinolytics
The antifibrinolytics used in pediatric cardiac surgery include aprotinin, ε-aminocaproic acid (EACA) and tranexamic acid (TXA). EACA and TXA are lysine analogs that reduce bleeding after cardiac surgery in adults and children, with apparent similar efficacy and safety. Doses of EACA and TXA for pediatric cardiac surgery have yet to be clearly established. Furthermore, in view of the disproportion between circulating blood volume and CPB prime volume, a drug target concentration that differs between neonates and children, as well as other differences in pharmacokinetics, suggest that different dosing schemes should be used in neonates and smaller children compared with older children.
Aprotinin is a serine protease inhibitor no longer available in many countries, and with only very limited availability in others, after its marketing license was withdrawn because of safety concerns. In fact, despite having been studied thoroughly in adults, its use remains a cause for great concern. Early evidence demonstrated that aprotinin reduces bleeding, reduces the time taken to extubation, shortens ICU stay, and reduces overall mortality rates. However, subsequent studies have contradicted these earlier findings. The same volume of evidence has not been published for children, although several studies suggest that it is effective in reducing bleeding and that it reduces the duration of postoperative mechanical ventilation. An increased risk of renal failure or stroke in adults undergoing revascularization surgery has been reported. The same investigators reported an increase in the 5-year mortality rate for adults after the use of aprotinin in revascularization surgery, mostly resulting from stroke and myocardial infarction. Aprotinin has increased the 30-day mortality rate by as much as one-third compared with TXA or EACA. However, it appears that the early data regarding increased death rates have not been supported by a subsequent study and that the benefits may outweigh risks in specific populations. While aprotinin use in adults is still surrounded by significant controversy, taking into account the differences in pathophysiology and underlying risk factors, it seems plausible that data about increased mortality from stroke and myocardial infarction have only limited relevance for the pediatric population. Similarly, it appears unlikely that the risk of renal failure associated with the use of aprotinin in children is the same as it is in adults, despite some concerns about its propensity to cause acute kidney injury (AKI) in children, which another study failed to demonstrate. An important additional safety consideration pertains to the risk of severe hypersensitivity reactions. The reported incidence of side effects in children varies. Even though anaphylaxis seems to be infrequent in pediatric patients after primary exposure, the risk of such a severe reaction is increased after reexposure, particularly if it occurs within 12 months after the most recent prior aprotinin exposure. This has led the manufacturer to issue a black box warning for aprotinin reexposure within 1 year of a prior exposure. The Food and Drug Administration (FDA) has also recommended that aprotinin should be administered only in the operative setting when CPB can be started quickly, in the event of a severe reaction. Uncertainty about the relative safety profiles of aprotinin and the lysine analogs is met with similar considerations about the effectiveness of these different drugs. Whereas there is some evidence that EACA or TXA are at least as effective as aprotinin, research has also suggested that aprotinin use may decrease the output of the chest drain, effect superior blood-sparing effect, as well as confer differences in other outcomes such as cytokine activation or early indexes of postoperative recovery. In fact, it appears that aprotinin may have unique antiinflammatory properties, which may benefit pediatric patients. Further research is needed to clarify issues concerning safety and relative effectiveness of the two classes of drugs, as well as proving benefits and improve effective dosing schemes in specific patient populations (see Chapter 20 ).
Topical Agents
The use of topical agents to promote clot formation and reduce bleeding after cardiac surgery is common. The most frequently used topical agents are fibrin sealants. Fibrin sealants mimic the stages of the blood coagulation process. Unlike the synthetic adhesives, they are biocompatible. Fibrin sealants are usually sourced from plasma components, and most contain virally inactivated human fibrinogen and thrombin with different quantities of factor XIII, antifibrinolytic agents, and calcium. When the fibrinogen and thrombin are mixed during the application process, the fibrinogen is converted to fibrin monomers. This results in the formation of a semirigid fibrin clot. By mimicking the later stages of the coagulation process, these sealants stop bleeding and assist in wound healing. They have significantly reduced bleeding in children undergoing heart surgery.
Ultrafiltration
Ultrafiltration is a process that results in the production of an ultrafiltrate by means of convection forces and a hydrostatic pressure gradient across a semipermeable membrane. Thus free water and low–molecular-weight substances are removed from a child during and after CPB. It provides many benefits, including increasing the hematocrit, concentrating the clotting factors and platelets, increasing blood pressure, reducing PVR, and removing inflammatory mediators in the ultrafiltrate. It has significantly reduced bleeding after cardiac surgery in children.
Desmopressin
Desmopressin acts by increasing plasma concentrations of factor VIII and von Willebrand factor (see also Chapters 10 and 12 ). It has been effective in reducing bleeding after CPB in adult cardiac surgery and its use is indicated in specific subgroups of patients. Unfortunately, studies in children failed to demonstrate a similar effectiveness in reducing bleeding or transfusion requirements.
Anesthesia Management for Surgery Requiring Cardiopulmonary Bypass
Monitoring
Noninvasive monitoring during pediatric cardiac surgery includes pulse oximetry, five-lead electrocardiography, an automated blood pressure cuff, a precordial or esophageal stethoscope, continuous airway manometry, inspired and expired capnography, anesthetic gas and oxygen analysis, multiple-site temperature measurement, and volumetric urine collection. The pulse oximeter is especially important when managing children with congenital cardiac disease. At least two probes should be placed on different limbs in the event that one fails during the procedure. In children with cyanotic heart disease, conventional pulse oximetry overestimates arterial oxygen saturation as saturation decreases ; this error tends to be exacerbated in the presence of severe hypoxemia. When monitoring children with a shunt across the ductus arteriosus, a probe should be placed on the right upper limb to measure preductal oxygenation, and a second probe should be placed on a toe to measure postductal oxygenation (children with a right-sided aortic arch may require the probe to be placed on a left upper limb). Children undergoing repair of coarctation of the aorta should be monitored with a pulse oximeter on the right upper limb, because it may be the only reliable monitor during the repair, and blood pressure cuffs should be placed before and after the coarctation. These two cuffs may be cycled and the differential documented before and after surgical correction.
Monitoring end-tidal carbon dioxide tension (P etco 2 ) is of value in most children. However, in children with cyanotic-shunting cardiac lesions, the P etco 2 measurement may be less reflective of Pa co 2 because of ventilation-perfusion mismatching. Arterial blood gases are the most accurate measure of the adequacy of ventilation and oxygenation. To provide rapid decision making, it is helpful to have the blood gas analysis machine located in or near the cardiac operating room.
Monitoring ionized calcium concentrations is essential during surgical procedures in which significant quantities of citrated blood are infused rapidly or when entire blood volumes are replaced. Neonates are particularly prone to disturbances in their ionized calcium concentration when citrated whole blood, FFP, or platelets are infused. Those with limited cardiac reserve tolerate ionized hypocalcemia poorly because of their greater sensitivity to the myocardial effects of citrate infusion (see Chapter 12 ). In isolation, the total serum calcium concentration is misleading.
Temperature monitoring during CPB is a critical guide to adequate brain cooling and to appropriate rewarming before separation from bypass. Because it is not practical to measure brain temperature directly, surrogate measuring sites including the tympanic membrane, nasopharyngeal, and rectum have been used. The nasopharyngeal site most closely matches true brain temperature and is the site at which temperature is most often monitored. The tympanic and rectal sites tend to overestimate the brain temperature. Measurement of skin temperature gives an indication about peripheral perfusion and provides information about adequate peripheral rewarming.
After induction of anesthesia, an arterial catheter should be placed in children who will undergo CPB. The radial artery may be percutaneously cannulated with relative ease, even in infants. In neonates, the femoral arteries are frequently used for arterial access, and the axillary arteries may also be used. The radial, femoral, and axillary arteries all seem to constitute suitable sites for arterial cannulation and invasive blood pressure monitoring, with complication rates similar to those in adults. The brachial artery is generally avoided because it is an end artery, lacking collateral circulation, although one retrospective series of 200 children reported complication rates similar to other arterial sites. Catheters placed in the dorsalis pedis or posterior tibial artery often provide inaccurate hemodynamic data, especially after separation from bypass, and it may become difficult to sample blood for laboratory testing. In the rare circumstance that peripheral arterial cannulation cannot be accomplished percutaneously, consideration should be given to obtaining arterial access by the cutdown method; alternatively, the surgeon may place a catheter in the internal mammary artery after sternotomy, and a sterile monitoring line may be passed over the drapes.
Central venous catheters are very useful for both central venous pressure monitoring and as a safe, reliable route for the administration of inotropes or vasopressors as well as potentially venoirritant solutions. For cardiac surgical procedures, there are two commonly used methods of obtaining central access. The decision of which to use may be determined in part by institutional bias. In the first method, the cardiac surgeons expose the heart quickly and have it available for inspection and estimation of filling pressures. Central lines can be readily established from the surgical field and handed off to the anesthesia team. These transthoracic central lines are useful but carry a small amount of risk. The second method is percutaneous insertion of central venous lines via the subclavian or internal jugular vein. This route is particularly useful for long, complex procedures, especially when access to the infant is limited or the heart is not exposed. It is important to appreciate that the internal jugular or subclavian route may fail or be associated with pneumothorax, hemorrhage, and hematoma formation after puncture of major arteries. Cannulation of the external jugular vein may avoid some of these serious complications when the catheter can be successfully threaded into the central circulation. Increasingly, ultrasound-guided techniques are being used to establish central venous access (see also Chapter 49 ). In the United Kingdom, the use of ultrasound for the placement of these lines is recommended by the National Institute of Clinical Excellence (NICE); ultrasound is used routinely for the placement of central lines.
In children with unrestrictive VSDs or ASDs, including hearts with a single ventricle or single atrium, central venous pressure is identical to left ventricular filling pressure. Cannulation of vessels that drain into the SVC should be approached with caution in children with univentricular anatomy who may undergo the Fontan procedure, because thrombosis of the SVC can be a devastating complication. In these children, the femoral veins may be the preferred sites for central venous access. Left-sided central venous lines in the SVC territory should also generally be avoided in cardiac patients. There is a greater risk of erosion and perforation from central venous catheters placed through the left internal jugular or left subclavian veins. Furthermore, in up to 10% of patients with CHD, these veins join a persistent left SVC that most often drains into the coronary sinus or left atrium, both undesirable locations for a central venous catheter tip.
Percutaneously inserted pulmonary arterial catheters in children with intracardiac defects usually provide information that is not substantively different from that of a simple central line, are difficult to insert without fluoroscopy, and may not provide meaningful measurements of cardiac output. As a result, they are rarely used in pediatric cardiac patients. In circumstances that would be deemed useful, it is probably preferable to insert them surgically. In some complex CHDs and procedures in which postoperative left ventricular dysfunction is expected, it may be valuable to have continuous monitoring of pressures in the left heart. Such measurements are usually obtained via an LA pressure monitoring line inserted by the surgical team.
Transesophageal Echocardiography
Use of perioperative echocardiography has become the standard of care in the United States for both adults and children undergoing heart surgery. In adult practice, anesthesiologists usually perform the transesophageal echocardiography (TEE), but in children, the TEE is more commonly performed by a pediatric cardiologist. This may reflect the increased complexity of congenital lesions and the difficulty in accurately assessing these lesions and their repairs. TEE is cost-effective since its use can have a significant impact on surgical and medical management. In one study, a second bypass run was undertaken in 7.3% of cases based on the findings of the TEE, surgical alteration in the management in 12.7% and medical alteration in 18.5% of cases. Pediatric cardiac anesthesiologists usually can perform TEE before and after bypass if they have received adequate training.
The introduction of small probes with multiplane capability has greatly increased the use of TEE, even in infants and neonates. In 1999, a survey of centers in the United States indicated that 93% used intraoperative echocardiography and that all but one used TEE. The American Society of Echocardiography and the Society of Cardiovascular Anesthesiologists have published guidelines for performing a comprehensive intraoperative TEE in adults and children.
Although the use of TEE in children is generally safe, complications do occur and may be more common in small infants. Complications include damage to the mouth, tongue, oropharynx, esophagus, and stomach. Other complications include hemodynamic disturbance as a result of compression of the left atrium or other structures; erroneous invasive blood pressure monitoring may result if the compressed structure is an artery proximal to the arterial line insertion site. Interference with the airway also occurs in a small number of cases. This includes inadvertent extubation, right main-stem bronchus intubation, and compression of the tracheal tube. However, the overall incidence is small, approximately 2%. Information gathered from the TEE examination takes place before and after bypass and may be divided broadly into two categories: hemodynamic assessment with monitoring and structural diagnostic information. Hemodynamic information includes information about ventricular function and filling. Diagnostic information relates to confirmation of preoperative findings and assessment of the surgical repair.
Near-Infrared Spectroscopy
Near-infrared spectroscopy (NIRS) allows real-time monitoring of tissue oxygenation. This technology is based on the principle of optical spectrophotometry, making use of the fact that body tissues are relatively transparent to light in the near-infrared wavelength range. The majority of NIRS monitors use reflectance-mode NIRS, in which a region underlying the sensor is interrogated by a transmitter optode and a receiving sensor. The value obtained is a reflection of the underlying heterogeneous tissue area, composed of arteries, veins, and capillaries, as well as other nonvascular tissues. Even though there are several reports of the applicability of this technology to monitor other tissue beds such as the renal and splanchnic circulations, cerebral NIRS has received the most attention in the context of pediatric cardiac surgery. This noninvasive monitoring is becoming widely used during CPB in children to assess the adequacy of oxygen delivery to the brain. This may lead to improved neurologic outcomes after cardiac surgery, although there is no clear evidence for target based NIRS values in humans. One algorithm suggested that a 20% drop from baseline bilaterally was significant and should trigger efforts to increase the cerebral saturation, such as optimizing the neck position, increasing mean arterial pressure, increasing arterial CO 2 , or increasing the hematocrit. If the change was unilateral, it may be related to incorrect aortic cannula positioning. To be most accurate, baseline readings should be undertaken prior to the induction of anesthesia as anesthesia itself may result in changes (see Chapter 52 ).
Induction of Anesthesia
In the United Kingdom, most children are anesthetized in an anesthesia induction room, which is a small room immediately adjacent to the operating room, and in most cases, the parents are present at the induction. Anesthesia is commonly induced while the child is sitting with or being held by a parent. It is possible to engage some older children to hold the mask themselves during the first stages of induction; alternatively, some parents can hold the mask for the child as he or she is anesthetized. After the child is asleep, he or she is transferred to the anesthetic trolley, where venous and arterial access is secured and the trachea is intubated. This contrasts with the practice in most centers in North America, where induction of anesthesia usually occurs in the operating room.
The method of induction, either intravenously or by inhalation, should be tailored to the child and the cardiac defect. When an IV induction is selected (e.g., mask induction is refused) but IV access appears to be difficult, ketamine may be given intramuscularly or orally to sedate the child during the attempts. Application of a local anesthetic cream such as EMLA (eutectic mixture of local anesthetics; AstraZeneca, Wilmington, DE) or Ametop Gel (Smith-Nephew, Mississauga, ON, Canada) also reduces the pain of injection. However, this requires close communication in that suitable veins should be identified during the preoperative visit and clear instructions are given to the parents or nursing staff regarding where and when the cream should be applied (1 hour for EMLA and 30 minutes for Ametop). When IV access is already present, an IV induction is preferred. In severely ill children, it is generally advisable to secure IV access before induction of anesthesia.
Sevoflurane is the most commonly used inhalational induction agent in children. Sevoflurane is very rapid acting and should be used with care in the child with CHD because high concentrations can produce bradycardia, hypotension, and apnea if not titrated carefully. Concentrations should be rapidly reduced after an adequate depth of anesthesia is achieved (remembering that the minimum alveolar concentration [MAC] in children is 2.5%) to limit myocardial depression. To facilitate establishing IV access when the concentration of sevoflurane must be restricted, application of local anesthetic cream is helpful because it allows cannulation at a much lighter plane of anesthesia. In children who are cyanotic with a right-to-left shunt and reduced pulmonary blood flow, inhalational inductions are slow. Moreover, in neonates and young infants with large right-to-left shunts, the desired depth of anesthesia may not be achieved; the end-tidal concentration does not accurately reflect the blood and brain partial pressures. Many include nitrous oxide during inhalational inductions for two reasons. First, it is odorless; therefore it can be started before the introduction of the sevoflurane to sedate the child before the stronger-smelling anesthetic is introduced. Second, it allows a smoother and more rapid induction compared with sevoflurane alone. Concentrations up to 70% nitrous oxide can be used to smooth induction of anesthesia even in cyanotic children, but the nitrous oxide should be replaced with air and oxygen or 100% oxygen as soon as IV access is obtained and a muscle relaxant is given. Some children do not want an inhalational induction out of fear of the mask. To address this problem, we put the mask aside and begin the induction by cupping our hands with the elbow of the breathing circuit between two fingers and slowly bringing our hands toward the face from under the chin (this gas mixture is heavier than air). It is important to warn the child about each event before it occurs (such as a mask applied to the face) and, when possible, to demonstrate the action on yourself, a parent, or a toy animal to avoid startling or scaring the child. Some children prefer to hold the mask themselves, or if the child is accompanied by a parent and unable to hold the mask, the parent may hold it. Good premedication often aids this process (see also Chapter 4 ).
For sick children in whom an IV induction is preferable, several options are available. In neonates, for example, those with coarctation of the aorta or with hypoplastic left heart syndrome who are not ventilated before coming to the operating room, one approach is to administer fentanyl in a dose of 2 to 3 µg/kg, followed by pancuronium and then by a very low dose (i.e., sedative dose) of sevoflurane or isoflurane. Fentanyl obtunds the hypertensive response to intubation, and the pancuronium maintains cardiac output by maintaining the heart rate. The very-low-dose inhalational agent provides the sedation or anesthesia. In older children, etomidate is an excellent choice as an induction agent, providing stable hemodynamics, although it does cause pain on injection. Ketamine is also widely used for IV induction in neonates and older children. Ketamine maintains or increases blood pressure, heart rate, and cardiac output. The exact mechanism of these effects of ketamine is unknown; ketamine may stimulate the release of endogenous stores of catecholamines, although it is a negative inotrope in the denervated heart. This negative inotropic effect may make ketamine a poor choice in children in whom catecholamine stimulation may already be maximal, such as in severe cardiomyopathy. It may also be a poor choice if tachycardia is undesirable, such as in the case of aortic stenosis.
Monitors should ideally be applied before induction begins, although applying monitors can upset the child, which can be detrimental (e.g., the child with TOF who begins to cry and precipitates a tet spell). A pulse oximeter probe may be the only monitor applied before induction of anesthesia. Sevoflurane and other halogenated agents may provide another advantage by offering a degree of ischemic preconditioning to the heart and to other organs, particularly the brain and kidney. In fact, sevoflurane use has decreased biochemical markers for myocardial and renal injury in coronary artery bypass grafting in adults. Current evidence suggests a role of inhalational anesthetic agents in improving outcomes after cardiac surgery, in particular for some subsets of patients. Further research is needed to clarify their protective role in different organs and systems, in noncoronary and noncardiac surgery, as well as recommended doses and timing of administration. It is thought that the same effect is observed in children. Sevoflurane, but also midazolam and propofol, protect against myocardial injury in pediatric cardiac surgery when using cardiac troponin T as a marker of such damage. One study has demonstrated definite cardioprotective effects from inhalational agents in children, although these effects do not seem to be universally applicable to all children undergoing heart surgery, suggesting the need for further investigations.
Maintenance of Anesthesia
Maintenance of anesthesia in children with CHD depends on the preoperative status and the response to induction of anesthesia. Whether inhalational agents, additional opioids, or other IV agents are used for maintenance depends on the tolerance of the child and postoperative plans for ventilation. If a primary opioid-based anesthetic is chosen, additional opioid should be administered on initiation of CPB to offset dilution from the pump prime and to maintain adequate opioid plasma concentrations. Awareness during adult cardiac surgery has been reported when amnestic agents were not used. Although small children may be unable to describe such events, the potential for awareness during pediatric cardiac surgery should not be underestimated. In effect, while it is unclear whether the incidence of awareness in children is more or less than in adults, anesthesiologists should be cognizant of the possibility of intraoperative awareness in pediatric anesthesia, and mindful of the potential short- and long-term psychological effects of such a complication. Recently, a national audit project in the United Kingdom (NAP5) suggested strategies to minimize the risk of awareness in pediatric cardiac surgery that in part may depend on the several factors, including the child's age, hemodynamic stability, predicted duration of surgery and CPB, and plans for postoperative ventilation. The choice of a specific strategy is often dictated by institutional or personal preferences. Different agents, singly or in combination, may prevent awareness: an inhalational agent may be administered through the membrane oxygenator with an anesthetic vaporizer; IV midazolam (0.2 mg/kg) may be administered at the institution of CPB; propofol may be given by infusion during the bypass period. More recently, the use of a dexmedetomidine infusion has been proposed to attenuate awareness as it attenuates the hemodynamic and neuroendocrine responses to surgical stress and CPB in pediatric cardiac surgery. Other benefits include decreased intraoperative anesthetic requirements and postoperative opioid consumption, which suggest it may have a role in reducing the possibility of awareness. However, dexmedetomidine is not a general anesthetic (conferring 0.5 MAC equivalence) and its effectiveness in preventing awareness has not been established. In one study, dexmedetomidine conferred a protective effect in the heart, brain, kidney, and lungs; the administration of dexmedetomidine may contribute to improved outcomes, a decrease in postoperative mortality, and a reduced incidence of complications and delirium in adults undergoing cardiac surgery. Dexmedetomidine also slows sinus and AV node conduction ; this may prove useful in those with junctional ectopic tachycardia (JET).
Institution and Separation From Bypass
Before initiation of CPB, the surgeon requests heparin to be given; after administration (preferably flushed through a central venous catheter) but before the initiation of bypass , the ACT should be determined . By convention, the ACT measurement should be at least three times greater than the baseline value or greater than 480 seconds. Despite significant interindividual variations in heparin dose requirements and multiple problems associated with its use, heparin remains the anticoagulant of choice for CPB. In fact, achieving an adequate balance between the appropriate amount of heparin to minimize the risk of thrombosis and platelet activation while reducing the risk of bleeding from overanticoagulation may be particularly challenging in children. Similarly, the use of the ACT as the sole metric of anticoagulation may hold a number of inaccuracies, based on the inconsistent relationships between plasma heparin concentrations, thrombin inhibition, and coagulation tests. Individualized management of anticoagulation and its reversal seem to result in less activation of the coagulation cascade, less fibrinolysis, and reduced blood loss and transfusion requirements. However, until further research defines the clinical impact of these findings, it is likely that the use of heparin and ACT measurement for anticoagulation management will remain the standard of care in most centers (see also Chapter 19 ). When bypass is started, additional anesthetic drugs should be administered to counteract the effects of dilution and adsorption by the CPB circuit. Ventilation should cease. Both hypertension and hypotension may complicate bypass. Blood pressure may be controlled within an appropriate range to ensure end-organ perfusion by using α-adrenergic agonists or blockers such as phenylephrine and phentolamine. The child is usually cooled at this stage, guided by the nasopharyngeal temperature. If the heart is to be stopped, cardioplegia is given by the perfusionist after the aorta is cross-clamped to provide myocardial protection during the period of ischemia. Cardioplegia is usually repeated every 20 to 30 minutes, although it is not required if the surgery is performed while the heart is beating. Myocardial damage is related to the duration of the aortic cross-clamping and the effectiveness of the myocardial protection.
At an appropriate time during the surgery, the cross-clamp is removed, and perfusion to the heart is restored. The heart usually starts to beat in normal sinus rhythm, although this is not always the case. In the early phase of reperfusion, it is possible for various degrees of heart block to occur. However, these are usually short-lived and as the effects of cardioplegia wear off, normal sinus rhythm is usually restored. Persistent heart block may result from damage to the conducting system during surgery.
After release of the cross-clamp, any inotropes or vasodilators that are required are usually started. Rewarming may have begun before release of the cross-clamp, but more commonly, the child is rewarmed after release of the clamp.
When the child has adequately rewarmed, as reflected by (1) a normal core and minimal core-peripheral temperature gradient, (2) inotrope(s) infusion as needed, (3) restoration of satisfactory heart function, and (4) the adequate ventilation of the child's lungs, the child is ready to be weaned from CPB. If a TEE probe is in place, the heart should be scanned for the presence of air. If air is present, additional attempts to de-air the heart should be attempted before separating from bypass. In the initial stages after coming off bypass, additional volume can be administered through the aortic cannula by the perfusionist, usually under the direction of the surgeon or anesthesiologist. Many centers institute modified ultrafiltration at this point, which involves taking arterial blood from the aortic cannula and passing it through the ultrafine filter. This blood, which is oxygenated and warm, is then reinfused into the right atrium. As previously discussed, reported benefits from the use of modified ultrafiltration include increasing the hematocrit, concentrating the clotting factors and platelets, increasing blood pressure, reducing PVR, and removing inflammatory mediators from the patient. When this process is complete, a thorough TEE examination can be undertaken.
When the team is satisfied with the TEE result, the perfusionist and the surgical team should be informed that protamine will be administered soon. The surgeon should remove any pump suckers from the field, and the perfusionist should stop all pump suction. This is done to ensure that no protamine enters the bypass circuit in case it is necessary to reestablish bypass for any reason, especially if this needs to be done in an emergency situation. Once these preliminary activities are complete, the surgeon asks for protamine to be administered to antagonize the circulating heparin. At this point, a blood gas analysis is performed and the ACT repeated; the ACT should return to prebypass levels. Required blood products may be given during modified ultrafiltration or after the administration of protamine, usually while the surgeons are achieving hemostasis. As soon as reasonable stability is achieved and the chest is closed (or the decision to leave the chest open has been made), the child can then be transferred to the ICU.
Control of Systemic and Pulmonary Vascular Resistance During Anesthesia
In some children with hypoplastic left heart syndrome (HLHS) who present for a Norwood procedure, excessive blood flow to the lungs resulting from a relatively low PVR and a relatively high SVR steals blood from the systemic circulation, leading to hypotension, poor tissue oxygen delivery, myocardial ischemia, and progressive acidosis. However, when the reverse occurs and the PVR is greater than the SVR, the child develops progressive excessive desaturation. Similar pathophysiology exists with other duct-dependent circulations and to some extent with other shunting lesions. It may prove difficult to manipulate the SVR and PVR predictably because control of PVR is poorly understood, vasoactive drugs usually are distributed on both sides of the circulation, and pharmacologic attempts to modify the degree and direction of shunting have produced unpredictable results. Despite these problems, several techniques have proved useful in manipulating the relative PVR and SVR. Increasing inspired oxygen to 100% and by hyperventilation to a pH of 7.6 or greater decreases the PVR in children. Positive end-expiratory pressure, acidosis, hypothermia, and the use of 30% or less inspired oxygen can increase PVR. Potent inhalational anesthetics reduce SVR more than PVR. Etomidate does not change the pulmonary blood flow in children with TOF, whereas ketamine increases the flow in children with limited cyanosis (presumably by dilating the pulmonary artery) and decreases the flow in children with moderate cyanosis (by constricting the pulmonary artery). Because vasoconstrictors such as phenylephrine increase SVR more than PVR, they are effective acutely in reducing right-to-left shunting and increasing left-to-right shunting in the operating room.
During cardiac surgical procedures, a direct method of selectively increasing PVR or SVR is to have the surgeon place partially obstructing tourniquets around pulmonary arteries or the aorta to increase resistance so that flow to the opposite side of the circulation increases. Although these are only temporary measures, they may reestablish a better relative balance of resistances and a more normal physiology in a deteriorating clinical situation.
Anesthetic Drugs Used in Pediatric Cardiac Anesthesia
Inhalational Agents
Sevoflurane
Sevoflurane is the induction agent of choice for inhalational inductions in pediatric anesthesia. It is associated with little myocardial depression or dysrhythmias, and there is a reduced likelihood of precipitating airway hyperreactivity than that observed with other inhalational agents. It has specific advantages over halothane when used in children with CHD, particularly in children younger than 1 year of age and in cyanotic children. In contrast to halothane, sevoflurane causes no reduction in heart rate at 1.0 and 1.5 MAC in healthy children compared with awake values. However, at greater concentrations, it can slow the heart rate and depress respiration. Both features are important in children with CHD because a slow heart rate reduces cardiac output and hypoventilation leads to hypercarbia and hypoxia, which can increase PVR. In the absence of nitrous oxide, sevoflurane depresses myocardial contractility to a lesser extent than halothane during induction of anesthesia. However, it does decrease left ventricular systolic function to a limited extent as well as SVR, but in common with halothane and isoflurane, it does not alter the degree of left-to-right shunting through an ASD or VSD at concentrations of ~1 MAC in 100% oxygen. Sevoflurane causes bradycardia in specific subsets of patients (e.g., trisomy 21) and conduction abnormalities in susceptible children, which may be clinically significant in children with marginal cardiovascular reserve. Sevoflurane should also be used with great caution in children with severe ventricular outflow tract obstruction.
Isoflurane
Isoflurane is not recommended for induction of anesthesia because the frequency of laryngospasm is greater than 20%. The inability to ventilate whether due to laryngospasm or other causes quickly leads to hypoxemia and hypercarbia, both of which increase PVR. This increase in PVR and the resulting pulmonary hypertension is poorly tolerated in small children with heart disease, especially in the presence of right-to-left shunting (see Chapter 7 ). Even though isoflurane depresses the hemodynamics in healthy neonates and infants to a similar extent as halothane at equipotent concentrations, isoflurane may hold an advantage in children with CHD, as it depresses myocardial contractility to a lesser extent than halothane.
Halothane
In the United States, Canada, and the United Kingdom, the use of halothane has all but ceased, but it is still widely used in other parts of the world. It is included here for completeness. Uptake of halothane in infants younger than 3 months of age is more rapid than it is in adults. This also is the case for the uptake of halothane by the myocardium. Although the precise effects of halothane on the human neonatal myocardium are unknown, young rodents have a reduced cardiovascular tolerance for halothane but require greater amounts for anesthesia. Studies in infants with normal cardiovascular systems have demonstrated a significant incidence of hypotension with bradycardia during induction with halothane. During induction of anesthesia in normal infants, halothane decreases the cardiac index to 73% of awake values at 1.0 MAC and to 59% at 1.5 MAC. The MAC for halothane in infants 1 to 6 months of age is the greatest of any age group. This increased anesthetic requirement in infants, combined with the immaturity of their cardiovascular system, explains in part the relative cardiovascular intolerance of halothane by infants. In fact, hemodynamic depression associated with halothane has been shown to be inversely related to age in pediatric patients. When compared with induction of anesthesia with sevoflurane, halothane decreased heart rate and systolic blood pressure in children of different age groups. As such, atropine intramuscularly before induction and IV atropine during anesthesia partially offset the myocardial depression by halothane by attenuating the severity of the bradycardia and hypotension and increasing cardiac output. Despite the hypotension caused by halothane, it increases the arterial saturation in children with cyanotic CHD.
A careful induction with sevoflurane is usually well tolerated in children with mild to moderate heart disease. However, large concentrations of potent inhalational agents may be an unwise choice for induction in young infants with severe cardiac disease. In children of any age with marginal cardiovascular reserve and in those with severe desaturation of systemic arterial blood due to right-to-left shunting, inhalational anesthetic-induced myocardial depression and systemic hypotension are poorly tolerated. A more appropriate use of these anesthetic agents in children with severe heart disease is the addition of low concentrations of the inhalational agent to provide amnesia and hypnosis, as well as to control possible hypertensive responses after an IV induction (see Chapter 7 ).
Nitrous Oxide
Nitrous oxide should be avoided for maintenance of anesthesia in children with CHD because of the risk of enlarging intravascular air emboli and the potential to increase the PVR. Nitrous oxide may expand microbubbles and macrobubbles, increasing obstruction to blood flow in arteries and capillaries. In all children with right-to-left shunts, there is a potential for these bubbles to be shunted directly into the systemic circulation and coronaries, a phenomenon designated by paradoxical embolization. The passage of air bubbles from the right to left sides is possible even in patients with predominantly left-to-right shunts, as the direction of shunting may transiently change under the influence of multiple factors during anesthesia and surgery. Consequently, care must be taken to ensure that no air bubbles are accidentally injected into the veins. Adverse outcomes after coronary air embolism are exacerbated by nitrous oxide. The hemodynamic effects of venous air embolism are increased by nitrous oxide, even without paradoxical embolization. In children with preexisting right-to-left shunts, paradoxical air embolism is clearly a potential problem; but even those with large left-to-right shunts can transiently reverse their shunts, as mentioned previously. This is particularly true during coughing or a Valsalva maneuver, when the normal transatrial pressure gradient is reversed. Right-to-left shunting of microbubbles of air after injection of saline into the right atrium has been demonstrated during these maneuvers. Because coughing and Valsalva maneuvers may occur during anesthesia induction, even the most rigorous attention to removing air bubbles from IV lines may not prevent small amounts of air from reaching the systemic circulation. Microbubbles have also been observed after CPB.
Nitrous oxide can increase PVR in adults. However, in a 50% inspired concentration, it does not appear to affect PVR or pulmonary artery pressure in infants. Nitrous oxide mildly decreases cardiac output at this concentration. Avoidance of its use has been suggested in children with limited pulmonary blood flow, pulmonary hypertension, or depressed myocardial function. In the well-compensated child who does not require 100% inspired oxygen, nitrous oxide (usually at concentrations of 50%) may be used during induction of anesthesia but discontinued before tracheal intubation. If a reduced inspired oxygen concentration is indicated to maintain an appropriate balance between PVR and SVR after tracheal intubation, air may be added to the inspired gas mixture.
Intravenous Induction Agents
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here