What the Anesthesiologist Should Know before the Operative Procedure

Mixing Lesions: Definitions of Anatomy and Physiology

Complete mixing lesions are defined as lesions in which there is obligatory mixing of oxygenated and deoxygenated blood. There are a number of different anatomic presentations demonstrating this pathophysiology including single ventricle hearts, truncus arteriosus and anomalies of pulmonary venous return.

In patients with complete mixing lesions the intracardiac communications allow you to think of the heart essentially as a common chamber with desaturated systemic venous blood and saturated pulmonary venous blood mixing thoroughly together. As a result, some degree of hypoxemia always exists in these patients. This single-chambered heart pumps to both the systemic and pulmonary circulations which exist as parallel circuits. The ratio of pulmonary blood flow (PBF) to systemic blood flow (SBF) or Qp:Qs is entirely dependent on the balance between pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR) when there is no fixed obstruction to outflow. Unrepaired patients can present along a broad physiologic spectrum ranging from increased PBF to decreased PBF. The amount of PBF determines the baseline degree of hypoxemia.

The arterial saturation can be helpful in determining where along the spectrum of PBF these patient lie. When the Qp:Qs is balanced at 1:1, the systemic arterial saturation should be approximately 80%-85% on room air. When the Qp:Qs is less than 1:1, the systemic arterial saturation will be lower than 80%-85%. Similarly when the Qp:Qs is greater than 1:1, the systemic arterial saturation will be greater than 80%-85%. Therefore, patients who exhibit systemic arterial saturations in the low to mid 90s have significantly increased PBF, pulmonary overcirculation, and may present with congestive heart failure. It is important to recognize that a normal saturation of 95%-100% is deleterious to these patients.

For example, truncus arteriosus is typically associated with increased PBF while pulmonary atresia is associated with decreased PBF. Because of the mixing nature of the lesion, both of these patients will have hypoxemia despite the increased PBF in truncus arteriosus and the decreased PBF in pulmonary atresia. It should also be noted that some patients will have obstruction of outflow from the ventricle as seen in hypoplastic left heart syndrome (HLHS) or obstruction to pulmonary venous return as seen in total anomalous pulmonary venous return (TAPVR) both of which will affect the Qp:Qs.

1. Possibilities for Presenting for Surgery

The majority of children with unrepaired mixing lesions are neonates. Ideally, elective noncardiac surgery should be postponed until their cardiac lesion had been addressed. However, there are certain circumstances when urgent or emergent surgery needs to be performed in a neonate with an uncorrected or nonpalliated lesion. Examples include management of tracheoesophageal fistula (TEF), laparotomy for acute abdomen or necrotizing enterocolitis, and operative repair of duodenal atresia. Urgent or emergent surgery can be accomplished in a safe and effective manner given a thorough understanding of the underlying cardiac anatomy and physiology. However, it should be clear that most of these neonates will benefit from transfer to a tertiary care center with pediatric cardiac experience.

2. Preoperative evaluation

A comprehensive history and physical is essential when evaluating these patients prior to an anesthetic. It should include a birth history, medical history up to the point of presentation for surgery, as well as knowledge of any previous cardiac operations. Knowledge of the birth history can provide information about comorbidities associated with prematurity including lung health and maturity and any neurologic events. The medical history can provide information about the child’s medical management up to that point and help clarify whether the child suffers from increased PBF and congestive heart failure or decreased PBF. Knowledge of previous cardiac operations is especially important as the patient may have had a palliative procedure with changes in the resultant anatomy and physiology.

While patients with truncus arteriosus or TAPVR typically undergo a complete repair as a neonate, patients with single ventricle disease are frequently palliated within the first few days to weeks of life. For example, HLHS is initially palliated by a stage 1 Norwood procedure with placement of either a systemic-pulmonary shunt (Blalock-Taussig shunt [BTS]) or ventriculopulmonary shunt (Sano shunt) as a source of pulmonary blood flow. Patients with tricuspid atresia or pulmonary atresia with hypoplastic right ventricles may also have undergone a BTS. Patients with an unbalanced atrioventricular septal defect who had unrestricted pulmonary blood flow may have undergone a pulmonary artery band procedure. Each of these palliative procedures will affect the Qp:Qs.

3. What are the implications of co-existing disease on perioperative care?

a. Chromosomal abnormalities

Children with conotruncal anomalies will commonly have associated chromosomal abnormalities. Truncus arteriosus is commonly associated with microdeletion of the 22nd chromosome as seen in DiGeorge syndrome or velocardiofacial syndrome. DiGeorge syndrome is characterized by conotruncal defects of the heart, hypoparathyroidism with hypocalcemia, and thymic hypoplasia with immune deficiency. As a result of the thymic hypoplasia, these children not only have a reduced number of T-cells but reduced function as well. The immunodeficiency requires the use of strict aseptic technique for invasive procedures and the use of irradiated blood products to prevent graft versus host disease.

Truncus arteriosus is also commonly associated with CHARGE syndrome. CHARGE syndrome includes coloboma, heart defects, choanal atresia, retardation of growth and mental development, genitourinary problems, and ear anomalies. These children also suffer from midface hypoplasia, small mouth, and micrognathia with an anterior larynx making mask ventilation and tracheal intubation difficult.

There are reports of an association between HLHS and Turner syndrome (monosomy X). Patients with Turner syndrome may have a difficult airway secondary to neck anomalies and micrognathia.

b. Cardiovascular

A thorough understanding of the aforementioned anatomy and physiology of the unrepaired patient as well as the palliated patient will be important in understanding the anesthetic management. In addition to the baseline anatomic and physiologic changes, one should gain an appreciation of the overall global myocardial function. These patients may also present with a history of arrhythmias including supraventricular tachycardias and atrial or ventricular ectopy. Antiarrhythmic medications or infusions should be continued throughout the perioperative period. Exacerbations of arrhythmias should be treated per ACLS protocols and consultation with an electrophysiologist can be extremely helpful.

c. Pulmonary

Pulmonary artery pressures (PAPs) can be elevated in neonates and children with unrepaired mixing lesions. The increased PAP can be secondary to increases in PBF or increases in PVR (Pressure = Flow × Resistance). While PVR falls at birth the pulmonary vasculature remains very reactive. The PVR typically falls to adult levels by approximately 6 months of age secondary to thinning of the medial smooth muscle layer of the arteries as well as ongoing proliferation of the alveoli. Congenital heart disease can impede this process and contribute to increases in PAP. If the pressures are high secondary to increased flow then they can be expected to decline after cardiac surgical repair. However, if they are high secondary to structural changes in the pulmonary vasculture, the pulmonary pressures may remain elevated even after surgical repair. Cardiac catheterization with and without pulmonary vasodilating agents, such as 100% oxygen and inhaled nitric oxide, is needed to delineate whether increases in PAP are secondary to flow or resistance and to determine if the vascular bed remains responsive to these pulmonary vasodilators.

Patients with heart failure are often prone to respiratory infections or may have signs and symptoms from their cardiac disease that mimic respiratory infections making the decision whether to proceed difficult. Recent infectious pulmonary disease (viral or bacterial) is associated with increased incidence of respiratory complications postoperatively. If the operation is purely elective it is prudent to delay surgery for 6-8 weeks.

These patients may also have coexisting structural pulmonary disease secondary to meconium aspiration or bronchopulmonary disease of prematurity. Efforts should be made to optimize lung function prior to any cardiac or noncardiac procedure.

d. Renal

Infants with congenital heart disease are often born with concomitant defects of the genitourinary tract including dysplastic, absent or pelvic kidneys, hypospadius, or various fistulae. In the face of anatomic renal abnormalities, perturbations of electrolytes or renal function should be thoroughly evaluated before proceeding with elective cardiac or noncardiac surgery.

e. Neurologic

Birth trauma with associated intraventricular hemorrhage can be seen especially in preterm infants. Development of hydrocephalus may result in the patient presenting for neurosurgical intervention in the form of a ventricular drain or shunt.

If the patient has undergone previous cardiac surgery using cardiopulmonary bypass, neurologic insult may present as evidence of stroke or seizure activity. Chronic seizures medications should be continued throughout the perioperative period. If the patient is being treated with barbiturates, the sedative effects may influence choice of premedication.

f. Endocrine

Thyroid disease may be seen in patients with cardiac disease. If possible, it is best for patients to be made euthyroid prior to surgery. Thyroid replacement therapy should be continued throughout the perioperative period.

Parathyroid disease is also often seen especially in patients with DiGeorge syndrome. Calcium levels should be diligently monitored and any deficiencies corrected.

Stress responses in these patients may lead to either hypoglycemia or hyperglycemia. Glucose should be monitored and normoglycemia be maintained.

g. Renal-GI:

A number of gastrointestinal diseases are commonly seen in neonates with congenital heart disease including reflux disease, intestinal obstructions, and TEF.

Reflux disease is very common in the neonatal period. Most patients are well controlled on medications that should be continued throughout the perioperative period. If reflux is poorly controlled or is combined with intestinal obstruction, a rapid sequence induction (RSI) should be performed.

In the case of intestinal obstruction, whether mechanical or functional, patients may present with significant dehydration and either a contraction metabolic alkalosis or a combined metabolic alkalosis and metabolic acidosis. Patients may also present with sepsis secondary to bacterial translocation from the gastrointestinal tract into the bloodstream. Volume resuscitation with correction of electrolytes is necessary before proceeding with elective surgery. Care needs to be taken with volume resuscitation in the patient with CHF as a consequence of their congenital heart disease and additional invasive monitoring may be required.

Alterations in the cerebrospinal fluid pH secondary to metabolic alkalosis affect the ventilatory response to carbon dioxide and need to be considered when planning for narcotic administration and emergence.

Changes in abdominal girth will reduce functional residual capacity (FRC) and can also increase the risk of regurgitation and aspiration of gastric contents. An RSI should be considered.

Patients with TEF commonly have associated congenital heart disease. These patients are taken to the operating room for either ligation of the fistula or primary repair of associated esophageal atresia. This is done prior to the cardiac repair because of the need for positive pressure ventilation.

4. What are the patient's medications and how should they be managed in the perioperative period?

Neonates and children with intracardiac mixing lesions and associated congestive heart failure are often treated preoperatively with a variety of medications including diuretics, digoxin, angiotensin-converting enzyme (ACE) inhibitors, and possibly carvedilol. Diuretics are often held the morning of surgery secondary to the potential for dehydration in the face of the patient being made nothing by mouth (NPO). ACE inhibitors, especially long-acting ones, are known to exacerbate hypotension during anesthesia and are also often held the morning of surgery. Other medications can be continued throughout the perioperative period.

Critically ill neonates and children may also be treated with inotropes and/or prostaglandin infusions, which should be continued throughout the perioperative period.

Other frequently encountered medications include calcium and vitamin D administration in patients with DiGeorge syndrome and antireflux medications for patients with gastroesophageal reflux disease (GERD). These medications should be continued throughout the perioperative period.

5. Prevention of Infective Endocarditis

Recommendations published by the American Heart Association for the prevention of infective endocarditis have been recently revised. They now recommend prophylaxis for fewer patients targeting those patients who are at significantly increased risk for poor outcome. For complete recommendations please see their publication in PUBMED:17446442.

Patients who should be considered for prophylaxis include those with previous infective endocarditis, those with prosthetic cardiac valves, those who have had a cardiac transplant with acquired valvulopathy, and those with the following congenital heart diseases: patients who are unrepaired including patients with palliative shunts and conduits, patients who are completely repaired with prosthetic material or devices during the first 6 months after the procedure, patients who are repaired but with residual defects at or adjacent to the site of prosthetic material thereby inhibiting endothelialization.

These patients should be given antibiotic prophylaxis when they present for any dental procedure that involves the manipulation of gingival tissue or periapical region of the teeth or perforation of the oral mucosa, for invasive respiratory tract procedures, or infected skin, skin structure, or musculoskeletal tissue procedures. Routine prophylaxis is no longer recommended for GU and GI tract procedures. Appropriate antibiotic choice, route of administration, and dosages can be found in the Circulation 2007 article.

Patients who are on chronic antibiotic therapy, for example heterotaxy patients who are on chronic penicillin therapy secondary to functional asplenia, should be treated with a different class of antibiotic (clindamycin, azithromycin, or clarithromycin) if they require prophylaxis.

6. How To modify care for patients with known allergies

Many of these young children have not developed allergies secondary to an immature and developing immune system. However the presence of allergies should be sought and known allergens should be avoided.

a. Latex allergy

It is important to be aware of any previous reactions to latex. Also consider latex precautions in neurologic or urologic patients in whom frequent future exposures could result in the development of an allergy.

b. Does the patient have a history of allergy to anesthesia?

It is important to review any prior anesthetics and discover if there were any deleterious effects from the anesthetic. It is also important to determine if there is a family history of problems with anesthesia that may indicate pseudocholinesterase deficiency or malignant hyperthermia.

7. Preoperative testing.

a. Pulse Oximetry

A baseline pulse oximetry value obtained when the infant is calm and breathing room air is essential information. If available, a series of values should be reviewed to discover the range of high and low within which the infant lives.

b. Laboratory Values

Hematocrit, hemoglobin and coagulation values are the most commonly obtained preoperative laboratory tests. In cyanotic heart disease, the hematocrit (Hct) is directly related to the degree of hypoxemia. Polycythemia, with a Hct greater than 65%, is a risk factor for neurologic events. Dilutional phlebotomy should be considered if the Hct is greater than 65%. Because cyanosis affects the coagulation system, it is important to check the prothrombin time (PT) or international normalized ratio (INR) as well as the activated partial thromboplastin time (aPTT). The PT can also provide information regarding the synthetic function of the liver. Appropriate blood products should be ordered for procedures where blood loss is expected.

Chemistry values can provide information regarding liver function and renal function. They may also demonstrate electrolyte abnormalities secondary to chronic diuresis.

c. Radiography

A recent chest radiograph provides invaluable information in the setting of cyanosis. The degree of pulmonary vascularity can distinguish between increased versus decreased PBF. Primary lung parenchymal diseases such as pneumonia, atelectasis, or pleural effusions are important to diagnose prior to coming to the OR.

d. Echocardiography

An echocardiogram is an essential component of the workup for a child presenting for noncardiac surgery with a history of a mixing lesion. It provides a noninvasive assessment of intracardiac anatomy, cardiac function, and blood flow patterns. Neonates presenting for an urgent noncardiac procedure should have had a recent in-hospital echocardiogram. Older infants who are repaired or who have had a palliative procedure should have had an echocardiogram within the past 3 months at the cardiologists office. Do not hesitate to contact the primary cardiologist to obtain recent office visit notes or recent echocardiogram results or to have a conversation with the cardiologist regarding the patient’s clinical status.

e. Cardiac Catheterization

Some, although not all, of these patients will have had a cardiac catheterization. A cardiac catheterization is usually performed when the anatomy remains unclear after echocardiogram or to evaluate PBF and PVR when there are concerns about increased pulmonary pressures. Catheterization data should be reviewed so that the anatomy and blood flow patterns are clearly understood. Saturations obtained during the catheterization and calculations of Qp:Qs should be noted. Manipulation of PVR by 100% oxygen and/or inhaled nitric oxide should be reviewed and the patient’s response noted.

Intraoperative Management: What are the options for anesthetic management.

There is no one anesthetic technique or formula that should be used in the child with a mixing lesion. The management technique should be selected keeping in mind the surgical procedure to be performed, the child’s overall state of health and the preference of the anesthesiologist involved. Nevertheless, for most of these children, a general anesthetic is the common choice. General anesthesia is widely accepted in these situations and has the following advantages: certainty of effect, comfort of the patient for the placement of invasive monitoring lines such as arterial or central venous lines, control of ventilation, control of hemodynamics and the option to remain intubated postprocedure.

1. Regional anesthesia

Regional techniques ranging from single-shot caudal blocks or indwelling epidural catheters to extremity blocks are acceptable in this patient population if indicated for the operation. Regional techniques can provide analgesia thereby reducing the need for narcotics that can lead to hypoventilation, hypoxemia, or undesirable increases in PVR.

In general, infants tolerate neuraxial anesthesia quite well demonstrating hemodynamic stability even when the block reaches the upper thoracic dermatomes. Nevertheless, one should be aware of the potential for a sympathectomy and its hemodynamic consequences.

Antiplatelet therapy is common in patients palliated with a prosthetic shunt. While aspirin is not a contraindication to regional anesthesia, other antiplatelet medications such as clopidigrel and other anticoagulants such as Coumadin or low molecular weight heparin may need to be held prior to proceeding with regional anesthesia.

Patients who are profoundly cyanotic with hematocrits greater than 60% often have abnormalities of coagulation that need to be considered before proceeding with regional techniques.

2. Monitored Anesthesia Care

Monitored anesthesia care can be used for certain nonsurgical diagnostic procedures such as CT scans, MRIs, and cardiac catheterizations. A combination of PO or IV medications can be used successfully. Careful monitoring is important because sedation may lead to hypercarbia, which can worsen pulmonary vascular resistance. This, in turn, results in further worsening of hypoxemia.

3. General Anesthesia

The author’s preference for patients with unrepaired, palliated or recently repaired mixing lesions for the majority of possible operations would be to proceed with general anesthesia. Once again, general anesthesia allows for certainty of effect, comfort of the patient for the placement of invasive monitoring lines such as arterial or central venous lines, control of ventilation, control of hemodynamics, and the option to remain intubated postprocedure.

a. Premedication

In the patient population we are discussing, children with mixing lesions, most patients will be less than 6 months of age and will not experience stranger anxiety. Thus, it is reasonable to not administer a premedication. Many of these patients may be already in the intensive care unit (ICU) at time of presentation for surgery and receiving IV or PO sedation. ICU sedation, if well tolerated by the patient, may be continued preoperatively.

b. Monitoring

All noninvasive standard ASA monitors should be used. A precordial stethoscope is a useful adjunct to monitor breath sounds, heart tones, and the quality of murmurs. Changes in any of the above may be indicative of early hemodynamic compromise.

The use of invasive monitors (arterial and central venous catheters) is influenced by the hemodynamic status of the child and the complexity of the operative procedure. In general most of these infants will benefit from arterial catheter placement for blood pressure monitoring and frequent blood gas analysis. Children undergoing operative procedures involving large fluid shifts will benefit from central venous pressure monitoring. Poor peripheral venous access is also an indication for central venous catheter placement.

Near infrared spectroscopy (NIRS) is becoming increasingly used in children with congenital heart disease and may be an early indicator of hemodynamic compromise. NIRS is a noninvasive optical monitor used to estimate the balance between oxygen delivery and consumption within the frontal cortex of the brain providing a measure of regional cerebral oxygen saturation (rSO2). Different than the pulse oximeter NIRS monitors a “mixed vascular bed” including arterial and venous blood flow. A normal cerebral rSO2 is 60%-80%, however this may not be true in a desaturated child. In desaturated children, it is important to determine the baseline rSO2 and observe any trends. NIRS may also be used in a somatic position most often over the flank to estimate renal perfusion.

c. Induction and Maintenance

i. Unrepaired mixing lesion

Neonates with unrepaired truncus arteriosus, TAPVR, or single ventricle disease are typically in the ICU and have established IV access allowing for an IV induction. Multiple agents may be used for induction although a combination of fentanyl, midazolam, and/or ketamine allows for minimal hemodynamic change. Nondepolarizing muscle relaxants are generally well tolerated.

Maintenance with a combination of fentanyl, midazolam, ketamine, and/or inhalation agent is appropriate. Keep in mind that the majority of anesthetics decrease PVR leading to increases in PBF. This can potentially worsen signs of heart failure. On the other hand, in patients with significantly decreased saturations indicative of decreased PBF, anesthetic agents may lead to improvement in PBF.

In patients with mixing lesions, it is important to remember that oxygen is a very potent pulmonary vasodilator and can lead to significant increases in PBF. Administration of oxygen should be directed by peripheral saturations and NIRS data with the goal of maintaining arterial saturations between 80%-85% with adequate cerebral rSO2 (NIRS) relative to baseline.

Throughout the case frequent blood gas monitoring should be used. A worsening base deficit or increase in lactate level can be an early indicator of decreased cardiac output and poor peripheral perfusion. The authors are relatively quick to correct a developing metabolic acidosis with sodium bicarbonate.

Changes in hematocrit should also be followed with the goal of maintaining the hematocrit between 35% and 40%.

ii. Palliated mixing lesion

Regardless of underlying single ventricle anatomy, the end result of initial palliation is a single ventricle that supplies both PBF, through either a systemic-pulmonary or ventriculopulmonary shunt, and SBF. After palliation, these children are no longer dependent on prostaglandins to provide PBF or SBF. However, the physiology is the same as a patient with an unrepaired mixing lesion. The amount of PBF continues to be dependent on the balance between PVR and SVR.

Infants who have undergone a ventriculopulmonary shunt (Sano shunt) as a part of their initial palliation can present in the postoperative period with decreased PBF secondary to dynamic muscular obstruction at the ventricular origin of the Sano shunt. The management then becomes centered on minimizing the dynamic obstruction analogous to treating a hypercyanotic episode in a patient with tetralogy of Fallot. It is important to keep the patient volume resuscitated with a slower heart rate and prevent noxious stimuli. Raising the SVR with phenylephrine can promote PBF, and beta-blockers may be helpful in controlling heart rate.

iii. Repaired truncus arteriosus

Truncus arteriosus is generally repaired in the neonatal period. If there is no evidence of decreased myocardial function or residual ventricular shunting on echocardiogram, the anesthetic management for additional noncardiac surgery can be the same as that of a patient with no cardiac disease.

iv. Repaired TAPVR

Timing of repair of TAPVR is often dictated by the type of TAPVR and the presence of any obstruction of pulmonary venous return.

The presence of obstruction dictates urgent or emergent repair of TAPVR. The presence of in utero obstruction has implications for the care of these infants in the postrepair period. Patients experiencing in utero obstruction will have persistent pulmonary disease as a result of the long-standing pulmonary hypertension and edema that were present during fetal development. As a result, these infants will have chronic problems with oxygenation and ventilation.

Additionally, left ventricular compliance is often reduced secondary to lack of left ventricular filling while in utero. While the left ventricle recovers quickly in the immediate postrepair period, attention to left ventricular function is very important if additional noncardiac surgery is required. Afterload reduction with adequate intraoperative sedation and analgesia is important. Milrinone may be a helpful inotrope as well as provide additional afterload reduction.

d. Emergence and Postoperative Planning

The emergence technique and location of postoperative care are dependent on the child’s hemodynamic status and the type and extent of surgery. Children who are repaired, hemodynamically stable, and undergoing operations with minimal physiologic perturbations may be extubated in the operating room and recovered in the postoperative anesthesia care unit. Conversely, children who are palliated or unrepaired may not tolerate immediate extubation.

Contraindications to extubation in the operating room include hemodynamic instability, coagulopathy, significant inotrope requirements, and high ventilatory requirements. Hypoventilation and hypercarbia will increase PVR and may negatively affect overall hemodynamics. These children may benefit from continued mechanical ventilation and invasive monitoring that is available in the ICU.

Conclusion

We recognize that there are a number of different anatomic variations that demonstrate mixing physiology. These patients can present to the operating room at a variety of different stages of repair or palliation and for a variety of different operations. Given the complexity of these children, they are often transferred and cared for at tertiary care centers where pediatric cardiac specialists are available. Careful preparation and an understanding of patient pathophysiology is paramount to a successful and safe anesthetic.

What's the Evidence?

Kussman, BD, DiNardo, JA, Holzman, RS, Mancuso, TJ, Polaner, DM. “The cardiovascular system”. A Practical Approach to Pediatric Anesthesia. 2008. pp. 306-374. (Textbook on pathophysiology, anatomy, and anesthetic management of various congenital cardiovascular lesions.)

Mahajan, A, Marijic, J, Andropoulos, DB, Stayer, SA, Russell, IA. “Hemodynamic management”. Anesthesia for Congenital Heart Disease. 2005. pp. 227-8. (Textbook on pathophysiology, anatomy, and anesthetic management of various congenital cardiovascular lesions.)

Miller-Hance, WC, Coté, CJ, Lerman, J, Todres, ID. “Anesthesia for noncardiac surgery in children with congenital heart disease”. A Practice of Anesthesia for Infants and Children. 2009. pp. 465-490. (Textbook on pathophysiology, anatomy, and anesthetic management of various congenital cardiovascular lesions.)

Kussman, BD, McGowan, FX, Davis, PJ, Cladis, FP, Motoyama, EK. “Congenital cardiac anesthesia: Non-bypass procedures”. Smith's Anesthesia for Infants and Children. 2011. pp. 674(Textbook on pathophysiology, anatomy, and anesthetic management of various congenital cardiovascular lesions.)

DiNardo, JA, Shukla, AC, McGowan, FX, Davis, PJ, Cladis, FP, Motoyama, EK. “Anesthesia for congenital heart surgery”. Smith's Anesthesia for Infants and Children. 2011. pp. 605(Textbook on pathophysiology, anatomy, and anesthetic management of various congenital cardiovascular lesions.)

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