Cardiovascular Surgery in the Newborn



Cardiovascular Surgery in the Newborn


Gary S. Kopf

Dennis M. Mello



Congenital heart disease (CHD) occurs in 0.8% to 0.9% of live births. Approximately 30% of these patients will need surgical treatment as a neonate. Table 54.1 lists the most common types of CHD requiring surgical intervention in the neonatal period. Primary cardiac repair or palliation in neonates now is accomplished with reasonably low morbidity and mortality for a variety of complex congenital heart defects in infants as small as 1.5 to 2.0 kg.








TABLE 54.1. COMMON CARDIOVASCULAR PROCEDURES PERFORMED IN THE NEONATAL PERIOD



































Diagnosis Operation
Transposition of Great Arteries Arterial Switch
Hypoplastic Left Heart Syndrome Norwood Procedure or Heart Transplant
Tricuspid Atresia Blalock-Taussig Shunt
Tetralogy of Fallot Complete Repair or Blalock-Taussig Shunt
Pulmonary Atresia with VSD Rastelli Type Procedure or Blalock-Taussig Shunt
Ventricular Septal Defect VSD Closure or Pulmonary Artery Banding
Truncus Arteriosus Rastelli Type Procedure
Total Anomalous Pulmonary Venous Drainage Total Repair
Coarctation Repair with Extended End-to-end Anastomosis
Patent Ductus Arteriosus PDA ligation

Procedures that result in normal physiology are preferred whenever feasible. Reparative operations commonly performed in neonates include the arterial switch for transposition of the great arteries, repair of truncus arteriosus, repair of total anomalous pulmonary venous return, closure of ventricular septal defect (VSD), and repair of coarctation. For other complex lesions, repair or palliation are options to be considered depending on individual anatomic substrates and the general condition of the patient. These include pulmonary atresia with VSD, and tetralogy of Fallot. Patients with single-ventricle physiology comprise about 20% of patients with CHD, and can only be palliated, short of transplantation. Palliation consists of a systemic–to–pulmonary artery shunt for most cyanotic patients, or a pulmonary artery (PA) banding for those in congestive heart failure with increased pulmonary blood flow. Neonates with left heart obstruction or hypoplastic left heart syndrome will require more complex palliations such as the Damus-Kaye-Stansel procedure or the Norwood procedure, described in later sections.

Prolonged medical treatment to promote growth often leads to complications with little weight gain and should be used only in mildly symptomatic or asymptomatic patients. Although neonates with CHD can be palliated with prostaglandin E1 (PGE1) infusion for weeks, complications are frequent and the growth seen is often insignificant. Noncardiac malformations, such as gastrointestinal obstruction, tracheoesophageal fistula,
or diaphragmatic hernia, usually require surgical repair before the underlying CHD can be addressed, and PGE1 infusion is useful in this group (Box 54.1).


Extreme low-birth-weight neonates (<1,500 g) with CHD are difficult to manage with repair or palliation and remain a challenging problem with high mortality.


GENERAL CONSIDERATIONS


Preoperative Management

Resuscitation and stabilization of cardiac, renal, and respiratory function is critical to optimize surgical outcome. Preoperative shock or renal failure is strongly associated with poor surgical outcome and should be corrected before surgical intervention. Neonates who are dependant on ductal patency for pulmonary blood flow (e.g., pulmonary atresia) or systemic perfusion [e.g., hypoplastic left heart syndrome (HLHS), interrupted aortic arch (IAA)] are treated with PGE1 infusion as early as possible to reverse acidosis, severe cyanosis, or low cardiac output. Inotropic agents, particularly dopamine, dobutamine, and milrinone, are used to increase cardiac output and renal blood flow. Mechanical ventilation is used to ensure adequate gas exchange and oxygenation. Nitrogen balance must be optimized with either enteral or intravenous nutrition if surgery is to be delayed more than a few days.

A diagnosis can be made expeditiously utilizing echocardiography and color flow Doppler. Cardiac catheterization is used only when important physiologic or anatomic questions remain after echocardiographic examination, or if catheter intervention (balloon atrial septostomy, balloon valvuloplasty) is contemplated. Following initial stabilization, surgery should be accomplished as soon as feasible. Additional delay in symptomatic patients for the purpose of weight gain is usually unjustified, because intervening complications usually outweigh the benefits of the often slow and insignificant growth.


Intraoperative Management


Monitoring

Arterial blood pressure is monitored via umbilical, radial, or femoral artery cannulation. Good intravenous access is important, but central venous monitoring is not necessary because transthoracic left and/or right atrial lines can be placed for postoperative monitoring. Urine output is monitored using a bladder catheter. Several electrocardiographic leads are monitored continuously. Temperature monitoring is a critical part of the operative management and is accomplished with tympanic, esophageal, and either bladder or rectal temperature probes.

Intraoperative monitoring using transesophageal echocardiography (TEE) is critically important to reassess anatomy pre-repair, and to check on the anatomic result post-repair. Probes now are available for low-birth-weight neonates.


Incisions

The median sternotomy incision is the standard approach for virtually all open-heart procedures and most palliative procedures in the neonate. The sternum is incised vertically using the sagittal saw. The thymus gland is subtotally resected to facilitate exposure. The pericardium is harvested for use as a vascular patch when necessary. The pericardium is treated with glutaraldehyde solution to increase strength if it will be subject to systemic pressure. Before cannulation for bypass, patients are heparinized with 3 to 4 mg/kg of heparin.

Coarctation repair and ligation of an isolated patent ductus arteriosus (PDA) are approached through the left chest. A left posterior-lateral thoracotomy is made, with entrance through the third or fourth intercostal space. The lung is gently retracted anteriorly to expose the mediastinum.


Cardiopulmonary Bypass

During cardiopulmonary bypass (CPB), venous blood is siphoned into the venous reservoir of the heart–lung apparatus via a single cannula placed in the right atrium, or two smaller cannulae inserted into each vena cava. After traversing a roller pump, membrane oxygenator, heat exchanger, and filter, the blood is returned to the ascending aorta, via an aortic cannula. To provide for the special needs of the neonate, each component of the system is specifically designed to minimize priming volume and blood trauma, and increase the efficiency of gas exchange and heat transfer. The time during which blood is continuously exchanged between the heart–lung apparatus and the patient is referred to as the total bypass time.

CPB results in a whole-body inflammatory response, which can lead to generalized edema and an increase in total body water. The priming volume of the heart–lung bypass circuit may be twice that of a neonates’ blood volume. The severe and sudden hemodilution decreases oncotic pressure and tends to increase the loss of intravascular fluid into third-space compartments. The inflammatory response to bypass surgery will increase membrane permeability, adding to this third-space effect. The hormonal stress response will result in increased levels of antidiuretic hormone and further contribute to the tendency to accumulate excess fluid. Diuretics, in significant amounts, are almost always needed postoperatively to rid the body of excess total body water. Dilution of clotting factors may contribute to severe coagulopathic states following bypass.


Surgical Strategy

The successful surgical repair of complex lesions requires meticulous attention to every detail. A bloodless, motionless field with a relaxed heart is mandatory. Once bypass is established, the ascending aorta between the root of the aorta and the aortic cannula is clamped. The heart is thereby isolated from the rest of the circulation and a cardioplegia solution can be infused selectively into the coronary circulation. Cardioplegia is a cold, hyperkalemic, physiologically balanced crystalloid or crystalloid–blood solution containing glucose, buffer, and electrolytes. With cardioplegia infusion into the coronary circulation, the heart becomes flaccid, and a bloodless, motionless surgical field is produced. A vent suction often is placed into
the left ventricle via the right upper pulmonary vein, which returns blood to the heart–lung circuit. This vent helps keep the field dry during cardiac procedures, removes air, and prevents ventricular distension during the rewarming phase after repair is completed. The amount of time during which the ascending aorta is clamped is called the aortic cross clamp time and is also the myocardial ischemic time. Current techniques of myocardial protection using cardioplegia, and with topical cooling around the heart as well as systemic cooling, allow for cross clamp times of up to 2 hours or more, with good preservation of myocardial function after bypass. The most complex repairs usually can be accomplished within this time frame.

At the onset of CPB, to protect the heart and other organs, body temperature is lowered using a heat exchanger. A core temperature below 20°C is referred to as deep hypothermia. At this temperature, pump flow can be reduced temporarily to as low as 25 to 50 cc/kg/minute, because metabolic demands decrease with hypothermia. This is known as the low-flow technique. Complete circulatory arrest is another way of creating ideal surgical conditions. Systemic perfusion can be stopped completely once systemic temperatures are below 18°C. The head is packed in ice, because the brain is organ most vulnerable to injury during circulatory arrest. The amount of time during which no systemic flow is present is the circulatory arrest time. Arrest times of less than 45 minutes are considered acceptable, but the incidence and severity of neurologic sequelae increase significantly with longer periods. Complete circulatory arrest, even for aortic arch repair, can be avoided by brachiocephalic artery perfusion; however, some surgeons still prefer using total circulatory arrest, which arguably may be just as safe for short periods of time.

Most neonatal procedures can be carried out with exposure through the atria and/or great vessels. A small ventricular incision in the pulmonary ventricle is necessary for some procedures. Incision in the systemic ventricle is poorly tolerated in the neonate and is avoided. Once off bypass, the adequacy of the repair or the existence of residual lesions, and the inotropic state of the heart is evaluated using TEE. Heart rate, volume status, and the contractile state of the ventricles are optimized. Adequate heart rhythm and rate can be adjusted with the use of temporary atrial and ventricular pacing wires, which are routinely inserted in all neonates. Optimum volume status is maintained. Optimal contractility may require an adjustment of inotropic support.

During bypass, extra fluid and electrolytes are partially removed utilizing ultrafiltration, a process similar to hemodialysis. After discontinuation of cardiopulmonary bypass, modified ultrafiltration (MUF) can be used to further remove excess fluid and raise the hematocrit. This is done by circulating the patients’ blood volume and the bypass circuit volume through the hemoconcentrator while the patient’s own heart supports the circulation. This process also may remove inflammatory or myocardial depressant proteins and often improves hemodynamics. After MUF is completed (usually 10 to 15 minutes), the circulating heparin is reversed with protamine.

Two or three chest tubes are inserted for drainage of blood and fluid from the pericardial and pleural spaces. In cases where renal function is questionable, a peritoneal dialysis catheter is placed for drainage of the peritoneum and for dialysis, if necessary. In some CHD centers, this catheter is routinely inserted in neonates.


Delayed Sternal Closure

Infants who have prolonged procedures or who have excessive bleeding in the operating room that requires the transfusion of significant amounts of clotting factors and blood often have generalized edema, which precludes sternal closure without hemodynamic compromise. In such cases, the sternum is left open and the wound closed by sewing a silastic or polytetrafluoroethylene (PTFE) sheet to the skin edges without tension. After 2 to 5 days of diuresis, the sternum usually can be easily closed, a procedure done at the bedside in the ICU. The incremental risk of infection is small.


Postoperative Care

Patients who undergo reparative surgery with a good hemodynamic result can be expected to make a quick recovery within 24 to 72 hours. In patients undergoing palliation, a good balance between systemic and pulmonary flow is the critically important factor. The ventricle should provide systemic flow unimpeded by any obstruction, whereas pulmonary flow is limited by the size of the shunt or by a PA band or other intrinsic obstruction to pulmonary blood flow. In lesions with complete mixing of systemic and pulmonary venous blood, oxygen saturations from 75% to 85% with PO2 values of about 40 torr are optimal. This generally equates to a pulmonary–to–systemic flow ratio in the range of 2 to 1.


Fluid Balance

Fluid administration is restricted to between one-half and two-thirds of maintenance levels. In the first 24 hours, supplemental red cells, plasma, or albumin may be needed to maintain cardiac output. Overdistension of the heart, with filling pressures above 10 to 12 mm Hg should be avoided, because this is poorly tolerated in the neonate. After the initial 12 to 24 hours following surgery, neonates usually require several days of intense diuresis to rid the body of the excess fluid accumulated perioperatively. Continuous furosemide infusion is effective.


Inotropic Support

Moderate doses of dopamine (3 to5 μg/kg/minute) and milrinone (0.3 to 0.5 μg/kg/minute) are commonly used. If additional inotropic support is needed, calcium and/or epinephrine may be added. A modest decline in ventricular function often occurs during the first 6 to 18 hours after surgery; this improves the next day. The contractile state can be assessed using transthoracic echocardiography.


Ventilatory Complications

Respiratory problems represent the most common postoperative complications, and frequent blood gas determinations are required to monitor and adjust ventilation. Adequate tidal volume must be maintained, with modest positive end expiratory pressure (PEEP) to prevent progressive atelectasis. Respiratory acidosis and alkalosis are avoided by appropriate changes in ventilator settings. Acidosis will depress cardiac function and increase pulmonary vascular resistance. Alkalosis also can decrease function and impair cerebral blood flow. Sudden, unexpected, severe decompensation in the postoperative patient usually is due to compromised ventilation, including dislodged, malpositioned, or obstructed endotracheal tubes, ventilator malfunction, pleural effusions, and pneumothorax, which must be recognized and treated immediately.


Bleeding

Normal red cell mass with a hematocrit value of 35% to 45%, or higher in cyanotic patients, is maintained to ensure optimum oxygen transport and oncotic pressure. Continuous bleeding, with chest tube outputs of greater than 10% of blood volume per hour, if not due to coagulopathy, may require re-exploration. Postpump coagulopathy is treated using platelets, cryoprecipitate, and fresh frozen plasma and red cells as indicated.



Arrhythmias

Temporary transthoracic bipolar atrial and ventricular pacing wires are placed in the operating room to help manage postoperative arrhythmias. A heart rate in the 120 to 160 beats per minute range is needed to maintain adequate cardiac output, because low neonatal ventricular compliance limits stroke volume expansion. Bradyarrhythmias, including an inappropriate sinus bradycardia or junctional rhythm, can be treated with single or dual chamber pacing. Supraventricular tachycardia is a frequent complication in the immediate postoperative period. The most common of these is junctional ectopic tachycardia. Intravenous amiodarone often is the first line drug treatment for uncontrolled supraventricular and ventricular tachyarrhythmias, and electrolyte disturbances must be treated, particularly low magnesium levels.

Deteriorating hemodynamic status not responding to supportive measures is usually a result of a residual anatomic lesion. The adequacy of surgical repair can be re-evaluated using echocardiographic examination. Cardiac catheterization may be necessary if the echocardiographic examination is not definitive. Once identified, the repair of significant residual lesions should be promptly undertaken, because continued nonoperative treatment usually is futile.


LEFT-TO-RIGHT SHUNTS


Patent Ductus Arteriosus

The ductus arteriosus is a fetal structure connecting the main PA to the descending thoracic aorta. It arises from the superior portion of the bifurcation of the PA, and connects directly to the descending aorta distal and medial to the left subclavian artery. Its length and diameter are variable. In the fetal circulation, over 60% of the cardiac output is directed through the ductus. After birth, when the pulmonary vascular resistance falls and oxygenation increases, the ductus constricts. Spontaneous complete closure occurs in most infants during the first days or weeks of life.

Persistent PDA commonly is seen in extremely low-birth-weight and premature infants. Left-to-right shunt through the ductus causes congestive heart failure, which exacerbates the pulmonary dysfunction of the premature infant. Low systemic output can lead to necrotizing enterocolitis, intracranial hemorrhage, and renal failure. Diagnosis is confirmed using echocardiography. Pulse wave Doppler shows left-to-right shunting and a large left atrium. Medical treatment with indomethacin usually is effective, but may be contraindicated in the presence of compromised renal function or intracranial hemorrhage. Prolonged courses of medical therapy in symptomatic infants can result in further deterioration and should be avoided, because surgical therapy is effective and well tolerated.


Patent Ductus Arteriosus Ligation

Because many of these patients are extreme low-birth-weight neonates weighing between 400 and 1,000 g and having multiple problems, surgery is done at the bedside, or in special procedure rooms in the NICU to avoid the significant hazards of transporting such patients to the operating room. Surgery is performed through a left postero-lateral thoracotomy, with the chest cavity entered through the third or fourth intercostal space. The lung is retracted medially, exposing the aorta and the posterior mediastinum. The ductus and aortic arch are carefully identified. Minimal dissection is carried out between the base of the ductus and the aorta. A small stainless steel clip can be applied to the base of the ductus at the aortic end. Care must be taken to identify and avoid injury to the recurrent laryngeal and phrenic nerves. A chest drain usually is not needed unless significant bleeding or air leak occurs.

Complications include hemorrhage from a friable ductus, injury to the recurrent laryngeal nerve that loops around the ductus near the area of dissection, and disruption of significant lymphatics, resulting in chylothorax. The latter usually responds to conservative treatment consisting of drainage, dietary therapy, and nutritional support.


Ventricular Septal Defect

VSD is the most common congenital heart anomaly. A large VSD often will produce congestive heart failure after the first 2 weeks of life, as pulmonary vascular resistance falls and left-to-right shunting increases. Symptomatic neonates with congestive heart failure who respond poorly to medical therapy should undergo prompt surgery.

Jul 24, 2016 | Posted by in PEDIATRICS | Comments Off on Cardiovascular Surgery in the Newborn
Premium Wordpress Themes by UFO Themes