PEDIATRICS FOR THE ANESTHESIOLOGIST

Medical Management of Pediatric Patients

Children are usually healthy when they come to the operating room for a surgical procedure. Because we work in a tertiary care center, however, we encounter a disproportionate number of children with chronic disease. These superimposed processes will affect the conduct of the surgical procedure and the anesthetic management of the child including the way that the child views you as a provider, the amount of stress that the parents feel, and sometimes even the developmental level of the child. An understanding of the natural history of some of the common diseases that affect infants and children is important for the anesthesia practitioner that sees children as a part of their practice.

As a rule, if a child is not growing, that is, following a specific population based growth curve, they are ill. Growth along a standard curve will assist the clinician in determining whether caloric expenditure is being met by energy intake. Chronic, untreated illness increases work and thus caloric expenditure. Similarly, it may prevent an infant or child from feeding appropriately. As examples, if a child has congenital heart disease, diabetes or asthma and if the child is not following some growth curve, then one possibility is that the therapy is not completely effective. (e.g. For children with congenital heart disease, a lack of growth may relate to cardiac failure or profound hypoxemia) Thus, an early signal to the practitioner that a child is ill is the lack of linear growth.

Social, emotional, and intellectual development may not meet the level that is recognized as appropriate for a patient’s age in the face of some chronic diseases. This can occur for a variety of reasons relating to frequent hospitalization, insults to the central nervous system during its development, or a mismatch in the development of the relationship between a child and their parents that are a consequence of the disease process. An example of this is a child with a myelomeningocoele. These kids suffer through countless operative procedures over the course of their childhood. They may appear mature in the holding room, largely because of their previous experience. Many will decompensate once they leave their parents in the same way that a child much younger than their age would.

The child with congenital heart disease

Consider a child with Tetralogy of Fallot. These children present withan over riding aorta, VSD, RVH and infundibular hyperplasia. Because of these anatomic abnormalities any increase in pulmonary vascular resistance may lead to decreased pulmonary blood flow and increased cyanosis. Like wise, a major increase in systemic vascular resistance will produce luxuriant flow left to right across the VSD. For these children, control of the flow of blood back and forth through the VSD is an important determinant of general health. Any hypoxemia will produce an increase in pulmonary vascular resistance. Concomitant increased autonomic tone can lead to infundibular spasm, further reducing pulmonary blood flow – a so-called “tet spell”. Tet spells represent episodes of profound hypoxemia and can be treated by increasing the systemic vascular resistance so that blood is preferentially shunted left to right through the VSD increasing blood flow to the lungs. Neosynephrine is the drug of choice to increase systemic vascular resistance.

Growth of infants with TOF reflects the balance of hypoxemia and myocardial failure. Pulmonary blood flow that is luxuriant will increase lung water and the work of breathing. Unusual restriction of pulmonary flowcan produce profound hypoxemia by creating an intracardiac shunt that is right to left.

Current practice is to band the pulmonary artery soon after birth to restrict blood flow from the VSD and diminish the risk of failure. An infant that comes to the operating room before a definitive repair can be evaluated by determining the resting respiratory rate and whether the child is meeting growth goals. Questions for the parents about whether the child can feed appropriately may demonstrate subtle changes in lung water by revealing gasping and seeming inability to coordinate suck and swallow. Administration of large volumes of fluid during a surgical procedure will increase pulmonary interstitial water and may result in postoperative respiratory failure. For these infants, dry is usually the best plan.

The child that has had a Fontan procedure for palliation of hypoplastic left heart or other congenital cardiac anomalies represents a special case for the pediatric anesthesia practitioner. These children are dependent on the passive filling of the right heart in order to maintain cardiac output. Reduction of the right sided filling pressure whether by dehydration or by an increase in pressure in the abdomen, such as during laparoscopy, will reduce the cardiac output dramatically. For these babies, excessive NPO status can have a profound impact on cardiac output. Most of these infants utilize the right ventricle for systemic and pulmonary blood flow.

The child that has pulmonary artery hypertension also represents a significant management problem for the anesthesiologist. PAH is common in children with untreated congenital heart disease in which the flow of blood left to right across an intracardiac defect is allowed to proceed unabated over time. Some children with significant pulmonary hypertension develop this aberration because of primary lung disease – cystic fibrosis, BPD, or chronic pneumonia. In these children, though the cardiac anatomy is normal the pathophysiology of what occurs is the same and the perioperative risk is substantial. Progressive remodeling of pulmonary artery vasculature with increases in the number and size of arteriolar smooth muscle results in increases in the pressure required to push blood through the pulmonary circuit. In these children, hypotension during induction or during the surgical procedure can and will reduce coronary filling and may produce an irreversible cycle of acidosis, further hypotension, and death.

Managing Infants and Children with Congenital Heart Disease

In order to successfully manage an infant with congenital heart disease, whether for cardiac or noncardiac surgery, there are essential questions that the clinician must answer.

1.         What are the flow characteristics of blood in the infant’s heart? Where does the deoxygenated blood come from and where does it go? Example: In patients with Truncus Arteriosus, blood returns from the body to the right atrium and then leaves the heart through a large common vessel that feeds the pulmonary and the systemic circulation.

2.         Which ventricle is the most affected? Which ventricle supplies blood flow to the pulmonary circulation? Which to the systemic circulation? Example: Patients with hypoplastic left heart do not have an effective left ventricle. In these cases, the right ventricle pumps blood to the systemic and the pulmonary circulation.

3.         Is the pulmonary blood flow normal? Is the pulmonary resistance elevated?

Example: Infants with a large VSD will have significant increases in pulmonary blood flow producing a clinical picture of pulmonary congestion.

4. Arethereabnormalcommunicationsbetweenthepulmonary circulation and the systemic circulation? Example: Infants with ASDs have an abnormal communication between the right and the left side of the heart. The predominant flow of blood is usually left to right unless pulmonary vascular resistance is significantly elevated.

5.         Are the valves normal? Are they competent as valves? Do they obstruct the flow of blood? Example: Congenital aortic stenosis obstructs the flow of blood out of the left heart.

6.         Is there a normal rhythm? Example: Infants with WPW can develop supraventricular tachycardia producing failure if it is allowed to continue.

Further characterization of congenital heart disease can be accomplished by clinically placing the lesion in one of four categories: 1. Cyanotic with normal blood flow, 2. Cyanotic with decreased pulmonary blood flow, 3. Acyanotic with increased pulmonary blood flow, and, 4. Acyanotic with obstructed pulmonary blood flow.

Examples of these categories:

Cyanotic with normal pulmonary blood flow: Single ventricle lesion and double outlet right ventricle after a Fontan procedure; Transposition of the great vessels (TGA) after a atrial septectomy

Cyanotic with decreased pulmonary blood flow: Tetralogy of Fallot after a BT shunt; TGA after BTS

Acyanotic with increased pulmonary blood flow: ASD, VSD, PDA, Aorto-Pulmonary Window

Acyanotic with obstructed pulmonary blood flow: Pulmonic stenosis, Aortic stenosis, Hypoplastic left heart syndrome, Coarctation of the aorta.

Infants with congenital heart disease suffer from abnormal anatomic variations as well as the usual issues that characterize the transition from fetal to neonatal life. Their metabolism demands a high heart rate, the left ventricle has immature muscle elements and there are anatomic communications that, while physiologically established, are not anatomically complete – PFO, PDA.

With this as background, the management of infants with congenital disease, repaired or not must give consideration to the conditions that are present during the operative procedure; conditions that relate to the anesthetics that are being given as well as the conditions that are created by the surgeon and the surgical procedure.

A reasonable example of how complex the management becomes relates to the patient who was born with a double outlet right ventricle and has been repaired with a Fontan procedure. This surgical procedure, the Fontan, was developed in the early 1970’s and has been modified many times since the original description. It is used for many patients with single ventricle physiology, including hypoplastic left heart syndrome. In this procedure venous blood is returned to the pulmonary circulation through a conduit that has no pumping function. Usually, the SVC and the IVC are connected to the right pulmonary artery, the IVC through a baffle created through the right atrial wall. A small defect is left in the baffle to decompress the right heart and this produces some systemic desaturation.

Patients that have had a Fontan are dependent on the central venous pressure for filling of the pulmonary circulation and are every sensitive to elevations in pulmonary vascular resistance. Increased intraabdominal pressure, produced by laparoscopy, can reduce venous return as can increased ventilating pressures. Hypoxemia and hypercarbia can elevate pulmonary vascular resistance. Either of these will reduce blood flow to the right side of the heart and subsequently cardiac output. As an aside, cannulation of the right internal jugular vein is contraindicated because of the risk of thrombosis.

Thus, evaluation of the anatomical conditions as well as the physiological conditions that have been created by palliative procedures must be considered in managing these infants. Communication with the patient’s cardiologist prior to taking a child to the operating room aids immeasurably in defining the clinical conditions that the anesthesia care provider will be presented with. By understanding the anatomy and the physiology, the astute clinician can plan for common eventualities and avoid most disasters.