Pediatric Cardiac Disorders

Fetal and Neonatal Circulation

During fetal development, blood oxygenated by the placenta flows to the fetus through the umbilical vein, bypasses the fetal liver through the ductus venosus, and returns to the fetal heart through the inferior vena cava. From the inferior vena cava, blood enters the right atrium and is preferentially shunted to the left atrium through the patent foramen ovale ( Fig. 165.1 ). Fetal pulmonary vascular resistance (PVR) is higher than fetal systemic vascular resistance (SVR); this forces deoxygenated blood to mostly bypass the fetal lungs (see Fig. 165.1 ). From the left atrium, blood flows to the left ventricle and the aorta. The oxygenated blood ejected through the ascending aorta is preferentially directed to the fetal coronary and cerebral circulations.

The proportion of returning deoxygenated blood from the superior vena cava that empties into the right atrium and then right ventricle is pumped into the pulmonary artery. This poorly oxygenated blood enters the aorta through the patent ductus arteriosus and mixes with the well-oxygenated blood in the descending aorta. The mixed blood in the descending aorta then returns to the placenta for oxygenation through the two umbilical arteries.

Once the infant is delivered and the umbilical cord is cut, expansion and aeration of the lungs cause a decrease in PVR, which enhances pulmonary blood flow. Increased global oxygenation causes a physiologic closure of the umbilical arteries, umbilical vein, ductus venosus, and ductus arteriosus. Increasing pulmonary blood flow to the infant’s left atrium promotes closure of the foramen ovale. Complete anatomic closure of the foramen ovale does not occur until about 3 months of age. Although the ductus arteriosus functionally closes at about 10 to 15 hours of life, complete anatomic closure does not occur until 2 to 3 weeks of life.

In the absence of any congenital cardiac defects, these transitional circulatory changes pose no physiologic problems to the infant. However, closure of the ductus arteriosus can be life-threatening in neonates with cardiac defects that depend on the patency of the ductus arteriosus for survival.

Pathophysiology of Cardiovascular Compensatory Responses

The young myocardium is inefficient and unable to increase contractility in response to demand. When more cardiac output is needed, infants and children respond with an increase in heart rate; therefore, bradycardia is an ominous sign that connotes a severely compromised cardiac output. Children develop the adult capacity to increase contractility by 8 to 10 years of age.

A decrease in stroke volume can be caused by a weak “pump,” decreased volume in the circulation, or both. As stroke volume decreases, a compensatory increase in the heart rate is needed to preserve normal cardiac output. The most common cause of decreased stroke volume in children is hypovolemia from dehydration. Other causes of decreased stroke volume in children are listed in Box 165.2 .

Pathophysiology of Cyanosis

Cyanosis is a clinical sign caused by the preponderance of deoxygenated blood in the capillary beds, most readily observed in the mucous membranes, conjunctiva, nail beds, and skin. For cyanosis to be evident clinically, there must be at least 4 to 5 g/dL of deoxyhemoglobin admixed in the blood; this usually correlates with an oxygen saturation of approximately 80% to 85%. However, children with anemia—even if hypoxic—may not show overt signs of cyanosis (i.e., the critical mass of deoxygenated hemoglobin is not met to manifest cyanosis clinically). Central cyanosis results from a decrease in pulmonary ventilation and oxygenation, a decrease in pulmonary perfusion, the shunting of deoxygenated blood directly into the systemic circulation, or the presence of abnormal hemoglobin. Cyanosis in the neonate may be due to a variety of cardiac, pulmonary, hematologic, or toxic causes. Cardiac causes of cyanosis include congenital lesions with right-to-left shunts and cardiac lesions with decreased or increased pulmonary blood flow. Common pulmonary causes of cyanosis include bronchiolitis, pneumonia, and pulmonary edema. Methemoglobinemia is a hematologic cause of cyanosis.

Clinical Features of Cyanosis

Central cyanosis involves the lips, tongue, and mucous membranes; peripheral cyanosis (acrocyanosis) involves the hands and feet. Acrocyanosis is a common mostly benign finding in neonates caused by cold stress and peripheral vasoconstriction. Infants with cyanosis due to a congenital heart defect may not exhibit as much respiratory distress compared with the infant with cyanosis due to a pulmonary cause. Thus, a cardiac cause of central cyanosis should be suspected in a child who appears “comfortably blue.” Another important clinical clue to the cause of central cyanosis is that cyanosis of cardiac origin usually worsens with crying, whereas cyanosis due to a pulmonary cause may improve. Cyanotic congenital heart defects with right-to-left shunting will demonstrate a minimal improvement with supplemental oxygen, whereas cyanosis of a purely pulmonary origin typically exhibits a significant improvement with supplemental oxygen ( Table 165.1 ).

History

Infants with an underlying congenital heart disorder may be detected with a thorough history targeting key question ( Box 165.3 ). Additional history including diaphoresis during feeds and poor weight gain may signal early congestive heart failure (CHF). The cause of the infant’s hypoxia—cardiac or pulmonary—may be ascertained by the age at onset and the events surrounding a change in color. For example, an infant who sweats during feeding may exhibit a splanchnic steal from anomalous coronary arteries, causing transient ischemia, pain, color change, and diaphoresis that resolve after eating. A child with an undiagnosed congenital heart defect resulting in CHF and pulmonary edema may take longer to feed, frequently pausing to catch his or her breath, with subsequent poor weight gain and gradually increasing work of breathing. Respiratory tract infections are common during childhood and may cause an acute deterioration in a child with an underlying cardiac disorder. In turn, children with congenital heart disease (CHD) with large left-to-right shunts and increased pulmonary blood flow tend to have a higher incidence of lower respiratory tract infections. Acute respiratory distress in these patients may be from a combination of pulmonary and cardiac factors (e.g., CHF).

Chest Pain

Common causes of pediatric chest pain are musculoskeletal chest wall pain, asthma exacerbation, pneumonia, pleurisy, gastritis, and gastroesophageal reflux. Precordial catch syndrome (also known as Texidor’s twinge ) presents as a sharp, focal pain, usually located in the left periapical area of the chest wall. It occurs suddenly, is often worsened by inspiration, and is not associated with dyspnea. The child may report that the pain “took my breath away” or that “I was afraid to move”; the pain typically resolves within a few minutes and is not associated with dysrhythmias or other sequelae.

Chest pain or syncope on exertion should be investigated for an underlying cardiac condition, especially if there is a positive family history of sudden unexplained death in young adulthood. Myocardial injury may also be secondary to drug abuse (e.g., cocaine, methamphetamines, synthetic or over-the-counter drugs of abuse). Pulmonary embolism is a possible cause of chest pain, especially in pregnant adolescent girls, patients taking oral contraceptive agents, or those with blood dyscrasias. The rare, though life-threatening, condition of aortic dissection should be considered as a cause of chest pain in a patient with physical examination findings suggestive of a collagen vascular disorder, such as Marfan syndrome.

Physical Examination

Hyperoxia Test

The hyperoxia test may help differentiate between cardiac and pulmonary causes of central cyanosis. This test consists of assessment of the rise in arterial oxygenation with the administration of 100% oxygen. An arterial blood gas is measured after several minutes on high-flow oxygen (100% oxygen). When the child is breathing high-flow oxygen, an arterial oxygen partial pressure (Pa o ₂ ) of more than 250 mm Hg virtually excludes hypoxia due to CHD—a “passed” hyperoxia test. An arterial oxygen reading of less than 100 mm Hg (in a child without obvious pulmonary disease) is consistent with a right-to-left shunt and is highly predictive of CHD—a “failed” hyperoxia test. Values of 100 to 250 mm Hg may indicate lesions with intracardiac mixing. Pulse oximetry is not an appropriate substitute for an arterial blood gas analysis; it is not sensitive enough to determine “pass” or “fail” of the test, because a child breathing high-flow oxygen and registering 100% on pulse oximetry may actually have a Pa o ₂ anywhere between 80 and 680 mm Hg. Prolonged administration of 100% oxygen may cause some theoretic problems, such as, closure of the ductus arteriosus in infants with critical left-sided heart obstructions or pulmonary vasodilation (which could potentially worsen pulmonary vascular congestion). However, oxygen should not be initially withheld in critically ill infants based on this concern alone; rather, clinicians should closely monitor the response to oxygen in infants with suspected CHD.

Laboratory Analysis

Patients with CHF exacerbation may exhibit respiratory acidosis (low pH and high Pa co ₂ ), in addition to a low Pa o _2, due to respiratory fatigue; this may initially be clinically subtle. In contrast, children with compensated cyanotic congenital heart defects may have a normal pH despite a (chronically) low Pa o _2. Chronic mild hypoxemia causes a chronic mild acid load on the respiratory, renal, and blood buffer systems; acute illness, such as a respiratory infection, can rapidly cause a decompensation in this fragile balance, resulting in a worsening acidosis. Patients with congenital heart defects who are not experiencing respiratory compromise are unlikely to exhibit elevation in Pa co ₂ .

Children with cyanotic CHD often partially compensate with an acquired polycythemia. Hemoglobin and hematocrit levels can also help determine if a child with CHF has pallor due to CHD or high-output failure from anemia. Serum electrolyte values may be helpful in the evaluation of children with acute dysrhythmias, suspected metabolic acidosis, or chronic diuretic therapy. We recommend that patients with a known congenital heart defect or an acquired cardiac disorder (e.g., Kawasaki disease, acute rheumatic heart disease, myocarditis, pericarditis, and cardiomyopathy) who present with dysrhythmias, syncope, chest pain, or unexplained shortness of breath be assessed with cardiac biomarkers and an ECG.

Chest Radiography

Three important features of the chest radiograph ( Fig. 165.2 ) are the cardiac size (cardiothoracic ratio), the cardiac shape (silhouette), and the degree of pulmonary vascular markings. The easiest method to gauge heart size in children is to determine the cardiothoracic ratio: compare the largest transverse diameter of the cardiac shadow on the posteroanterior view of the chest radiograph with the widest internal diameter (measured from the inside rib margin at the widest point above the costophrenic angles) of the chest. The films should be obtained during maximal inspiration whenever feasible. Of note, the cardiothoracic ratio is not very accurate in preverbal children, in whom a good inspiratory view is rarely obtained.

The normal cardiothoracic ratio in children is 50% to 55%. A cardiac silhouette that is larger than normal may be due to a shunt lesion, cardiomegaly, or pericardial effusion. An enlarged heart shadow on a chest radiograph more reliably reflects a volume overload rather than pressure overload. The cardiac size can be falsely increased in infants by the presence of the thymus, seen in the mediastinum on the chest radiograph from birth until about 5 years of age. The thymic borders are typically wavy in appearance and sometimes can be seen as the classic “sail sign” along the superior right border of the heart ( Fig. 165.3 ). The thymic shadow may not be visible radiographically in infants during times of physiologic stress, but should reappear when the infant recovers.

The three classic cardiac silhouettes seen in patients with congenital heart defects are the “boot-shaped heart” of tetralogy of Fallot ( Fig. 165.4 ), the “egg-on-a-string” silhouette of transposition of the great arteries, and the “snowman-shaped” or “figure-of-eight” heart of total anomalous pulmonary venous return.

The degree of pulmonary vascular markings is a key factor in the differential diagnosis of congenital heart defects. Increased pulmonary vascularity is present when the pulmonary arteries appear enlarged and are visible in the lateral third of the lung fields or the lung apices. Another marker of increased pulmonary vascularity is seen on the posteroanterior view of the chest radiograph: the diameter of the right pulmonary artery in the right hilum is wider than the internal diameter of the trachea. The differential diagnosis of a cyanotic infant with decreased vascular markings includes Tetralogy of Fallot, pulmonary atresia, and tricuspid atresia. The cyanotic infant with increased vascular markings may have transposition of the great arteries, total anomalous pulmonary venous return, or truncus arteriosus. Increased vascular markings in an acyanotic infant are suggestive of an endocardial cushion defect, VSD, ASD, or patent ductus arteriosus.

In a normal left-sided aortic arch, the aorta descends to the left of the midline and slightly displaces the tracheal air shadow toward the right of midline above the level of the carina. With a right-sided aortic arch, the tracheal air shadow may be midline or deviated toward the left. A right-sided aortic arch is found in up to 25% of the children with tetralogy of Fallot. Rib notching secondary to increased collateral blood flow along the intercostal vessels can sometimes be appreciated between the fourth and eighth ribs in older children with undiagnosed coarctation of the aorta ( Fig. 165.5 ) but is rarely visualized in children with coarctation of the aorta who are younger than 5 years old.

Electrocardiography

Electrocardiographic findings in infants and children change with the child’s age ( Table 165.3 ). At birth, muscle mass of the right ventricle is greater than that of the left ventricle; this is demonstrated by right axis deviation on the neonatal ECG. By the end of the first month of life, the left ventricle assumes dominance. By 6 months old, the left ventricular to right ventricular mass ratio is 2 : 1, which then reaches the adult ratio of 2.5 : 1 by adolescence. The durations of the PR interval, QRS complex, and QT intervals increase with age.

Left axis deviation is present when the QRS axis is less than the lower limit of normal for the child’s age; it occurs with left ventricular hypertrophy and left bundle branch block. Right axis deviation is present when the QRS axis is greater than the upper limit of normal for the child’s age; this is seen with right ventricular hypertrophy and right bundle branch block. A “superior” QRS axis (0 to −180 degrees with an S wave in aV _F greater than the R wave) may be suggestive of an endocardial cushion defect or tricuspid atresia.

Some common indications for ECG in a pediatric patient include chest pain, dyspnea, syncope, palpitations, suspected dysrhythmias, or an underlying cardiac disorder. A rare but potentially fatal congenital cardiac abnormality detected by ECG, anomalous origin of the left coronary artery from the pulmonary artery (ALCAPA), will show ischemic changes. Infants may have a history of poor feeding, irritability, and failure to thrive. Older children and adolescents may have acute-on-chronic ischemic symptoms. Anyone of any age with ALCAPA may suddenly present with cardiogenic shock secondary to myocardial ischemia. Evidence of volume overload seen on ECG includes: right atrial enlargement (also seen with ASD, atrioventricular [AV] canal defects, tricuspid atresia, Ebstein anomaly, and severe pulmonary stenosis); and right ventricular hypertrophy (also seen with pulmonary stenosis, Tetralogy of Fallot, transposition of the great arteries, VSD with pulmonary stenosis or pulmonary hypertension, coarctation of aorta [CoA] in the newborn, pulmonary valve atresia, and hypoplastic left heart syndrome).

Biochemical Markers

As in adults, cardiac troponin T (cTnT) and cardiac troponin I (cTnI) are highly sensitive and specific in children for myocardial damage. Reference values are slightly higher for neonates younger than 3 months of age; normal and indeterminate values will depend on the bioassay used. The indications for troponin testing in children include suspected cardiac ischemia (of any etiology), myocarditis, and myocardial dysfunction in sepsis syndrome. Several studies have supported the use of plasma B-type natriuretic peptide (BNP) levels in the assessment and management of CHF in adults. Elevated BNP levels have demonstrated a similar correlation in children with CHF. BNP levels also correlate with the clinical symptoms of heart failure and ejection fraction.

Congenital Heart Disease