Laboratory Evaluation

Published on 27/03/2015 by admin

Filed under Pediatrics

Last modified 27/03/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 2446 times

Chapter 417 Laboratory Evaluation

417.1 Radiologic Assessment

Daniel Bernstein

The chest x-ray may provide information about cardiac size and shape, pulmonary blood flow (vascularity), pulmonary edema, and associated lung and thoracic anomalies that may be associated with congenital syndromes (skeletal dysplasias, extra or deficient number of ribs, abnormal vertebrae, previous cardiac surgery). Variations are due to differences in body build, the phase of respiration or the cardiac cycle, abnormalities of the thoracic cage, position of the diaphragm, or pulmonary disease.

The most frequently used measurement of cardiac size is the maximal width of the cardiac shadow in a posteroanterior chest film taken mid-inspiration. A vertical line is drawn down the middle of the sternal shadow, and perpendicular lines are drawn from the sternal line to the extreme right and left borders of the heart; the sum of the lengths of these lines is the maximal cardiac width. The maximal chest width is obtained by drawing a horizontal line between the right and left inner borders of the rib cage at the level of the top of the right diaphragm. When the maximal cardiac width is more than half the maximal chest width (cardiothoracic ratio >50%), the heart is usually enlarged. Cardiac size should be evaluated only when the film is taken during inspiration with the patient in an upright position. A diagnosis of “cardiac enlargement” on expiratory or prone films is a common cause of unnecessary referrals and laboratory studies.

The cardiothoracic ratio is a less useful index of cardiac enlargement in infants than in older children because the horizontal position of the heart may increase the ratio to >50% in the absence of true enlargement. Furthermore, the thymus may overlap not only the base of the heart but also virtually the entire mediastinum, thus obscuring the true cardiac silhouette.

A lateral chest roentgenogram may be helpful in infants as well as in older children with pectus excavatum or other conditions that result in a narrow anteroposterior chest dimension. In these situations, the heart may appear small in the lateral view and suggest that the apparent enlargement in the posteroanterior projection was due to either the thymic image (anterior mediastinum only) or flattening of the cardiac chambers as a result of a structural chest abnormality.

In the posteroanterior view, the left border of the cardiac shadow consists of three convex shadows produced, from above downward, by the aortic knob, the main and left pulmonary arteries, and the left ventricle (Fig. 417-1). In cases of moderate to marked left atrial enlargement, the atrium may project between the pulmonary artery and the left ventricle. The outflow tract of the right ventricle does not contribute to the shadows formed by the left border of the heart. The aortic knob is not as easily seen in infants and children as in adults. The side of the aortic arch (left or right) can often be inferred as being opposite the side of the midline from which the air-filled trachea is visualized. This observation is important because a right-sided aortic arch is often present in cyanotic congenital heart disease, particularly in tetralogy of Fallot. Three structures contribute to the right border of the cardiac silhouette. In the view from above, they are the superior vena cava, the ascending aorta, and the right atrium.

Enlargement of cardiac chambers or major arteries and veins results in prominence of the areas in which these structures are normally outlined on the chest x-ray. In contrast, the electrocardiogram (ECG) is a more sensitive and accurate index of ventricular hypertrophy.

The chest roentgenogram is also an important tool for assessing the degree of pulmonary vascularity. Angiocardiographic studies have shown that the hilar shadows seen on the plain chest roentgenogram are mainly vascular. Pulmonary overcirculation is usually associated with left-to-right shunt lesions, whereas pulmonary undercirculation is associated with obstruction of the outflow tract of the right ventricle.

The esophagus is closely related to the great vessels, and a barium esophagogram can help delineate these structures in the initial evaluation of suspected vascular rings, although this has largely been supplanted by CT. Echocardiographic examination best defines the morphologic features of intracardiac chambers, cardiac valves, and intracardiac shunts. CT is used as an adjunct to echo to evaluate extracardiac vascular morphology. MRI is used to quantitate ventricular volumes, cardiac function, and shunt and regurgitant fractions.

417.2 Electrocardiography

Developmental Changes

The marked changes that occur in cardiac physiology and chamber dominance during the perinatal transition (Chapter 415) are reflected in the evolution of the ECG during the neonatal period. Because vascular resistance in the pulmonary and systemic circulations is nearly equal in a term fetus, the intrauterine work of the heart results in an equal mass of both the right and left ventricles. After birth, systemic vascular resistance rises when the placental circulation is eliminated, and pulmonary vascular resistance falls when the lungs expand. These changes are reflected in the ECG as the right ventricular wall begins to thin.

The ECG demonstrates these anatomic and hemodynamic features principally by changes in QRS and T-wave morphologic features. It is recommended that a 13-lead ECG be performed in pediatric patients, including either lead V3R or V4R, which are important in the evaluation of right ventricular hypertrophy. On occasion, lead V1 is positioned too far leftward to reflect right ventricular forces accurately. This problem is present particularly in premature infants, in whom the electrocardiographic electrode gel may produce contact among all the precordial leads.

During the 1st days of life, right axis deviation, large R waves, and upright T waves in the right precordial leads (V3R or V4R and V1) are the norm (Fig. 417-2). As pulmonary vascular resistance decreases in the 1st few days after birth, the right precordial T waves become negative. In the great majority of instances, this change occurs within the 1st 48 hr of life. Upright T waves that persist in leads V3R, V4R, or V1 beyond 1 wk of life are an abnormal finding indicating right ventricular hypertrophy or strain, even in the absence of QRS voltage criteria. The T wave in V1 should never be positive before 6 yr of age and may remain negative into adolescence. This finding represents one of the most important, yet subtle differences between pediatric and adult ECGs and is a common source of error when adult cardiologists interpret pediatric ECGs.

In a newborn, the mean QRS frontal-plane axis normally lies in the range of +110 to +180 degrees. The right-sided chest leads reveal a larger positive (R) than negative (S) wave and may do so for months because the right ventricle remains relatively thick throughout infancy. Left-sided leads (V5 and V6) also reflect right-sided dominance in the early neonatal period, when the R : S ratio in these leads may be <1. A dominant R wave in V5 and V6 reflecting left ventricular forces quickly becomes evident within the 1st few days of life (Fig. 417-3). Over the years, the QRS axis gradually shifts leftward, and the right ventricular forces slowly regress. Leads V1, V3R, and V4R display a prominent R wave until 6 mo to 8 yr of age. Most children have an R : S ratio >1 in lead V4R until they are 4 yr of age. The T waves are inverted in leads V4R, V1, V2, and V3 during infancy and may remain so into the middle of the 2nd decade of life and beyond. The processes of right ventricular thinning and left ventricular growth are best reflected in the QRS-T pattern over the right precordial leads. The diagnosis of right or left ventricular hypertrophy in a pediatric patient can be made only with an understanding of the normal developmental physiology of these chambers at various ages until adulthood is reached. As the left ventricle becomes dominant, the ECG evolves to the characteristic pattern of older children (Fig. 417-4) and adults (Fig. 417-5).

Ventricular hypertrophy may result in increased voltage in the R and S waves in the chest leads. The height of these deflections is governed by the proximity of the specific electrode to the surface of the heart; by the sequence of electrical activation through the ventricles, which can result in variable degrees of cancellation of forces; and by hypertrophy of the myocardium. Because the chest wall in infants and children, as well as in adolescents, may be relatively thin, the diagnosis of ventricular hypertrophy should not be based on voltage changes alone.

The diagnosis of pathologic right ventricular hypertrophy is difficult in the 1st wk of life because physiologic right ventricular hypertrophy is a normal finding. Serial tracings are often necessary to determine whether marked right axis deviation and potentially abnormal right precordial forces or T waves, or both, will persist beyond the neonatal period (Fig. 417-6). In contrast, an adult electrocardiographic pattern (see Fig. 417-5) seen in a neonate suggests left ventricular hypertrophy. The exception is a premature infant, who may display a more “mature” ECG than a full-term infant (Fig. 417-7) as a result of lower pulmonary vascular resistance secondary to underdevelopment of the medial muscular layer of the pulmonary arterioles. Some premature infants display a pattern of generalized low voltage across the precordium.

The ECG should always be evaluated systematically to avoid the possibility of overlooking a minor, but important abnormality. One approach is to begin with an assessment of rate and rhythm, followed by a calculation of the mean frontal-plane QRS axis, measurements of segment intervals, assessment of voltages, and, finally, assessment of ST and T-wave abnormalities.

P Waves

Tall (>2.5 mm), narrow, and spiked P waves are indicative of right atrial enlargement and are seen in congenital pulmonary stenosis, Ebstein anomaly of the tricuspid valve, tricuspid atresia, and sometimes cor pulmonale. These abnormal waves are most obvious in leads II, V3R, and V1 (Fig. 417-8A). Similar waves are sometimes seen in thyrotoxicosis. Broad P waves, commonly bifid and sometimes biphasic, are indicative of left atrial enlargement (Fig. 417-8B). They are seen in some patients with large left-to-right shunts (ventricular septal defect [VSD], patent ductus arteriosus [PDA]) and with severe mitral stenosis or regurgitation. Flat P waves may be encountered in hyperkalemia.

QRS Complex

Right Ventricular Hypertrophy

For the most accurate assessment of ventricular hypertrophy, pediatric ECGs should include the right precordial lead V3R or V4R, or both. The diagnosis of right ventricular hypertrophy depends on demonstration of the following changes (see Fig. 417-6): (1) a qR pattern in the right ventricular surface leads; (2) a positive T wave in leads V3R-V4R and V1-V3 between the ages of 6 days and 6 yr; (3) a monophasic R wave in V3R, V4R, or V1; (4) an rsR′ pattern in the right precordial leads with the 2nd R wave taller than the initial one; (5) age-corrected increased voltage of the R wave in leads V3R-V4R or the S wave in leads V6-V7, or both; (6) marked right axis deviation (>120 degrees in patients beyond the newborn period); and (7) complete reversal of the normal adult precordial RS pattern. At least 2 of these changes should be present to support a diagnosis of right ventricular hypertrophy.

Abnormal ventricular loading can be characterized as either systolic (as a result of obstruction of the right ventricular outflow tract, as in pulmonic stenosis) or diastolic (as a result of increased volume load, as in atrial septal defects [ASDs]). These two types of abnormal loads result in distinct electrocardiographic patterns. The systolic overload pattern is characterized by tall, pure R waves in the right precordial leads. In older children, the T waves in these leads are initially upright and later become inverted. In infants and children <6 yr, the T waves in V3R-V4R and V1 are abnormally upright. The diastolic overload pattern (typically seen in patients with ASDs) is characterized by an rsR′ pattern (Fig. 417-9) and a slightly increased QRS duration (minor right ventricular conduction delay). Patients with mild to moderate pulmonary stenosis may also exhibit an rsR′ pattern in the right precordial leads.

Left Ventricular Hypertrophy

The following features indicate the presence of left ventricular hypertrophy (Fig. 417-10): (1) depression of the ST segments and inversion of the T waves in the left precordial leads (V5, V6, and V7), known as a left ventricular strain pattern—these findings suggest the presence of a severe lesion; (2) a deep Q wave in the left precordial leads; and (3) increased voltage of the S wave in V3R and V1 or the R wave in V6-V7, or both. It is important to emphasize that evaluation of left ventricular hypertrophy should not be based on voltage criteria alone. The concepts of systolic and diastolic overload, though not always consistent, are also useful in evaluating left ventricular enlargement. Severe systolic overload of the left ventricle is suggested by straightening of the ST segments and inverted T waves over the left precordial leads; diastolic overload may result in tall R waves, a large Q wave, and normal T waves over the left precordium. Finally, an infant with an ECG that would be considered “normal” for an older child may, in fact, have left ventricular hypertrophy.

ST Segment and T-wave Abnormalities

A slight elevation of the ST segment may occur in normal teenagers and is attributed to early repolarization of the heart. In pericarditis, irritation of the epicardium may cause elevation of the ST segment followed by abnormal T-wave inversion as healing progresses. Administration of digitalis is sometimes associated with sagging of the ST segment and abnormal inversion of the T wave.

Depression of the ST segment may also occur in any condition that produces myocardial damage or ischemia, including severe anemia, carbon monoxide poisoning, aberrant origin of the left coronary artery from the pulmonary artery, glycogen storage disease of the heart, myocardial tumors, and mucopolysaccharidoses. An aberrant origin of the left coronary artery from the pulmonary artery may lead to changes indistinguishable from those of acute myocardial infarction in adults. Similar changes may occur in patients with other rare abnormalities of the coronary arteries and in those with cardiomyopathy, even in the presence of normal coronary arteries. These patterns are often misread in young infants because of the unfamiliarity of pediatricians with this “infarct” pattern, and thus a high index of suspicion must be maintained in infants with dilated cardiomyopathy or with symptoms compatible with coronary ischemia (e.g., inconsolable crying).

T-wave inversion may occur in myocarditis and pericarditis, or it may be a sign of either right or left ventricular hypertrophy and strain. Hypothyroidism may produce flat or inverted T waves in association with generalized low voltage. In hyperkalemia, the T waves are commonly of high voltage and are tent-shaped (Fig. 417-13).

Buy Membership for Pediatrics Category to continue reading. Learn more here