EVALUATING THE CYANOTIC PATIENT

Clinical cyanosis is defined as tissue-threatening hypoxemia. If not promptly recognized and the cause is not correctly diagnosed and effectively treated, multisystem end-organ dysfunction will occur. Clinical workup and initial management will depend on prenatal diagnosis, obstetric course, comorbid symptoms, physical examination, and preliminary diagnostic imaging.

Initial diagnostic imaging in most algorithms begins with chest radiography. Single- and two-projection chest radiographs afford fundamental cyanotic cardiac disease evaluation, including heart size and contour, degree of pulmonary vascularity, and aortic arch sidedness. If the heart is enlarged, analysis should address which chambers may be dilated. In parallel, the airway, lung parenchyma, pleura, and thoracic skeleton are analyzed for alternative or comorbid noncardiovascular causes.

Recognition of cardiomegaly with decreased pulmonary vascularity on the chest radiograph signals that there is significant pulmonary inflow obstruction associated with intracardiac right-to-left shunting of venous deoxygenated blood away from the lungs and into the systemic circulation. Flow obstruction may occur at the tricuspid valve, infundibulum, pulmonary valve, or a combination thereof, whereas the shunting may occur at the atrial or ventricular septal levels. Specific anatomic pathologies include Ebstein anomaly, pulmonary atresia with ventricular septal defect, a severe variant of tetralogy of Fallot, isolated critical pulmonary stenosis, and pulmonary atresia with intact ventricular septum.

For anatomic obstructive lesions, the ductus arteriosus is often patent, providing retrograde flow into the pulmonary circulation. Bronchial arteries and other aortopulmonary collaterals (APCs) may also provide systemic perfusion into the pulmonary circulation. The degree of decreased PBF and size and involvement of the ventricular and atrial chambers will depend on the level of obstruction, level of shunting, collateral flow, right ventricular pressures, and tricuspid valve competence.

When further diagnostic imaging is required to evaluate cyanotic structural congenital heart lesions, decisions should reflect the cardiac imaging goals, presence of comorbid synchronous disorders in other body systems, and the patient’s sedation, anesthesia risk, and radiosensitivity. Options include echocardiography, right and left heart catheterization with angiography, right heart catheterization with or without angiography, magnetic resonance imaging (MRI), and magnetic resonance angiography (MRA), and computed tomography angiography (CTA).

Echocardiography is ideal as the next step for most algorithms. It is portable, relatively inexpensive, and does not expose patients to ionizing radiation or iodinated contrast medium. Real-time two-dimensional, multiprojectional, transthoracic echocardiography with standard gray-scale and color Doppler techniques readily evaluates the cardiac chambers, interatrial and interventricular septa, valves, systemic and pulmonary venous connections, central pulmonary arteries, and thoracic aorta (including arch sidedness). In select applications, such as with valvular disease, three-dimensional echocardiography can offer greater structural detail. Concurrent with the structural analysis, flow, function, and hemodynamics are all assessed. Echocardiography, however, is operator-dependent, may have limited sonographic windows, and cannot evaluate peripheral thoracic vascular anatomy.

In the diagnostic algorithm at many centers, catheterization with angiography follows echocardiography. The objective is not primarily to confirm the anatomy, but rather to obtain flow and pressure dynamics directly. However, this approach is invasive, exposes the patient to ionizing radiation, uses iodinated contrast medium and, in the current workflow, is not cost-effective. Radiation dose can be reduced and contrast medium obviated if the procedure is limited to a right heart catheterization without angiography.

MRI, MRA, and CTA are alternative noninvasive modalities to catheterization and may provide comprehensive evaluation of cardiac and pulmonary morphology and function, which is key to diagnosing the cyanotic patient with decreased pulmonary vascularity (Figs. 48-1 and 48-2). Both can generate cardiac chamber volume and qualitative and quantitative functional data. Image postprocessing can readily be facilitated for data sets from both modalities.

FIGURE 48-1 A full-term neonate presented in respiratory distress on day 2 of life. An anteroposterior chest radiograph reveals an enlarged right atrium and ventricle associated with mild decreased pulmonary vascularity, consistent with a right-sided obstructive lesion. Echocardiography was subsequently performed, demonstrating critical pulmonary stenosis.

FIGURE 48-2 The first few hours postdelivery of a term neonate was complicated by respiratory distress. A, An anteroposterior chest radiograph reveals moderate cardiomegaly with a dominantly enlarged right atrium (arrowheads), an absent main pulmonary artery segment (asterisk), and decreased peripheral pulmonary vascularity. Echocardiography confirmed pulmonary atresia with an intact ventricular septum. B, CTA (volume rendered image) was performed to map out the aortopulmonary collateral arteries (arrow).

MRI and MRA are ideal because no ionizing radiation is required, there is no use of iodinated contrast medium, and only direct assessment of flow dynamics can be made. MRI and MRA examination times, however, may average 30 to 40 minutes, necessitating sedation or general anesthesia in the pediatric population. Furthermore, MRI does not afford comprehensive lung evaluation. Assessment of body systems outside of the thorax will add considerable examination time. During surveillance imaging, MRI and MRA may be contraindicated or nondiagnostic because of the presence of implanted devices and ferrous material.

A typical MRI protocol for evaluating cyanotic congenital heart lesions begins with standard bright and dark blood sequences. The goal is to survey and define the thoracic and upper abdominal cardiovascular and noncardiovascular morphology. Cine sequences should then be obtained for qualitative and quantitative cardiac chamber and valve functional analysis. Next, velocity maps are obtained for hemodynamic analysis, targeting at a minimum the ventriculoarterial valves and supravalvular segments. Regions of interest may also be placed on the branch pulmonary arteries, atrioventricular valves, central systemic veins, and pulmonary veins. Gadolinium-enhanced three-dimensional MRA is subsequently performed for further structural analysis. MRA is most useful for evaluating the pulmonary arteries, aorta, and systemic and pulmonary veins. Time-resolved MRA is advantageous in this instance because it provides real-time angiographic flow patterns.

Despite its dependence on ionizing radiation and iodinated contrast medium, CTA can be used for complete evaluation of the cyanotic cardiac disorders. Advantages include short examination times, maintenance of high diagnostic image quality in the presence of devices and metallic material, and nonthoracic cardiovascular and multisystem organ evaluation with the same bolus of contrast medium. With current CT scanner technology, sedation and anesthesia often can be eliminated. As with all CTA applications, dose reduction strategies are the standard of care. Voltage and amperage should be reduced, balancing the expected diagnostic and image qualities. If possible, coverage should be targeted to the pertinent thoracic anatomy. For most pediatric applications, pertinent morphology can be evaluated in a single series without electrocardiographic gating. Noncontrast and delayed acquisitions are rarely required.

EBSTEIN ANOMALY

Ebstein anomaly is a rare cardiac disorder, accounting for 0.04% to 0.93% of congenital heart malformations.^1–4 The distinguishing pathology is dysplastic tricuspid valve leaflets, which extend into and attach to the right ventricle, rather than the annulus at the atrioventricular junction, leading to tricuspid valve incompetence. Usually, the septal and posterior leaflets are involved, with insertion at the margin of the inlet and trabecular right ventricular zones, either directly or by way of anomalous chordae tendinae and papillary muscles. The degree of septal and posterior leaflet displacement and abnormal morphology has wide variability.

In distinction to the septal and posterior leaflets, the anterior leaflet typically has dysplastic morphology, but maintains normal attachments. It is enlarged, with a sail appearance and fibrous strands, which may be muscularized. Rarely, the anterior leaflet may have abnormal displacement along the right ventricular anterior free wall, leading to tricuspid stenosis. In 10% of cases, the anterior leaflet may have downward displacement, obstructing the ventricular inlet, leading to an obstructing, imperforate tricuspid valve.^5,6

Downward leaflet displacement divides the right ventricle into a basilar, atrialized inlet portion and a small functional true right ventricle. Both portions are typically dilated, with a loss of myocytes, leading to decreased contractility.⁶ Less commonly, only the atrialized portion is dilated. The native functional right ventricle has reduced right ventricular (RV) filling capacity and cardiac output. RV preload volume is further reduced by virtue of the atrialized portion functioning as a false aneurysm such that as the native right atrium contracts, the abnormal segment above the functional right ventricle expands, collecting blood. In addition to these factors, tricuspid insufficiency and possible anterior leaflet stenosis or obstruction decrease forward flow, contributing to the diminished right ventricle filling volumes.

Hemodynamically, the decreased diastolic filling and myocardial contractility lead to diminished PBF, elevated right atrial and systemic venous pressures and, ultimately, right-sided failure. With elevated right atrial pressures, right-to-left shunting will occur across a patent foramen ovale (PFO) or secundum atrial septal defect (ASD), leading to cyanosis.^5,7 Increased pulmonary vascular resistance in the early neonatal period increases afterload, further exacerbating cyanosis in these patients.⁸ As the intrapulmonary vascular resistance falls, cyanosis may improve.

Electrical conduction pathways may be abnormal with regard to the location and morphology of native pathways and the presence of accessory conduction pathways (Wolf-Parkinson-White; WPW). The conduction pathway abnormalities lead to dysrhythmias; the most common are atrial tachycardias and supraventricular and ventricular arrhythmias.^9,10

In most cases of Ebstein anomaly, there is visceral situs solitus. The ASD or PFO has been confirmed by select angiography or autopsy to be present in at least 33% to 56% of patients.^9–11 Additional cardiac anomalies may be present in at least 22% of cases. Among these anomalies, common lesions include pulmonary stenosis (35%), ventricular septal defect (VSD; 21%), patent ductus arteriosus (PDA) (20%), and pulmonary stenosis with VSD (9%).

Diagnostic Imaging

The degree of cardiomegaly on chest radiography is moderate to massive, with dominance of the right atrium (Fig. 48-3). This reflects the degree of elevated RV pressure and tricuspid insufficiency.¹⁰ Cardiac size decreases with age, so that the mean cardiothoracic ratio is 0.73 as a neonate, 0.57 during infancy and childhood, and 0.55 during adolescence and adulthood.¹¹ Pulmonary vascularity may be normal to decreased, and directly correlates with the degree of valvular deformity, right-to-left shunting, and diminished right ventricular cardiac output,

FIGURE 48-3 A 20-day-old neonate presented with respiratory difficulty and feeding intolerance. An anteroposterior chest radiograph demonstrates marked cardiomegaly, with a massively enlarged right atrium and ventricle. Echocardiography confirmed Ebstein anomaly.

Echocardiography, MRI, MRA, and CTA can all readily diagnose Ebstein anomaly. In a four-chamber projection, apical leaflet displacement into the right ventricle is identified by recognizing that it extends beyond the atrioventricular level of the mitral valve insertion (Fig. 48-4). Atrialized and native portions of the right ventricle are defined based on the insertion level. The size of the enlarged atrium, atrialized right ventricle, and small functional native right ventricle are characterized qualitatively and quantitatively. An atrial septal defect with right-to-left interatrial septal bowing is recognized with a short-axis or four-chamber projection.

FIGURE 48-4 Cardiac MRI (four-chamber projection) in a patient with Ebstein anomaly demonstrates anterior displacement of the tricuspid apparatus (black arrowheads) relative to the normally positioned mitral valve (white arrowhead), resulting in atrialization of the right ventricle. Note the decreased size of the functional right ventricle.

Color Doppler echocardiography and cine or phase contrast MRI will confirm tricuspid insufficiency and right-to-left shunting. Retrospective or single-phase CTA can also define right-to-left shunting, but detection is dependent on the contrast medium bolus and acquisition timing.