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echocardiographic investigation, most such neonates will already be receiving prostaglandins intravenously Consequently, the arterial duct will be relatively large Although the presence of a patent arterial duct would lead one to suspect the presence of interruption of the aortic arch, ductal patency may rarely be present in association with a normal arch.28 The high left parasternal section demonstrates the duct, and color flow will usually demonstrate the flow through it to be bidirectional Pulsed Doppler interrogation reveals that systolic flow is from the pulmonary arteries to the descending aorta, albeit that, provided that the pulmonary vascular resistance is low, there will be reversal of flow during diastole The suprasternal parasagittal sections will reveal the site of aortic interruption, if present, relative to the origin of the brachiocephalic, left common carotid and left subclavian arteries Cardiac Magnetic Resonance and Computerized Tomographic Imaging These modalities are rarely used in the initial investigation of patients with common arterial trunk before surgery because, in most instances, echocardiography provides adequate diagnostic information Nonetheless, there may be some instances, for example in the examination of complex problems of the aortic arch or anomalies of the drainage of the pulmonary veins, where they may make a contribution to the preoperative investigation Magnetic resonance imaging makes a more significant contribution to the investigations of patients after surgery, when it permits the quantitative assessment of not only the performance of the right and left ventricles, but also of the severity of regurgitation of the neoaortic and pulmonary valves, along with measurement of residual intracardiac shunts and imaging of the aortic arch.29,30 Cardiac Catheterization and Angiocardiography It is now possible to refer patients with typical noninvasive findings directly for corrective surgery However, should there be any doubt about any aspect of the presentation, cardiac catheterization should be considered, particularly if there is any suggestion of pulmonary vascular obstructive disease.31,32 The systemic consumption of oxygen should be measured while samples are obtained from the superior caval, the inferior caval vein, and the pulmonary veins, as well as from the pulmonary and systemic arteries for measurement of their oxygen content The effect of administration of 100% oxygen, or pulmonary vasodilators, should be measured Entry to the pulmonary arteries is achieved most readily by retrograde arterial catheterization, looping the catheter in the truncal root so that it can pass upwards into the pulmonary arteries Oximetry will reveal an increase in the saturation of oxygen in the pulmonary arteries, compared with the caval veins, indicative of a net left-to-right shunt (Fig 40.19) Commonly, the saturation of oxygen in the pulmonary arteries may be lower than in the aorta, reflecting the streaming of blood preferentially from the right ventricle to pulmonary arteries, and from the left ventricle to the aorta Pulmonary venous saturations of oxygen cannot be assumed because pulmonary edema or chest infection can result in pulmonary venous desaturation
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