pulmonary vascular resistance in patients for whom there is a clinical suspicion of pulmonary vascular disease, or for additional anatomic or physiologic information Magnetic Resonance Imaging Although transthoracic echocardiography is typically sufficient, preoperative magnetic resonance imaging may provide useful complementary information in the assessment of the anatomy It is particularly useful in the assessment of the mass of the left ventricle in the infant presenting late (Fig 37.14) This may assist clinicians in assessing the suitability of an individual patient for an arterial switch operation In a novel application, four-dimensional sequences have been used to assess streaming of flow in both repaired and unrepaired patients.38,39 Further discussion of postoperative use of magnetic resonance imaging follows in this chapter FIG 37.14 Magnetic resonance angiogram in a 9-month-old patient who presented from a developing country with an intact ventricular septum The aorta (Ao) can been seen arising from the right ventricle (RV) and the pulmonary trunk (PT) from the left ventricle (LV) Medical Management of Neonates Postnatal Stabilisation Careful and timely management of the neonate with transposition is critical to minimize preoperative mortality, which ranges from 3% to 10%.40–42 Optimal management requires thoughtful consideration of the risks and benefits related to atrial septostomy, mechanical ventilation, the use of prostaglandin, and the timing of surgery.43 Neonates diagnosed in utero should be delivered, preferably at term, in a highrisk obstetric unit, with rapid access to advanced cardiac care, including the availability of atrial septostomy.43 Among many other benefits, term delivery allows for more complete maturation of the brain in this patient population in whom it is known that oxygen delivery to the fetal brain is significantly diminished compared with normal Venous access should be obtained immediately after delivery, and an infusion of prostaglandin E should be at least readied, if not started empirically The common side effects of prostaglandin should be anticipated, including apnea, hypotension, and fever Lower starting doses (0.0125 µg/kg per minute at our institution) are usually adequate Prompt transthoracic echocardiography will define the nature of the anatomy and guide therapy Patients with severe acidosis or hypoxemia, usually attributable to inadequate mixing at the atrial level, may require an urgent balloon atrial septostomy This is the case in up to 12% of neonates.41 In others, a nonurgent septostomy is performed if there are signs of inadequate intracardiac mixing, including acidosis, decreased systemic perfusion, or hypoxemia, or if echocardiographic findings suggest a risk for inadequate mixing As discussed later, the threshold for performing an elective septostomy is institution dependent As previously mentioned, one must not be misled by the lack of a demonstrable pressure gradient from left-to-right atrium on the echocardiogram, and, if other indicators of inadequate mixing are present, a septostomy should be considered Although traditional teaching has advised against the use of supplemental oxygen, modest amounts may be beneficial during the early postnatal hours by compensating for any alveoloarterial gradient and reducing pulmonary vascular resistance Atrial Septostomy Following its introduction by Rashkind and Miller in 1966,44 the balloon atrial septostomy has become well accepted in the preoperative management of patients with transposition and selected other lesions The procedure requires passage of a balloon-tipped catheter from the right-to-left atrium, crossing the oval foramen The balloon is then inflated with saline and, with a quick, short motion, is pulled into the right atrium Although a femoral venous approach was originally described, it is now common to use the umbilical vein to enter the heart through the venous duct Fluoroscopic screening was previously used to guide the procedure (Fig 37.15) but has largely been replaced by cross-sectional echocardiography (Fig 37.16, Video 37.3) This provides a simple and reliable method for monitoring the position of the balloon and assessing the success of the septostomy It also allows the procedure to be performed outside of the cardiac catheterization suite FIG 37.15 Fluoroscopic images from a patient undergoing an atrial septostomy Although the balloon is clearly visible, its precise relationships with important structures, including the mitral valve and the pulmonary veins, are difficult to discern