FIG 35.21 Multidetector cardiac computed tomography of a patient with unoperated tetralogy of Fallot showing the malaligned muscular outlet septum and the narrowed subpulmonary infundibulum (A), the overriding aorta (B), and the concordance of ventriculoatrial (V-A) connections (C) (A–B, Courtesy Professor Tony Hlavacek.) Cardiac Catheterization and Angiography In most large units, diagnostic cardiac catheterization is now rarely performed prior to palliative or corrective surgery (Fig 35.22) Patients with multiple aortopulmonary collateral arteries are a frequent exception to this rule, although the origin and course of such vessels will often have been demonstrated by prior MRI or CT The role of cardiac catheterization in such patients is therefore to assess the hemodynamics within and connections between the individual vessels when necessary Interventional catheterization remains applicable for some patients Dilation and stenting of the right ventricular outflow tract, with prior radiofrequency perforation if atretic, has a role in some instances Similarly, preoperative balloon dilation of the pulmonary arteries, stenting of the arterial duct, and interventions on the aortopulmonary collateral arteries may be part of a combined surgical and medical program of management, often with the need for additional postoperative interventions in the more challenging cases FIG 35.22 Angiography of the right ventricle performed in the right anterior oblique projection The outlet (infundibular) septum (OS), profiled in cross section, together with hypertrophied septoparietal trabeculations of the right ventricular free wall, produce muscular subvalvar stenosis The pulmonary trunk (PT) is seen, together with the size of the right and left pulmonary arteries (RPA and LPA, respectively) The aorta (AO) has been opacified by contrast passing through the ventricular septal defect RV, Right ventricle Hemodynamics and Physiology The hemodynamic consequences of the lesion are dominated by the severity of obstruction within the subpulmonary right ventricular outflow tract superimposed upon the presence of a large ventricular septal defect In almost all patients, the interventricular communication is large and nonlimiting This results in equalization of right and left ventricular systolic pressure regardless of the severity of pulmonary stenosis The relative flows in the pulmonary and systemic circuits depend on the relative resistances, or impedances, to emptying of the right and left ventricles When right ventricular outflow obstruction is minimal and the pulmonary vascular resistance is normal, the flow of blood to the lungs will exceed that in the systemic circuit There will be a dominant leftto-right shunt, and the clinical picture will resemble closely that of a ventricular septal defect Under these circumstances, resting cyanosis may be absent When obstruction to right ventricular emptying is similar to that provided by the systemic vascular resistance, a balanced situation exists There will be no overall shunting in either direction, although transient small right-to-left and left-to-right shunts occur during each cardiac cycle Pulmonary and systemic flows, at least at rest, will be equal On exercise, however, a fall in systemic vascular resistance, with or without an increase in infundibular stenosis, will result in a right-to-left shunt with cyanosis The systemic flow will exceed the pulmonary flow With increasing degrees of obstruction in the subpulmonary right ventricular outflow tract, a dominant right-to-left shunt develops Cyanosis becomes a constant feature Such severe obstruction may be present from birth More usually, increasing infundibular stenosis develops coincidentally with progressive right ventricular hypertrophy Cyanosis is dependent on the degree of pulmonary stenosis but unrelated to the degree of aortic override Typically the mean pulmonary arterial pressure is lower than normal, consequent upon the reduced flow of blood to the lungs The pulmonary arterial systolic pressure, though lower than that in the right ventricle, may be higher