Andersons pediatric cardiology 1112

FIG 42.11 Image taken from an episcopic dataset of a mouse sacrificed at embryonic day 13.5 The aortic root has been transferred to the left ventricle The distal ends of the cushions in the intermediate component of the outflow tract are now excavating to form the leaflets of the pulmonary valve This section replicates the parasternal long-axis echocardiographic cut The walls of the sinuses are formed by additional ingrowth of nonmyocardial tissues from the second heart field The significance of the cells derived from the neural crest was demonstrated by experimental ablation of the neural crest in the chick This resulted in failure of septation and persistence of a common arterial trunk The tissues derived from the second heart field were also subsequently shown to be important in differentiation and separation of the ventricular outflow tracts.17 In terms of maldevelopment of the outflow tract, it is now established that the bicuspid aortic valve can be produced either by exuberant fusion of the peripheral components of the major outflow cushions or by fusion of the end of one of the major cushions with the intercalated cushion, thus producing a conjoined leaflet.18 It is also feasible that failure of formation of the intercalated cushion could produce a valve with only two leaflets The same processes must be capable of producing the pulmonary valve with two leaflets Stenosis of the pulmonary valve, however, develops as an acquired condition during intrauterine life.19 Interrogation of the developing fetus has now shown how progression of stenosis can produce pulmonary valvar atresia with an intact ventricular septum The degree of severity of obstruction seen at birth then depends on the extent of the process during gestation It is also well established that peripheral pulmonary arterial stenosis may be caused by congenital rubella Comparable changes are also seen in recognizable syndromes of malformation, such as the syndromes of Williams and Alagille Clinical Diagnosis Presentation The presentation of pulmonary stenosis depends on age and severity In the neonate, critical pulmonary stenosis presents with life-threatening cyanosis and atrial-level right-to-left shunting The differential diagnosis would include other forms of neonatal cyanosis, including transposition of the great vessels and the various forms of pulmonary atresia Such patients are likely to depend on the arterial duct to provide the flow of blood to the lungs Hence palliation in the short term, by maintaining ductal patency with intravenous infusions of prostaglandin, is lifesaving until a more definitive diagnosis can be made and an appropriate intervention planned Outside of the neonatal period, mild or moderate pulmonary stenosis is not likely to cause major symptoms, unless there are associated lesions or other factors For example, patients with the typical phenotype of Noonan syndrome may have difficulty with feeding and gaining weight, unassociated in most cases with the severity of the pulmonary stenosis Discovery of disease of mild or moderate severity is most likely when physical signs are detected during consultations for other matters, most commonly when a cardiac murmur is heard Although some patients with mild to moderate pulmonary valve stenosis may present with exertional dyspnea, this may not necessarily be related to inability of the right ventricle to increase the stroke volume across a fixed right ventricular outflow tract obstruction but instead may represent the patient's physical deconditioning While De Meester and colleagues found a reduced peak workload and peak oxygen uptake in a study evaluating 19 adults with mild to moderate pulmonary valve stenosis, they also identified a lower baseline heart rate and heart rate reserve as well as a lower threshold of workload reached at anaerobic threshold and peak exercise compared to healthy controls, suggesting that physical deconditioning may play a role in explaining these findings.20,21 This is important to keep in mind when patients with mild to moderate pulmonary valve stenosis are being managed so as to avoid inappropriate limitations from physical activities and to encourage them to maintain a healthy lifestyle with regular exercise The ability to increase cardiac output with exercise beyond increasing the heart rate in patients with mild to moderate