Hypertrophied muscle bundles develop in the RV cavity, creating RVOT obstruction.S4.3.3-5,S4.3.3-6 It is com- monly associated with a VSD. Double-chambered right ventricle can be missed on TTE if not sought specifically, and alternative imaging or cardiac catheterization is of- ten required to confirm the diagnosis and establish the hemodynamic impact. See Section 3.3 for recommendations on who should perform surgeries, cardiac catheterization, and other procedures in these patients; Section 3.4 for rec- ommendations on diagnostic evaluation; and Table 25 for routine testing and follow-up intervals. Recommendation-Specific Supportive Text 1. Surgery typically involves transatrial or transven- tricular resection of obstructing muscle bundles and VSD closure if present. Occasionally, patch enlargement of RVOT may be necessary to ade- quately relieve obstruction. Table 24. Branch and Peripheral PS: Routine Follow-Up and Testing Intervals Frequency of Routine Follow-Up and Testing Physiological Stage A* (mo) Physiological Stage B* (mo) Physiological Stage C* (mo) Physiological Stage D* (mo) *See Tables 3 and 4 for details on the ACHD AP classification system. Routine TTE may be unnecessary in a year when CMR imaging is performed unless clinical indications warrant otherwise. CMR may be indicated for assessment of branch PS. Baseline study is recommended with periodic follow-up CMR, with frequency of repeat imaging determined by anatomic and physiological findings. CCT may be used if CMR is not feasible and to evaluate cross-sectional imaging status and post-stent therapy for peripheral PS; the frequency should be weighed against radiation exposure. 6-minute walk test or CPET, depending on clinical indication. ACHD indicates adult congenital heart disease; CCT, cardiac computed tomography; CMR, cardiovascular magnetic resonance imaging; CPET, cardiopulmonary exercise test; ECG, electrocardiogram; PS, pulmonary stenosis; and TTE, transthoracic echocardiogram. CLINICAL STATEMENTS AND GUIDELINES 2. 