SubAS may occur in isolation or as part of a suite of abnormalities. In adults with Shone complex or its variants, subAS may be one of several LV obstructive lesions, including vari- ants of congenital mitral stenosis, supravalvular mitral stenosis, valvular aortic stenosis, supravalvular aortic stenosis, and CoA.S4.2.3-7 SubAS tends to recur, particularly when initial resec- tion is needed in childhood. Surgical repair for subAS carries a 10% to 15% risk of complete heart block.S4.2.3-6 SubAS may be first diagnosed in adulthood and may be confused with hypertrophic obstructive cardiomyopa- thy when LV hypertrophy of sufficient severity has de- veloped such that the subaortic membrane is less evi- dent on imaging. The recommendations in this guideline apply to subAS caused by a discrete membrane or tunnel-like obstruction. Similar principles may apply to more com- plex causes of subAS, but insufficient data exist to sup- port recommendations for more complex lesions, and extrapolation needs to take the additional anatomic complexity into account. Turbulent flow created distal to the subaortic ob- struction may cause barotrauma to the adjacent aortic valve leaflets and result in progressive AR, which may itself become clinically significant. Resection of the sub- aortic obstruction ideally delays or prevents the even- tual need for aortic valve replacement, and concomi- tant aortic valve repair could also help delay the need for aortic valve replacement in these cases. 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 18 for routine testing and follow-up intervals. Recommendation-Specific Supportive Text 1. Exercise stress testing may be reasonable in the assessment of exercise capacity, stress-induced arrhythmias, and ischemia in patients with subAS and may be considered as an adjunct to echocar- diographic imaging. 2. 