Up to 80% of patients with prior CoA in- tervention manifest an abnormally elevated upper extremity exercise blood pressure response, and peak blood pressure is correlated with increased LV Moreover, restenosis of the previously repaired or stented region may be identified by increased peak blood pressure response, increased upper to lower extremity blood pressure gradient with exercise, and increased Doppler velocity across the coarctation site during exercise TTE. 6. Significant native or recurrent aortic coarctation has been defined as follows: upper extremity/ lower extremity resting peak-to-peak gradient >20 mm Hg or mean Doppler systolic gradient >20 mm Hg; upper extremity/lower extremity gradient >10 mm Hg or mean Doppler gradient >10 mm Hg plus either decreased LV systolic function or AR; upper extremity/lower extremity gradient >10 mm Hg or mean Doppler gradient >10 mm Hg with collateral flow.S4.2.6-2,S4.2.6-8,S4.2.6-12 This should be coupled with anatomic evidence for CoA, typ- ically defined by advanced imaging (CMR, CTA). The best evidence to proceed with intervention for CoA includes systemic hypertension, upper Table 21. CoA: 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 aortic size and aortic arch/coarctation repair site anatomy. 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 post-stent therapy for coarctation of the aorta; the frequency should be weighed against radiation exposure. 