ACHD Surveillance Program/Observational Cohort (Aim 1 and Aim 2): To investigate the first two aims of this proposal, the study investigators will design and analyze an observational cohort of subjects \>18 years of age with CHD from 14 PCORnet sites. Data is collected retrospectively (5 years prior to initial data query). Each subject with a diagnosis of CHD, \> 18 years of age, and at least 1-3 years of retrospective information will be included in the study. Retrospective data will be obtained from the previous 1-3 years to ascertain adherence to recommended care by the ACC/AHA, specific to CHD subtype. Retrospective data collection will be to 1-3 years, with 2 years likely being sufficient to identify our patient cohorts. The investigators will still perform a retrospective data search for 5 years, but will not exclude patients if they do not have data sets for the full five year period.

The study team will specifically investigate frequency of gaps in recommended cardiology follow-up. Gaps in recommended care will be determined by the individual subtype of disease. The investigators will also examine advanced testing and imaging, but that will not be the primary measure of adherence to recommended care. In this cohort, the main outcomes of interest are key comorbidities and healthcare utilization. The investigators will be performing 2 cycles of data searches (March 2022 and January 2024), including the initial search for one year of follow-up after the subject's last cardiology visit.

Of note, with the COVID-19 pandemic, healthcare utilization has been affected throughout the population, not just those patients with rare diseases. To mitigate some of these issues, the study investigators will institute the following. For the data pertaining to the proposed study of loss to care, the retrospective data search in January 2020 (pre-COVID19) will be used and adjusted for lesion follow-up as described. Given the unique circumstances, the study investigators do feel that obtaining data during the COVID-19 pandemic is extremely important for this patient population, especially to identify the factors that might make some patients more vulnerable to loss to follow-up. Therefore, study investigators will analyze the retrospective data from February 2020 - June 2021 as a separate cohort in order to better identify the effects of the pandemic on this population of patients. Many centers (including all within the proposed study) began implementing telehealth visits, and will include this in the data capture.

Prospective cohort through CHI registry linkage (Aim 3): To investigate the third aim of the study, study investigators will design and analyze a prospective cohort design with the goal of establishing a diverse panel of CHD patients who are either (1) adhering or (2) not adhering to recommended care guidelines. Patients with qualifying CHD (ICD9/10 codes) who are \> 18 years of age will be included in the study. Once patients are identified, site-specific PIs and their teams will facilitate enrollment into the Congenital Heart Initiative registry through the Eureka Research Platform with an electronic patient recruitment and engagement platform used with several large research projects. Specifically, patient prioritized outcomes (quality of life, mental health, and physical health \& functioning) will be assessed among registrants who will be stratified by disease complexity and gaps in care.