Materials and Methods
A complete extract of prospectively collected data from the National Adult Cardiac Surgery Audit (NACSA) was obtained from the National Institute of Cardiovascular Outcomes Research (NICOR) central cardiac database and retrospectively analysed. The definitions of the database variables used for this study are available at https://www.nicor.org.uk/national-cardiac-audit-programme/adult-cardiac-surgery-surgery-audit/. The NICOR registry prospectively collects demographic, as well as pre- and post-operative clinical information, including mortality, for all major adult cardiac surgery procedures performed in the UK. The flow of the data from surgeon-input to analysis has been described elsewhere (13). Briefly, data entered locally by surgeons are validated at the unit-level by database managers prior to upload via a web-portal to NICOR. At this stage, further validation is performed according to logical rules and missing data reports are generated for primary variables (e.g., EuroSCORE risk factors, patient identifiers and outcome data). The data are then forwarded to an academic healthcare informatics department for data cleaning. The complete data cleaning process has been previously described (13). Duplicate records are removed, transcriptional discrepancies re-coded and clinical and temporal conflicts resolved. Missing data are resolved during the validation stages of the data transfer from individual centres. Missing and conflicting data for in-hospital mortality status are backfilled and validated via record linkage to the Office for National Statistics (ONS) census database. The overall percentage of missing data for baseline information is very low (1.7%). Missing categorical or dichotomous variable data were imputed with the mode while missing continuous variables data imputed with the median.
For this study, we used the NACSA dataset study all adult patients who underwent cardiac surgery between 2010-2018. We included elective and urgent isolated coronary revascularization and valvular procedures (aortic valve replacement, mitral valve replacement/repair). We excluded minor procedures, aortic arch surgery, heart transplantation and emergency and salvage surgery.