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.