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Carr et al. Vessel Plus 2020;4:12 I http://dx.doi.org/10.20517/2574-1209.2020.01 Page 3 of 21

Historically, the process of choosing logistic regression eligible (“candidate”) risk variables was different for
each CABG registry. As this pre-selection candidate variable approach may have introduced subjectivity
and biased model results, CABG risk models (such as those developed by the VA, Society of Thoracic
Surgeons, and EuroSCORE teams) have been derived in recent years using a standardized approach with a
core set of model eligible variables. Beyond this core set, however, each database incorporates an expanded
set of population-specific risk variables in their risk modeling processes.

Over the past 30 years, nearly countless CABG risk models with various designs and complexity have been
developed to predict the likelihood of death at pre-specified time periods. As the standard ST endpoint
used, operative mortality was defined as death within 30 days or within the index hospitalization. As
operative mortality avoids any potential post-discharge referral bias (e.g., post-CABG hospital discharge
to a separate sub-acute care facility), this endpoint was determined to be the most clinically relevant
performance metric; it is commonly used to assess the quality of the surgical procedure. Other models
have considered LT death during longer periods of follow-up, investigating the durability of the CABG
procedure and importance of other risk factors. For ST and LT published risk models, therefore, this
study describes the patterns in pre-CABG factors differentially impacting ST vs. LT mortality. Until this
report, these patterns had not been previously described. Moreover, this novel report identifies additional
opportunities to improve future CABG risk models.

METHODS
An advanced literature review was undertaken to document published risk factors associated with post-
CABG mortality. In February 2019, PubMed was searched for all Medline publications using the following
terms: “CABG” (Title) OR “coronary artery bypass” (Title) AND “mortality” (Title) OR “risk” (Title) OR
“death” (Title) OR “survival” (Title). This yielded 1904 publications. Following a review of all articles for
pre-stated inclusion/exclusion criteria, there were a total of 125 included articles with 156 CABG mortality
models. Only papers reporting risk models for mortality after an isolated CABG procedure were included;
inclusion criteria were otherwise left intentionally broad so as to gather a wide variety of models. Models
requiring data from the postoperative period were excluded for the purpose of this review, whereas those
employing only preoperative variables [as opposed to preoperative and intraoperative variables (e.g.,
cardiopulmonary bypass time)] were identified for sub-analysis review. For the 125 publications meeting
all inclusion/exclusion criteria, their reference lists were also carefully reviewed for relevant publications to
augment the original search strategy’s findings.

Working collaboratively under the senior co-authors’ guidance, the majority of literature search screening
and data extraction were performed primarily by one author (BC). To permit meaningful model
comparisons, risks were classified into 91 different common clinical categories. Clinically relevant composite
variables were reported based upon database-specific definitions (e.g., “critical preoperative state” and “extra-
cardiac arteriopathy”). Named risk indices (e.g., “Elixhauser Comorbidity Index”) were analyzed using their
assigned name as a group, rather than being recorded based upon the indices’ subcomponents. For the 125
publications evaluated, the set of risk factors identified to be associated with post-CABG ST or LT mortality
were compared. Time trends in models’ risk factors reported were evaluated across three time periods until
1997, 1998-2007, and 2007-2017.

RESULTS
One hundred fifty-six post-CABG mortality risk models were identified within 125 different papers. In
Appendix A, the full listing of these papers and models can be found in Supplementary Tables 1 and 2.

Of these models, 133 predicted ST CABG mortality. Operative mortality was the most commonly reported
ST endpoint, defined as death occurring during the index hospitalization and/or up to 30 days after the