2.2. Study population
A total of 312,736 patients who underwent a
health
checkup between 2005 and 2013 were selected from the Korean NHIS-Senior
database, and their follow-up data were reviewed until December 2014.
The exclusion criteria were as follows: (i) those with valvular heart
disease (mitral stenosis, prosthetic heart valves, valve replacement, or
valvuloplasty) (n=1948); (ii) those who had AF before enrollment
(n=8167); (iii) those who had a pacemaker implantation before enrollment
(n=217); and (iv) those who had SSS before enrollment (n=175). Finally,
302,229 subjects were included (Figure 1).
The
International Classification of Disease (ICD)-10th Revision, code I48
was used to diagnose AF. Patients were defined as having AF if it was
present at the time of discharge, or its presence was confirmed at least
twice in the outpatient clinic. This definition of AF diagnosis was
previously validated in the NHIS database with a positive predictive
value of 94.1%.1,
2, 4
For both the AF and the AF-free patients, the time at risk was estimated
from the index date or the day of their enrollment in the study. Effect
of incident AF was analyzed as a time-varying factor. Participants were
censored on the date of diagnosis of SSS, death, or at the end of the
study period, defined as the last date of follow-up or December 31,
2014, whichever came first.
The patients were considered to have comorbidities, as previously
described using data from the NHIS database, if their condition was
diagnosed at the time of discharge or was confirmed at least twice in an
outpatient setting (eTable 1 in the
Supplement).1,
2, 4