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