Study variables and covariates

The classification of social phobia was based on the diagnostic ICD-10 codes F40.1. Participants’ first dates of diagnosis with F40.1 by doctors were regarded as the diagnoses dates for social phobia. Participants with ICD-10 codes I47, I48, and I49 were considered to have arrhythmias. For each participant, we identified the date of diagnosis of arrhythmia or the end of the study period (December 30, 2013) as the final follow-up date. Various demographic variables that might affect the occurrence of social phobia or arrhythmia, such as age, sex, social security status, residential region, disability, income status, and the Charlson comorbidity index (CCI), were included in the regression model as covariates. CCI score is the index for assessing the participants’ comorbidities which can alter the risk of mortality for use in longitudinal studies. The score was calculated by weighting 1 ~ 6 scores for 19 comorbid diseases. The comorbid disease categories included in CCI score consists of myocardial, vascular, pulmonary, endocrine renal, gastrointestinal, cancer/immune and neurologic comorbid diseases [21]. Dementia is the only psychiatric comorbid disease included in the CCI evaluation and categorized into neurologic comorbid disease. We used the ICD-10 codes for each comorbid disease to calculate the CCI score of participants [22].. Participants were divided into three groups according to their CCI scores: 0–2, 3–4, ≥5. The diagnoses of diabetes, hypertension, chronic obstructive pulmonary disease (COPD) and hyperthyroidism obtained by using ICD-10 codes were also included in the analysis. Age was divided into seven 10-year groups (0–9, 10–19, 20–29, 30–39, 40–49, 50–59, ≥60) to distinguish between risks of arrhythmias among age groups. Region was divided into three categories according to population density: metropolitan, city, and rural. Social security status was categorized by participants’ health insurance status as per the criteria of South Korea’s National Health Insurance system within which self-employed or privately employed individuals are covered by National Health Insurance. Medical aid beneficiaries consist of individuals who have an income below the government-defined poverty threshold, or disabilities which enable them to receive free inpatient and outpatient care through the government. For the dependent variable subgroup analysis, arrhythmias were sub-grouped into three different categories: paroxysmal tachycardia (I47), atrial fibrillation/flutter (I48), and other cardiac arrhythmias (I49).