Study population
In NHANES III, 6990 participants (aged 40–90 years) with normal sinus rhythm and no evidence of bundle-branch block or intraventricular conduction delay (QRS ≥120ms) were recruited. Exclusion criteria included: (1) Participants with CVD, including HF (N = 306), myocardial infarction (N = 262) and stroke (N = 185). (2) Electrocardiographic data were unreliable (N = 41) and follow-up data were missing (N = 8). After exclusion, 6188 eligible participants were classified into four groups, depending on PR interval and QRS axis (Figure 1).