Fig. 1
![Fig. 1](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs13075-017-1240-1/MediaObjects/13075_2017_1240_Fig1_HTML.gif)
Study design and grouping of patients: normal electrocardiogram (ECG), non-ECG-cardiovascular disease (ECG-CVD), ECG-4, and ECG-5. Non-ECG-CVD defined as the presence of any of the following ECG abnormalities: right axis deviation, arrhythmia, sinus tachycardia, sinus bradycardia, atrioventricular blocks, atrial ectopic rhythm, and atrial enlargement