2.2. Data Collection

All ECGs in this study were measured by the standard 12-lead ECG Philips machine (PH080A) with a 500 Hz sampling frequency for 10 s and stored in a digital format. Due to the “black box” property of DLMs, 8 quantitative ECG measures and 31 diagnostic pattern classes were also collected from the structured findings statements based on the key phrases that are standard within the Philips system. These features were used to train an extreme gradient boosting (XGB) model, and the DLM was trained via raw ECG traces. Pattern classes included the abnormal T wave, atrial fibrillation, atrial flutter, atrial premature complex, complete AV block, complete left bundle branch block, complete right bundle branch block, first degree AV block, incomplete left bundle branch block, incomplete right bundle branch block, ischemia/infarction, junctional rhythm, left anterior fascicular block, left atrial enlargement, left axis deviation, left posterior fascicular block, left ventricular hypertrophy, low QRS voltage, pacemaker rhythm, prolonged QT interval, right atrial enlargement, right ventricular hypertrophy, second degree AV block, sinus bradycardia, sinus pause, sinus rhythm, sinus tachycardia, supraventricular tachycardia, ventricular premature complex, ventricular tachycardia, and Wolff–Parkinson–White syndrome. The detailed items have been described in previous works [27,28]. Blood BNP and pBNP were based on central laboratory methods. Since there are no universal cutoff points for BNP and pBNP, we divided the data into three categories based on BNP/pBNP by the same values: normal BNP/pBNP (<500 pg/mL), mild abnormal BNP/pBNP (500–999 pg/mL), and severe abnormal BNP/pBNP (≥1000 pg/mL).

We additionally collected demographic information, including sex, age, and body mass index, and disease histories, including diabetes mellitus (DM), hypertension (HTN), hyperlipidemia (HLP), chronic kidney disease (CKD), acute myocardial infarction (AMI), stroke (STK), coronary artery disease (CAD), heart failure (HF), atrial fibrillation (Afib), and chronic obstructive pulmonary disease (COPD), based on the corresponding International Classification of Diseases, Ninth Revision and Tenth Revision (ICD-9 and ICD-10, respectively) from EMRs. The detailed look-up tables for ICD codes and diseases have been described previously [29,30,31,32,33].

All-cause mortality events were also collected in this study. The survival time was calculated with reference to the date of the ECG test. Patient status (dead/alive) was captured through the EMRs. Data for alive visits were censored at the patient’s last known hospital alive encounter to limit bias from incomplete records. The end of follow-up in this study was 31 September 2021. The median follow-up periods were 25.2 months (interquartile range [IQR]: 6.3–46.5 months) and 15.3 months (IQR: 2.0–38.2 months) in the internal and external validation sets, respectively, with maximum follow-up times of 123.8 months and 120.7 months. The number of mortality events was 463 (3.93 per 1000 person-years) and 666 (4.59 per 1000 person-years) in the internal and external validation sets, respectively.