Participants studied
The China National Diabetes and Metabolic Disorders Study was a population-based cross-sectional study carried out from June 2007 to May 2008 [8]. The details of its sampling methods have been described previously [8, 9]. Briefly, 47,325 participants (18,976 men and 28,349 women aged ≥20 years old) from 152 urban street districts and 112 rural villages in 14 provinces completed the study [8, 9]. First, 1,086 persons were excluded due to the missing of demographic information or glucose level data. Second, ECG data from some subcenters (including all subcenters from Beijing, two subcenters from Hunan, two subcenters from Jiangsu and one subcenter from Xinjiang) were not well recorded; thus, data from these subcenters (data from 10,951 persons) were excluded from the analysis. Third, 100 persons missed smoking history, 93 persons missed BMI records, while 141 persons missed lipid data. Thus, 323 persons were excluded due to the missing of smoking history records, BMI records or lipid data. Ultimately, 34,965 participants (13,983 males and 20,982 females) were included in this analysis. This study was approved by the institutional review board and the ethics committee of local institutions [8].
Study-outcome definitions
The design and methods of the China National Diabetes and Metabolic Disorders Study were reported previously [8, 9]. Briefly speaking, interviews were conducted and standard questionnaires were completed to obtain the information about demographical characteristics, lifestyle risk factors, personal medical history, treatment of diseases and family disease history. Fasting blood samples were collected from the participants to test liver function, renal function and lipid levels. ECGs and measurements of blood pressure, waist circumference, height and weight were conducted for participants after an overnight fast by well-trained clinical staffs. Participants then received a standard oral glucose tolerance test. Diabetes mellitus was defined as fasting plasma glucose ≥7.00 mmol/L, 2-hour plasma glucose ≥11.10 mmol/L or using glucose-lowering medications with a history of diabetes. Prediabetes was defined as fasting plasma glucose 6.10-6.99 mmol/L or 2-hour plasma glucose 7.80-11.09 mmol/L without any evidence of diagnosis of diabetes. Hypercholesterolemia was defined as total cholesterol ≥6.22 mmol/L, LDL cholesterol ≥4.14 mmol/L or using cholesterol-lowering medications with a history of hypercholesterolemia. Hypertension was defined as systolic blood pressure ≥140 mmHg, diastolic blood pressure ≥90 mmHg or using antihypertensive medications with a history of hypertension. Obesity was defined as BMI ≥28 kg/m2, while overweight was defined as BMI 24-27.9 kg/m2 according to the criteria adopted by the Chinese Society of Endocrinology. The definition of smoking history was having smoked at least 100 cigarettes in the past. Rural areas referred to rural villages. Urban areas referred to city districts. The selection of geographic regions was described in a previous article [8].
Twelve-lead ECGs were conducted with the subject in the supine position. ECGs were read and recorded by two trained physicians in each subcenter. ECG data were classified based on the Minnesota coding (MC) criteria [1, 2, 5]. Major arrhythmias included atrial fibrillation or flutter (MC 8-3), complete left bundle branch block (LBBB, MC 7-1), complete right bundle branch block (RBBB, MC 7-2), nonspecific intraventricular conduction delay
(IVCD, MC 7-4), Mobitz Type II or III atrial-ventricular (AV) conduction defects (MC 6-1, 6-2), supraventricular or ventricular rhythm/tachycardia (MC 8-4-1, 8-4-2, 8-2-2, 8-2-3), Wolff-Parkinson-White (WPW, MC 6-4) and artificial pacemaker (MC 6-8). Minor arrhythmias mainly included sinus bradycardia (MC 8-8), sinus tachycardia (MC 8-7), atrial or junctional or ventricular premature beats (MC 8-1-1, 8-1-2, 8-1-3), incomplete RBBB (MC 7-3), Mobitz Type I AV conduction defect (MC 6-3) and short PR interval (MC 6-5). Other types of ECG abnormalities were classified and analyzed as well, including ST depression and T abnormalities (MC 4-1, 4-2, 4-3, 4-4, 5-1, 5-2, 5-3 or 5-4), Q wave abnormalities (MC 1-1, 1-2), Q wave abnormalities plus ischemic ST-T abnormalities (MC ‘1-1 or 1-2’ plus ‘4-1, 4-2, 4-3, 4-4, 5-1, 5-2, 5-3 or 5-4’), ST elevation (MC 9-2), tall R wave left (MC 3-1 or 3-3), tall R wave right (MC 3-2), left/right atrial hypertrophy (MC 9-3 or 9-6), left axis deviation (MC 2-1), right axis deviation (MC 2-2) and low voltage (MC 9-1).
Statistical methods
The prevalence calculation and significance evaluation, performed using SUDAAN software (version 10, Research Triangle Institute) in this study, were weighted to represent the population of Chinese adults (≥20 years old) based on the Chinese population distribution data in 2006 [8]. The age- and gender- standardized prevalences of ECG abnormalities were calculated for the whole population, for males and females, and for different age groups. Multivariate logistic regression analysis was conducted using SUDAAN software (version 10, Research Triangle Institute) to investigate associations of gender, age, metabolic factors, smoking history and rural/urban areas with the odds of ECG abnormalities. Factors simultaneously adjusted to calculate the odds ratios included gender, age classes, smoking history, hypertension, blood glucose classes, BMI classes, hypercholesterolemia and rural/urban areas.