A total of 7119 participants were recruited into this study, 3092 ones of whom (43.43%) were male. The mean age of the participants was 48.59±9.34 years, and there was no significant age difference between males and females. Comparison of the characteristics of all study participants by gender has been presented in Table 1. The results revealed significant differences between males and females with respect to all variables, except for age and amount of sleep per day (p<0.001).
Correlation between age and ECG parameters
The results of the linear regression model showed that age was positively associated with ECG parameters, including PR (ms) (B=0.275, p<0.001), QRS (ms) (B=0.047, p=0.005), QTC (B=0.331, p<0.001), and RaVL (mm) (B=0.003, p<0.001). As such, increase in age by one year after the age of 35 years increased the average ECG parameters, including PR (0.275), QRS (0.047), QTC (0.331), and RaVL (0.003 m/s), but significantly decreased SV3 (mm) (b=-0.002, p<0.001) and QRS axis (b=-0.454, p<0.001) (Tables 2-7).
The relationship between gender and ECG parameters
The results of multivariate analysis showed that gender was significantly associated with all ECG parameters, except for S in V3. After adjusting for the effects of other variables, the average intervals between PR, QRS, and QRS axis parameters were reduced by -6.03, -4.070, and -8.43 units, respectively among females (p<0.001). Females also displayed an increased QTC interval and RaVL amplitude by 16.82 and 0.096 units, respectively compared to males (p<0.001) (Tables 2-7).
The relationship between marital status and ECG parameters
According to the results presented in tables 2-7, marital status showed no significant relationship with any of the ECG parameters (p>0.05) (Tables 2-7).
The relationship between BMI and ECG parameters
After controlling the effects of other variables, BMI was significantly associated with R in Avl, S in V3, QRS axis, QTC, and QRS. Accordingly, increase in BMI by one point elevated the average interval of R in aVL (0.016, p<0.05), QTC (0.316, -1.42), and QRS (0.235, p<0.001), but decreased the interval of S in V3(-0.003) and QRS axis (-1.42) (p<0.001) (Tables 2-7).
The relationship between education level and ECG parameters
The effect of education level in the multivariate model showed that this variable was not significantly linked to any of the ECG parameters (p>0.05) (Tables 2-7).
The relationship between wrist circumference and ECG parameters
The findings of the present study showed a significant relationship between wrist circumference and PR, QRS, S in V3, QRS axis, QTC, and R in aVL. Based on the results, increase in wrist circumference by one cm increased the average interval of PR, QRS, QTC, and R in aVL by 1.501, 0.855, 1.32, and 0.040 units, respectively (p<0.001). Increase in wrist circumference by one cm also resulted in a decrease in the QRS axis and S in V3 by 3.45 and 0.008 units, respectively (p<0.001) (Tables 2-7).
The relationship between hip circumference and ECG parameters
The results indicated that hip circumference significantly associated with S in V3, R in aVL, QRS, QTC, and PR parameters. After controlling the effects of other variables, increase in hip circumference by one centimeter increased the mean interval of R in aVL, QTC, QRS, and PR parameters by 0.006, 0.152, 0.151, and 0.253, respectively (p<0.001), but decreased the mean interval of S in V3 and QRS axis by -0.001 and -598, respectively (p<0.001).
The relationship between waist circumference and ECG parameters
Among Iranian adults, waist circumference was shown to be significantly associated with all ECG parameters. After controlling the effects of other variables, increase in waist circumference by one centimeter increased the mean interval of PR (0.190), QRS (0.101), and R in aVL (0.006) (p<0.001), but decreased the mean interval of QTC (-0.234), QRS axis (-0.553), and S in V3 (-0.001) (p<0.001) (Tables 2-7).
The relationship between MET and ECG parameters
In the present study, MET showed a significant relationship with QTC, QRS, and R in aVL. Accordingly, increase in MET by one unit decreased the mean interval of QTC and R in aVL by -0.234 and -0.0005, respectively (p<0.05), but increased the mean interval of QRS axis by 0.096 (p=0.023) (Tables 2-7).
The relationship between diabetes and ECG parameters
Among the participants of this cohort, diabetes was not significantly associated with any of the ECG parameters in the multivariate model (p>0.05) (Tables 2-7).
The relationship between hypertension and ECG parameters
In this study, after adjusting for the effects of other variables, hypertension showed a significant relationship with PR, QRS, S in V3, QRS axis, QTC, and R in aVL. Accordingly, PR, QRS, QTC, and R in aVL increased by 2.40, 0.998, 2.423, and 0.081 units, respectively among hypertensive individuals (p<0.05). Besides, the mean intervals of QRS and S in V3 parameters were reduced by 3.48 and -0.030 units, respectively in hypertensive people compared to non-hypertensive ones (p<0.05) (Tables 2-7).
The relationship between cardiac ischemic disease and ECG parameters
The patients with cardiac ischemic disease showed a significant relationship only with R in aVL, such a way that the mean interval of R in aVL increased by 0.022 in these patients compared to the healthy individuals after controlling the effects of other variables (p=0.011). This variable showed no significant associations with other ECG parameters (p>0.05) (Tables 2-7).
The correlation between the total amount of sleep in 24 hours and ECG parameters
Among the Iranian adults of this cohort, the total amount of sleep in 24 hours was not significantly associated with any of the ECG parameters in the multivariate model (p>0.05) (Tables 2-7).