Statistical Analysis
SPSS for Windows version 21.0 (SPSS Inc., IL, USA) was used for statistical analyses. Kolmogorov-Smirnov test was used for evaluating if the continuous data were distributed normally. Continuous data that were expressed as mean ± SD or median (25-75 IQR) were compared using the Student t or Mann-Whitney U tests, while categorical data that were expressed as the number and the percentage were compared using the Chi-square test.  The correlation analysis was performed by using the Pearson or Spearman correlation coefficients. In order to predict the severity of COPD by using the frontal QRS-T angle, the cut-off value was determined using the receiver operating characteristics (ROC) curve analysis. The independent predictors of frontal QRS-T angle were determined using the linear regression analysis. Value ofp <0.05 was considered statistically significant.