Introduction
The PR interval measured from the surface electrocardiogram (ECG) is considered an indicator reflecting atrial and atrioventricular node myocyte depolarization and conduction1. PR prolongation, usually defined as PR interval >200ms, is a relatively common electrocardiographic finding with prevalence of 2-6% in general population2–4. However, the prognosis value of PR prolongation is controversial due to the many factors that can influence the PR interval5. Previous studies have demonstrated that PR prolongation is associated with the occurrence of atrial fibrillation and left ventricle dysfunction6–8. The prolonged PR interval is also related to an increased risk of death in patients with cardiovascular disease (CVD) including heart failure (HF), coronary artery disease (CAD) and hypertrophic cardiomyopathy (HCM)9–11. Since this correlation is less striking in the general population, abnormal PR interval is less well appreciated especially in the population free of CVD2,12,13.
The P-wave duration is a crucial component of the PR interval, and the link between P-wave duration prolongation and adverse outcomes, including increased risk of atrial arrhythmias and mortality, is well established14. Soliman et al using the data from the Third National Health and Nutrition Examination Survey (NHANES III) suggested that the prognostic significance associated with PR prolongation is largely determined by P-wave duration15. Meanwhile, since specific information on the P-wave has been rarely reported in the studies addressing PR prolongation, this opinion makes the clinical utility of PR interval be questioned. But there is no denying that hemodynamic abnormalities were indeed identified in this population with PR prolongation. In case of PR prolongation, atrial systole occurs much earlier than the onset of left ventricular (LV) systole, resulting in the impaired effective LV diastolic filling time16. Another important implication of PR prolongation on hemodynamics is diastolic mitral regurgitation. Increased mitral regurgitation, decreased LV compliance as well as impaired relaxation are inextricably linked17–19.
Starting from these observations, we proposed that LV diastolic function may be a significant factor for PR prolongation risk stratification. Meanwhile, the QRS axis is an important ECG parameter to reflect LV diastolic function20. We therefore designed this study to identify those people with PR prolongation who have poor prognosis in the population without CVD by using QRS axis.