This nested case-control study was designed in order to investigate the possible correlation between existence of fQRS and left ventricular dysfunction in apparent healthy people. It is noteworthy that Shiraz Heart Study is the first cardiovascular-oriented cohort in one of Iran's metropolitan city 15. Although the correlation between fQRS and cardiac disorders has been demonstrated in several diseased status 6,20, but the importance of this QRS alteration has been ignored in general population. The main finding of the present study is that in apparent healthy subjects with normal EF, those with fQRS had lower GLS than those without fQRS.
Scarring of the myocardium following by zigzag pattern of electrical conduction produces fQRS spikes 21. fQRS is known as an indicator of previous myocardial injury and warns possible future adverse cardiac events 21. It was reported that fQRS possibly is the only evidence of silent MI in high risk individuals 3. Moreover, fQRS was known as a sign of premature ventricular contractions in individuals without obvious structural heart diseases 22. It was shown to be superior than Q wave for detecting myocardial scar in terms of sensitivity and negative predictive value, but not of specificity 23. However, in a more recent study, higher sensitivity and specificity of fQRS than Q wave was declared 24. Also, in case of disappearance of MI-related Q wave due to revascularization therapies, fQRS would be a validated replacement 20.
Existence of fQRS in different EKG leads simply translates into tissue scarring in different segments of the heart and is associated to the higher incidence of cardiac death and hospitalization 21. Severity and complexity of CAD was reported to be in relation with the number of EKG leads with fQRS 25. Accordingly, fQRS could be a guiding tool to identify regions of interest for ablation, those of prone for ventricular arrhythmias 20. The potential of fQRS in predicting arrhythmic events, need for revascularization, MI, cardiac death, and all-cause mortality was shown in subjects with different cardiac disorders 23,26,27. Also, the prognostic significance of fQRS was seen in stable CAD and acute MI. However, there are contrary reports which consider the role of fQRS with doubt in myocardial scar detection, predicting arrhythmic events, and mortality 28-33. Some of these studies expressed that fQRS was not a good predictor of arrhythmic events 4,34.
EF, which is a popular means through assessment of LV function, is only able to reflect moderate to severe impairment in the ventricles. Also, this parameter suffers several limitations. Of note, EF mostly contributes to the myocardial changes in radial axis while longitudinal deformations are being neglected 35. Strain is the more developed and accurate measurement than volumetric parameter of EF. It demonstrates fine myocardial deformations in longitudinal, circumferential, and radial axis and also, changes in torsion 35. Among strains, GLS is of eminent importance due to its sensitivity and robustness 36,37. The association of mortality with GLS was stronger than LVEF 38. GLS, which is obtained by STE, measures myocardial deformations via tracing of speckles' displacement 5,16,39. Reduction in absolute GLS value is an indicator of a myocardial disease in most cases and portends future adverse events 36.
In an investigation on patients with systemic sclerosis, fQRS was present while LVEF and LV dimensions were normal. Importantly, GLS was significantly lower in these patients than control group 40. In a comparison within apparent healthy individuals, GLS was significantly lower in those with fQRS than those without fQRS despite normal similar EF. Further evaluation showed impairment in ventricular diastolic function as well as greater thickness in epicardial adipose tissue in subjects with vs. without fQRS 41. Although GLS reduction is a sign of LV malfunction, but GLS is also affected by other factors such as age, gender, and ethnicity 42-45. Also, changes in physiological parameters like heart rate affects GLS in healthy individuals 46. Hypertension, obesity, dyslipidemia, diabetes and medications were also considered as factors that modifies GLS value. As all these factors are known as cardiovascular risk factors, but of interest is that smoking do not change GLS. Vendor-specific disparities and timing of measurements should also be considered in GLS evaluation 36.
fQRS in individuals with normal EF may be due to the existence of myocardial fibrosis of subclinical scale which in turn boasts fQRS sensitivity 22,24. EKG as the mainstay is a convenient, cost-effective, and informative instrument. It seems that EKG-born fQRS could play a critical role in identifying individuals among general population who are prone to LV systolic dysfunction and consequent heart failure. A simple EKG has the potential to draw cardiologists' attention for further assessment of the heart function with more sophisticated tools and parameters such as STE and GLS to find minor, but life-threatening events.