This nested case-control study was done in the setting of a prospective cohort, Shiraz Heart Study (SHS), which is conducted on general population of Shiraz city aiming to analyze cardiovascular risk factors (15). The present study has been conducted in accordance with the declaration of Helsinki and has been approved by the Ethical Committee of Shiraz University of Medical Sciences. All the study subjects provided written informed consent.
Inclusion criteria were negative past medical history, normal lipid profile, normal blood pressure, normal anthropometric indices as well as non-smokers and non-diabetics. Exclusion criteria were history of CAD, history of major risk factors for CAD (hypertension, diabetes mellitus, and hyperlipidemia), angina pectoris, acute coronary syndrome, cardiomyopathies, receiving any cardiovascular-related medications, implantation of pacemaker, heart valve disease, atrial fibrillation and flutter, rheumatism, renal disease, malignancy, pulmonary hypertension, and chronic obstructive pulmonary disease.Among those who met inclusion and exclusion criteria, random sampling was utilized in order to select 500 subjects.
A resting 12-lead EKG has obtained previously from the all the participants as the cohort scheduled procedure (filter settings: 0.5–150 Hz, 25 mm/s, 10 mm/mV). The EKGs of 500 subjects were thoroughly evaluated by two independent cardiologists seeking for QRS fragmentation. Notching in the R or S wave in the absence of a branch block, or an RSR' pattern additional to the original QRS wave (< 120 ms) were defined as fQRS (16). Upon disagreement on interpretation of an EKG, it was referred to a third cardiologist. Existence of fQRS was confirmed in conference in twenty subjects (case group). Similarly, out of remaining480, twenty age-matched subjects without fQRS were assigned as control group. Fragmentation was classified based on its location to anterior (V1 to V5), inferior (DII, DIII,aVF), or lateral (DI, aVL,V5, V6) leads.
Subjects in the two groups were asked to attend in the clinic. EKGs were repeated in order to find any possible new changes or arrhythmias by an expert who was blinded to grouping. Then, STE was performed with a commercially available ultrasound scanner (Vivid E9, General Electric Medical Systems, Horten, Norway) with a 2.5-MHz transducer by a single blinded echoman cardiologist. Echocardiograms were obtained in three-, two- and four-chamber apical views at a rate of 50 to 70 frames/s with the patient holding their breath during at least three cardiac cycles. Endocardial borders were automatically marked and tracking was applied to each image. In satisfactory tracking, the entire cardiac wall (endocardium through myoepicardial border) was covered. The LV was divided into four segments in 3-chamber view, and six segments in 2- and 4-chamber view, totally 16 segments were assessed. If the segments were marked by the software automatically, the obtained data were recorded. Otherwise, they were corrected manually. Image analysis was done by AFI system.
Peak systolic longitudinal strains (LS) of different segments were calculated and then, average LS for each view was produced. GLS was the arithmetic mean of LSs in three apical views. GLS of >20% was assumed to be normal (17). Dimensions and volumes of the left ventricle were measured according to the guidelines of the American Society of Echocardiography. Also, LVEF was calculated by Simpson rule (18). Preserved EF was considered as EF ≥50% (19).
The statistical analysis was done in SPSS for Windows (release 14.0, SPSS Inc., Chicago, Illinois). Categorical variables are expressed as number (percentages) and continuous variables as mean±standard deviation (SD). Comparison between variables were done using chi square, independent samples t test or ANOVA when appropriate. P value of less than 0.05 was considered statistically significant.