“Epidemiology of Short QT Interval in Phase I of Kherameh Cohort Study: A Population-based Study on 4,363 Subjects in Southern Iran”

Aims: Short QT-interval is a condition that bear the suspicion of short QT syndrome (SQTS). SQTS is known to increase risk of life-threatening arrythmias and sudden cardiac death (SCD). Due to the insucient population-based studies and use of various QT cut-off values it accounts for as an undiagnosed condition. In this study, we sought for prevalence of short QT interval in Kherameh cohort study, one of the southern sectors of the Prospective Epidemiological Research Studies in Iran (PERSIAN). Methods: Data of 4,363 adult subjects were analyzed from phase 1 of the cohort during 2014-2017. The corrected QT (QTc) intervals were calculated and electrocardiograms (ECGs) with QTc of less than 370 milliseconds (msec) were reanalyzed for bradycardia, early repolarization, atrial brillation (AF), arrhythmias, and other electrical conduction abnormalities. Results: Seventy-two subjects (1.65%) had a QTc of less than 370msec (mean QTc of 360.72±11.72). A male predominance and a lower mean heart rate observed in SQTS susceptible group (M/F of 1/0.26 vs. 1/1.145, p-value<0.0001; 58.389±9.787 vs. 70.899±11.775; p-value<0.0001) compare to the subjects with normal QTc. At least, 2 subjects with high-probability SQTS and 3 with intermediate-probability SQTS identied. The frequency of AF, syncope, bradycardia, early repolarization, low voltage ECG, and infantile SCD in rst- and second-degree relatives were 16.67, 4.17, 33.33, 11.11, 6.94, 11.11%, respectively. Conclusion: The prevalence of short QT interval in our cohort was in line with previous studies. The higher proportions of cardiac symptoms, familial SCDs and ECG derived specic ndings amongst SQTS-susceptible index persons than non-short QT interval normal population might implicate in gene study and family screening.


1-2-Study population
We performed a population-based cross-sectional study on Kherameh cohort study, which is one of the southern sectors of the larger nationwide cohort program, Prospective Epidemiological Research Studies in Iran (PERSIAN) lunched in 2014 (Its objectives, structure and infrastructures is described further [17] ). All methods were performed in accordance with relevant guidelines and regulations. Kherameh county is located in East-central Fars province, with estimated population of 61,580 people mostly from "Fars" ethnicity. The main objective of Kherameh cohort study was to determine the prevalence and risk factors of non-communicable diseases (NCDs) during a follow up period of 10 to 15-years. We included data of 4,363 subjects that posed the baseline ECG assessment out of 10,663 subjects aged 40-70 years from phase 1 of the cohort during 2014-2017. In addition to general exclusion criteria including non-cooperation for evaluation, intellectual disability, and unwillingness to participate into the study, we excluded those without baseline ECG, or subjects who had corrected QT (QTc) interval less than 370msec but their SQTS survey data were unavailable. This study was approved by the local Ethics Committee of Shiraz University of Medical Sciences (code: IR.SUMS.MED.REC.1399.544). All the subjects signed the written informed consent.

2-2-Assessments
The upper limit of QTc in a well-studied topic, but the de nite cut-off values for SQTS is remained a puzzle nonetheless [18] . It should be noticed that to diagnose SQTS a diagnostic panel might be a better tool rather than single QTc; since, affected and healthy subjects pose a wide overlapping QTc range. In the present study, we applied Gollob criteria [18] and European Society of Cardiology criteria [19] for case nding; however, ndings were reported based on Gollob criteria (Table 1). 1-QTc < 340msec, or 2-QTc < 360msec and (1) con rmed pathogenic mutation, or (2) family history of SQTS, or (3) family history of SCD at 40 years of age, or (4) Survival from a VT/VF episode in the absence of heart diseases European Society of Cardiology criteria [19] Standard 12-lead ECGs (CARDIAX computerized ECG, Medusoft Pty. Ltd., Australia) were obtained at a paper speed of 25mm/sec and a gain of 10 mm/mV. First, machine-derived QTc intervals were recorded, and recheck manually through calculating Bazett's equation (QTc = QT/ (√RR). Then, ECGs with QTc of less than 370msec -labeled as SQTS susceptible group -were reanalyzed by an experienced cardiologist with special quali cation in clinical electrophysiology (MHN) from the original paper, and recorded ECG ndings i.e., bradycardia, early repolarization, atrial brillation (AF), arrhythmias, other electrical conduction abnormalities, etc. Furthermore, demographic variables, history of SCD in subjects and their rst-and second-degree relatives, history of sudden infantile death, history of syncope, cardiac diseases in rst-and second-degree relatives were recorded. These data were extracted from PERSIAN cohort database or online survey through dedicated platform by local residents who had relevant collage education and underwent practical training through a number of workshops.

3-2-Statistical analysis
Statistical analysis was carried out using statistical package for social sciences (SPSS) (IBM Corp. Released 2019. IBM SPSS Statistics for Windows, version 26.0. Armonk, NY: IBM Corp.). Qualitative variables and quantitative variables were described using frequency (percent) and mean ± standard deviation (SD) or median [interquartile range (IQR)], respectively. Independent t-test and Fisher's exact test were applied to compare QTc > 370msec group and QTc < 370msec. P ≤ 0.05 was considered statistically signi cant.

3-Results
Forty-three hundred and sixty-three subjects were analyzed ( Table 2). According to Gollob et al. (2011) [   cardiac death (SCD) in her 18-year-old sister and another sister during infancy Subject #59: a 53-year-old male with QTc of 369 msec who expired due to SCD two years after performing ECG Table 3 depicts detailed data of the 72 subjects with varying degrees of SQTS probability. The frequency of AF and syncope were 16.67% and 4.17%, respectively. Moreover, bradycardia, early repolarization and low voltage were prevalent on their ECG; 33.33%, 11.11% and 11.11%, respectively. Also, subject #64 was suspicious to have type II Brugada syndrome. His ECG showed a saddleback ST segment elevation in lead V1 and a prominent R wave in lead aVR, however its amplitude was not more than 0.3mV-it was 0.2mV-and did not ful lled R wave sign (Figures 1 and 2).   Interestingly, a noticeable proportion-equal to 11.11%-reported the infantile SCD in their rst-and second-degree relatives, which might represent the importance of genetic and pedigree patterns studies; for example:

4-Discussion
We presented the clinical pro le and 4-year follow-up of 72 adult (≥ 40 years) subjects with short QT interval in one of the southern sectors of the PERSIAN cohort. Findings were as following: (1) the prevalence of short QT interval in our normal adult population was 1.65% (n = 72, SQTS-susceptible group) with varying degrees of SQTS probability, which was within the range obtained by Japanese [14] , U.S. [13] and Finnish [16] cohorts (0.37 to 2.88% at 360msec cut-off).
Also, 4 subjects had QTc < 330ms; (2) up to 27.78% of subjects in SQTS-susceptible group had symptoms, including AF (16.67%), palpitations (6.94%), unexplained syncope (4.17%), and SCD (1.39%), respectively; (3) at least, 2 subjects with high-probability SQTS and 3 with intermediate-probability SQTS identi ed according to Gollob's criteria; (4) based on the European Society of Cardiology criteria, an underestimated crude prevalence of 0.18% was calculated for SQTS in our cohort; (5) bradycardia and early repolarization recognized in ECGs of 33.33% and 9.72% of SQTS-susceptible group; (6) SQTS-susceptible group had a signi cantly lower mean heart rate; (7) infantile SCD was found in 11.11% of the rst-and second-degree relatives of SQTS-susceptible group, re ected that family screening for SQTS must carry out to avoid SCDs in mostly-asymptomatic subjects; (8) A signi cant male predominance observed in SQTSsusceptible group; (9) one suspicious case of Brugada syndrome was identi ed. Similar to in-line studies [14,16] , we found a male predominance in short QT interval group, supporting that QT interval is generally longer in females [20][21][22] . Although we could not assess this male predominance for SQTS, several studies have shown that the same is true for SQTS, which support the role of sex-speci c parameters like sex hormones (especially testosterone level during puberty) modulation of potassium currents, genes located on the X chromosome, membrane ion channel availability, and intracellular signal transduction in pathogenesis of SQTS [10,14,[23][24][25] .
In Miyamoto et al. [27] cohort, the rate of AF was lower than our study (9.1% vs. 16.67%). Obviously, this is signi cantly higher than the 2.8% reported for adults in Iran [28] and 2% in the general population in the world [29] , showing the presence of the same mechanism of an accelerated repolarization, shorter refractory period, and decreased action potential in the atrium. Several reports on childhood slow rate AF diagnosed with SQTS support this [30][31][32][33] .
Early repolarization usually accounts for as a benign ECG nding in normal population with a heterogenous prevalence, range 1-13% [34][35][36][37][38][39][40] . However, early repolarization might be associated with an increased risk of arrhythmic events like idiopathic ventricular brillation and the short QT interval [37,40] . That is, an uneven increase of early repolarization with a signi cant male precedence has been reported in short QT interval cases (6.1-30%) [14,39,40] and SQTS patients (65%) [40] ; nevertheless, this nding was reached up to 11.11% in SQTS-susceptible group of our study, predominantly involving inferolateral leads that was in agreement with previous reports [30,40] . The proposed common underlying mechanism for early repolarization is a reduction in inward repolarizing currents by loss of function mutations and/or an increase in outward repolarizing currents by gain-of-function mutations. It decreases the action potential duration (short QT interval), and increases the risk of reentrant mechanisms that can lead to AF and VF [27,41,42] . Noticeably, early repolarization is found in both short QT interval patients with mutations [43] and without mutations in the known genes [42] , re ecting a variable genetic background for the association between short QT interval and early repolarization. By and large, the role of early repolarization, particularly inferior/inferolateral early repolarization, in risk strati cation of cardiac events in individuals with short QT interval or SQTS is still unanswered, and studies on the association between long-term outcomes and presence of early repolarization in different QT interval groups are warranted.
The overlapping syndromes, concomitant Brugada-like (atypical) and SQTS, in a single patient with the same mutation and positive family history of SCD are reported in less reported variants including voltage-dependent calcium channel subunits (CACNA2D1, CACNA1C, CACNB2) [42] and sodium channel protein (SCN5A) [44] . We identi ed a suspicious symptomatic (AF) case of Brugada-like ECG with short QT interval. This less understood varying genotype-phenotype relationship is believed to modulate by external factors (such as medication, fever, or electrolyte disorder) [45] and genetic modi ers (such as ethnicity) [46] .
Another nding was bradycardia in one third of SQTS-susceptible group as well as a lower mean heart rate. One explanation might be the indecisive performance of correction formula. Whatsoever, some common physiologic effects i.e., ion channel gain or loss-of-function, hormonal effects or autonomic nervous system alterations (co-existence of lower heart rate and lower blood pressure in Anttonen et al. [16] , study) may incorporate in short QT interval, repolarization and attenuated sinus node activity.
In our cohort, infantile SCD was found in 11.11% of the rst-and second-degree relatives of short QT interval group.
Giustetto et al. [11] , showered that approximately half of SQTS patients have history of familial SCDs. In addition, SCDs in a close relative diagnose late in half of the families [47] . Furthermore, while most of cases never experience symptoms, SQTS can be highly malignant with a highest mortality rate preceding productive age [5] . As a result, SQTS represents as a "self-extinguishing" and "neglected" disease; that is, rst-and second-degree relatives of an index patient must be screened and clinically assessed to avoid SCDs [7,48] .
Our study has limitations: (1) The calculated number of SQTS patients was a case for underestimation in our study.
First, our cohort did not include subjects younger than 40 years, which rise the chance of underestimation. Previous studies have shown that SQTS might be more prevalent at the two age extremes, partially explained by two distinct underlying mechanisms [14,15] . Furthermore, the arrhythmogenic and malignant forms of SQTS are more likely to cause infantile SCDs and SCDs during 20 to 40 years of age [47] , and it appears that a short QT interval in a normal middleaged subject may be considered as a benign condition [16] . This fact might be linked to the low rate of SCDs and symptomatic patients as well as lack of cases with extremely short QT interval (< 300ms) and documented ventricular tachyarrhythmias in our cohort. Albeit, it should be noticed that in a SQTS cohort SCD did not occur in any of the patients during follow-up [49] . Second, Although Gollob's diagnostic criteria relies on four items, we could not evaluate subjects for two items of both scoring criteria (items 3 and 4); (2) Due to the limited resources, we could not perform gene study and assess SQTS among rst degree relatives of intermediate to high probability SQTS cases and symptomatic subjects with short QT interval. This could be a potential source of bias because these scenarios increase the chance of identifying more cases for such a disease that shows a robust familial clustering and most of cases diagnose incidentally, till being symptomatic or developing life-threatening events; (3) Although QT interval is genderdependent, lower limit of QT interval is not described for males and females, separately. Hence, using a single cut-off value might exert bias in case nding, as this is expected to be longer in females than males. We partially dealt with this issue by using a higher cutoff value (370ms). In addition, owing to the nature of our cohort-a normal adults' population-we applied Bazett correction formula, which is shown to yield a consistent result at the range of heart rates encountered in healthy adults at rest [50] .

Conclusions
In summary, the prevalence of short QT interval in our cohort was in line with previous studies. Nevertheless, higher proportions of cardiac symptoms, familial SCDs and ECG derived speci c ndings amongst SQTS-susceptible index persons than non-short QT interval normal population might implicate in gene study and family screening. Furthermore, the seriousness of short QT interval as well as contribution of clinical and paraclinical variables in risk strati cation is still an open question. We believe that a long-term follow-up cohort of young population will be elucidative.

Declarations
Competing interest:  QTc, bradycardia (HR: 50), low voltage ECG and history of syncope; D [case #59] a 53 Y/O male with a score 1 QTc, relative bradycardia (HR: 59), history of AF, and counter clockwise rotation who expired due to SCD during the cohort; E [case #64] a 54 Y/O male with a score 1 QTc and relative bradycardia who was suspicious to the type II Brugada syndrome (a saddleback ST segment elevation in lead V1 and a prominent R wave in lead aVR, but not ful lled R wave sign). Brugada test was not performed.