4.3 Cause of Heterogeneity
In this study, obvious heterogeneity was reported in two meta-analyses
of comparing EAT volumes and EAT thickness in two groups with and
without AF occurrence. Given the obtained heterogeneity, the random
effect model was used. However, f the classification of studies into
three categories of LA-EAT volume, total EAT volume, and EAT thickness
reduced heterogeneity. Because of small sample sizes, a subgroup
analysis or meta regression could not find the possible sources among
various factors, such as female gender, cardiac surgery types, EAT
evaluation methods as well as quality of studies based on STROBE
Checklist. Moreover, BMI and age, as well as other classical AF risk
factors were additional possible sources of heterogeneity. According to
the sensitivity analysis (Table 2-a), it is suggested that the study of
Ertugrul et al. 21 be one of the sources of
heterogeneity. The endpoint of other studies was AF occurrence, which
was defined according to the guidelines for the management of atrial
fibrillation of European Society of Cardiology. But in this study, AFACS
was defined as arrhythmic episodes that deteriorate hemodynamics, lasts
more than 20 minutes per episode, or lasts more than 60 minutes during a
24-hour follow up, which underestimates the occurrence of AFACS. The
study by Ertugrul et al. omitted, the sensitivity analysis results
showed there was no heterogeneity resource (Table 2-b).
4.4 Study Limitations
The studies observational nature limits the study that by itself had no
high position in the Evidence-Based Medicine Pyramid. And lacking access
to patient detailed data, it didn’t allow us to adjust for other
covariates that may influence EAT including age, gender, and AF risk
factors of hypertension, obstructive sleep apnea hypopnea syndrome, BMI
and so on. Included studies were single-center trials with a small
sample size, which might result in unreliable data. The inclusion of
only 2 studies evaluating EAT thickness was limited because of the
potential lack of power and the inability to draw firm conclusions.
Finally, there was a significant degree of heterogeneity in the study,
which might be a limitation that has been demonstrated in other
published EAT meta-analyses with I2 values
>90% 51-53.