2.4 Data synthesis
We chose the ischemic heart disease as the main disease to analysis. The
studies included were summarized according to their RRs with
corresponding 95% CI . The study-specific log RRs were
weighted by the inverse of their variance, and the weighted values were
used to assess the association between hysterectomy and risk of CVD.
Furthermore,
pooling was also performed for studies on different ovaries status
(hysterectomy with oophorectomy or hysterectomy with ovarian
preservation) and studies on age at hysterectomy (divided by 50 years
old).
In
our analysis, hysterectomy with unilateral oophorectomy was classified
as the category of hysterectomy with ovarian preservation, and
hysterectomy with bilateral oophorectomy was classified as hysterectomy
with oophorectomy. Overall RRs with 95% CI in each study,
instead
of
stratum-specific values (eg, separate estimates for hysterectomy with
oophorectomy and hysterectomy with ovarian preservation), were used to
assess total pooled RR of this meta-analysis. If an overall value
was not presented in the
article,
the stratum-specific values were used following the principle that the
populations in the different strata did not overlap.
Assessment for among-study heterogeneity was performed by calculatingI2 which represents no, low, moderate and high
heterogeneity when its values are 0, 25, 50 and 75%
respectively.33 The fixed-effect model was used if
moderate or lower heterogeneity (I2 ≤50%) was
found. If I2 >50%, the
random-effect model was adopted. Pre-specified characteristics including
publication year, continent and study design for exploring potential
sources of heterogeneity were used in meta-regressions and subgroup
analyses.34 The sensitivity analysis which useI2 >50% as the criteria was
applied to evaluate the excluded studies that have substantial impact on
between-study heterogeneity.35 We also assessed the
relationship between hysterectomy with or without oophorectomy and CVD,
age at operation and CVD. The influence analysis was performed with one
study removed at a time to assess whether the results could have been
affected markedly by a single study.36 The small-study
effects were detected by Egger test37 and funnel plot.
All statistical analyses were carried out with STATA version 15.0 (Stata
Corporation, College Station, TX, USA). All reported 2-tailed P values
were considered statistically significant when P≤0.05.