Study design
We conducted a single-center retrospective cohort study using data of first-admitted patients 50 years or older with ICD-10-CM codes of acute myocardial infarction (AMI) (ICD- 10 code
I21) that extracted from the hospital information system (HIS) of Shahid Rajaei hospital in Karaj city, Iran, over a 4-year period.
The study population
The study population was all inpatients 50 years or older who were admitted for the first time with a diagnosis of AMI (with or without ST segment elevation) in our hospital from 23th March 2013 to 1th January 2020. As novel revascularization techniques including percutaneous coronary intervention (PCI) and coronary artery bypass grafting (CABG) have been performed in our hospital since 2013, the study was restricted to this period.
Karaj city is the center of Alborz Province, Iran. Karaj, with about 1.97 million inhabitants as recorded in the 2016 census, is the fourth-largest city in Iran. Also, Karaj is located 20 kilometers (12 mi) west of Tehran and is a large suburb of it.
Shahid Rajaei is a referral hospital providing outpatient, inpatient and emergency services for patients with AMI in Karaj city.
Management of acute myocardial infarction in Shahid Rajaei hospital is according to the most updated guidelines of the American College of Cardiology and American Heart Association and of the European Society of Cardiology.
Data Sources and Collection
Data on demographic characteristics, medical history, and ICD-10-CM codes of diagnoses and procedures for each patient are recorded and stored in the hospital information system (HIS) by trained staffs.
In this study, we retrieved some baseline characteristic of included patients, including age (years), gender, presence of comorbidities, type of MI, performed procedures, discharge status (dead or alive), date of admission, discharge or death from the HIS database. Patient’s age was categorized into three groups: 50–64, 65–74, and ≥ 75 years. Length of the stay in the hospital was calculated in days for each patient.
The clinical data were extracted by using International Classification of Diseases, tenth Revision, Clinical Modification (ICD-10-CM) codes.
The type of MI was determined according to the ICD-10-CM classification as follows: STEMI (codes I21.0-3), unspecified (code I21.9) and NSTEMI (code I21.4)
Using the ICD-10- CM codes, we identified the presence or absence of specific comorbidities at the time of the admission including heart failure (code I50), hypertension (code I10), diabetes (codes E10–E14), and chronic kidney disease (code N18). Besides, we also used ICD-10-CM codes 00.66and 36.01 to 36.09 to identify patients who underwent PCI, and ICD-9-CM codes 36.10 to 36.19 to identify patients who underwent CABG surgery.
The outcome measurement
The main outcome was in-hospital mortality in patients with AMI. As data on the exact cause of death was not available, in-hospital death was defined as death due to any cause during the hospital stay.
Statistical analyses
We used descriptive statistics, mean (SD) for continuous variables and as number (percentage) for categorical variables to summarize demographic and clinical data of the cohort. Characteristics of males and females were compared using two-tailed t-tests or chi-square tests, as appropriate.
We also used Univariable and Multivariable Cox proportional hazard regression models to assess the effect of being female on the in-hospital mortality after AMI. The results is presented as a hazard ratio (HR) 95% confidence interval [CIs]). Proportional hazard assumption was evaluated using both graphical methods (ln-ln S [t] graphs) and statistical tests containing continuous time-interaction terms (Cox tests) [8].
Various cox regression models were constructed according to adjustment strategy: model I, unadjusted; model II, adjusted for patient age; and model III, adjusted for age, diabetes, arterial hypertension.
We also compared the survival distribution between males and females using the log-rank test. Follow-up period defined as time (days) from the date of admission until the date of discharge or in-hospital death whichever occurred first. The probability of survival for males and females was depicted in Kaplan-Meier plots. For all analyses a p-value < 0.05 (two-tailed) was considered statistically significant. Analyses were performed using SPSS version 18.0 (SPSS, Chicago, IL, USA) and STATA V.12.0 (Stata Corp, College Station, Texas, USA).