Setting
Data were obtained from the Medical Information Mart for Intensive Care III version 1.4 (MIMIC-III, Version 1.4), which is a public and freely-available intensive care unit (ICU) database[16]. Briefly, the MIMIC-III database contains comprehensive, time-stamped information for more than 60,000 ICU patients (medical, surgical, coronary care and neonatal) admitted to Beth Israel Deaconess Medical Center (Boston, MA, USA) from June 1st 2001 to October 31st 2012 (single center), representing more than 46000 patients. Since the database was approved by the Institutional Review Boards (IRB) of Beth Israel Deaconess Medical Center (Boston, MA) and the Massachusetts Institute of Technology (Cambridge, MA), IRB approval from our institution was exempted. The study was reported according to the REporting of studies Conducted using Observational Routinely collected health Data (RECORD) statement[17].
Study subjects
Patients for this study were selected from all persons in MIMIC-III aged ≥18 years at ICU admission with cardiogenic shock (ICD-9-CM diagnosis codes 785.51 or 998.01), plus any of the following criteria: minimum systolic blood pressure (SBP) <90 mmHg, or need of vasopressors therapy (any of dopamine, epinephrine, norepinephrine, phenylephrine, vasopressin), or signs of hypoperfusion (first 24 hours of urine output <400 ml or maximum blood lactate >2 mmol/L). Of these patients, we excluded patients with ≤0 days or ≥100 days between ICU admission and discharge, defined as the earliest of recorded ICU discharge, hospital discharge or time of death. Patients initiated milrinone therapy after 48 hours of ICU entry were also excluded. If patients who had multiple admissions to ICU, only the first ICU admission was included for analysis.
Demographical and laboratory variables
The following variables were extracted from the MIMIC-III database for the first day of ICU admission: age at the time of hospital admission, gender, acute coronary syndrome (ACS), Acute Physiology Score III (APS III), Simplified Acute Physiology Score II (SAPSII), maximum creatinine, maximum bilirubin, maximum INR (international normalized ratio), mean heart rate, mean of mean BP (blood pressure), minimum SBP, urine output first day after ICU entry, use of mechanical circulatory support (MCS), dobutamine, vasopressors, mechanical ventilation, and renal replacement therapy (RRT). If a variable was measured more than once in the first 24 hours, the maximum value was used. Patients with age ≥300 years was corrected as a median age of 91.4 years.
The primary endpoint was the hospital mortality, which was defined as the status of patient survival at the time of hospital discharge.
Statistical analysis
The study population was categorized into the milrinone (intervention) and non-milrinone (control) groups according to milrinone treating status after ICU entry. Categorical variables were expressed as the number of percentage. They were compared between milrinone and non-milrinone groups with Chi-square or Fisher’s exact test as appropriate. Continuous variables were expressed as mean (standard deviation) or median [interquartile range (IQR)] as appropriate.
We selected these potential confounders on the basis of their associations with the outcomes of interest or a change in effect estimate of >10% or P-values <0.1 in univariable analyses. Cox regression model was used to adjust for imbalance by including parameters and potential confounders judged by clinical expertise.
Propensity score matching (PSM) was estimated with the use of a nonparsimonious multivariable cox-regression model[18], with CS with the use of milrinone as the dependent variable and select the baseline characteristics that was included in the multivariable cox regression model (including age, gender, APSIII, SAPSII, ACS, maximum creatinine, using MCS, mechanical ventilation, RRT, dobutamine, and vasopressors) as covariates. Matching was performed with the use of a 1:2 matching protocol with a caliper width equal to 0.2 of the standard deviation of the logit of the propensity score.
A two tailed P value <0.05 was considered to be statistically significant. EmpowerStats version 2.17.8 (http://www.empowerstats.com/cn/) and R software version 3.42 were used for all statistical analysis.