2.1 Data source
All data in our study were extracted from the Medical Information Mart for Intensive Care Database III version 1.3 (MIMIC-III v1.3), a large, open, free and single-centered database including information from more than 50,000 adult patients admitted to various critical care units at Beth Israel Deaconess Medical Center (Boston, MA, USA) from 2001 to 2012 14. The setting and use of this database were approved by the institutional review boards of the Massachusetts Institute of Technology (Boston, MA) and Beth Israel Deaconess Medical Center (Cambridge, MA). All personal information included in the database have been de-identified to safeguard privacy.
2.2 Population selection criteria
More than 50,000 ICU admissions to the MIMIC-III database were recorded, and only patients diagnosed with CS were extracted. Among these patients, we selected those who attained more than 16 years of age at first admission while remaining in the hospital for more than 48 hours. Exclusion criteria were as follows: (1) patients who were diagnosed with hematologic neoplasms, including leukemia, multiple myeloma, and others; (2) more than 10% individual data were missing; (3) individual data values exceeded the mean ± 3 times the standard deviation (SD).
CS was determined on the grounds of International Classification of Diseases, Ninth Revision (ICD-9), the definitions in the clinical practice guidelines including the ESC Guidelines 15 as well as clinical trials, including SHOCK 16 and IABP-SHOCK II 17.
2.3 Date extraction
Data were extracted through Structured Query Language (SQL) 18 with MySQL tools (version 5.6.24) from MIMIC-III. The extracted data contain demographic parameters, basic vital signs, laboratory indicators and scoring systems.
Demographic parameters included age, gender and ethnicity, while basic vital signs included heart rate (HR), systolic blood pressure (SBP), diastolic blood pressure (DBP), mean blood pressure (MBP), respiratory rate (RR), temperature and percutaneous oxygen saturation (SPO2). The following laboratory indicators were extracted: neutrophils, albumin, platelets, partial thromboplastin time (PTT), prothrombin time (PT), international normalized ratio (INR), serum bicarbonate, serum sodium levels, serum potassium levels and serum glucose levels. We defined the NAR as the ratio of the neutrophil percentage to the serum albumin level. We additionally extracted relevant comorbidities, for example, coronary heart disease (CAD), congestive heart failure (CHF), atrial fibrillation (AF), stroke, chronic obstructive pulmonary disease (COPD), pneumonia, acute respiratory distress syndrome (ARDS) and other diseases listed in Table 1. Severity-of-illness scores, including the Sequential Organ Failure Assessment (SOFA) 19 score and the Simplified Acute Physiology Score II (SAPS II) 20 were also calculated for every individual. The SOFA score, designed to describe a sequence of complications in the critically ill, is the sum of points obtained from evaluation of respiration, coagulation, liver, cardiovascular system, central nervous system and kidney. The SAPS II, a scoring system developed to estimate the risk of death, included 12 physiology variables (HR, SBP, body temperature, partial pressure of arterial oxygen or fraction of inspired oxygen ratio, urinary output, serum urea or serum urea nitrogen level, WBC count, serum potassium level, serum sodium level, serum bicarbonate level, bilirubin level and Glasgow Coma Score 21), age, type of admission (scheduled surgical, unscheduled surgical or medical) and three underlying disease variables (acquired immunodeficiency syndrome, metastatic cancer and hematologic malignancy). All scores were assessed and calculated on the basis of published recommendations and accepted formulas.
The initiation of our study was the time when the patient admitted to ICU. The outcomes were 30-day, 90-day and 365-day mortality, in which 30-day mortality was the primary outcome. Baseline characteristics were all recorded within 24 hours after admission to the ICU.
2.4 Statistical analysis and modeling strategy
Baseline characteristics delaminated by NAR were presented in Table 1. Categorical data were shown as frequency (percent), while continuous ones as mean (SD) or median(IQR). We did comparisons between groups by the chi-square test 22 or Fisher’s exact test 23 for categorical variables and the variance analysis or the Kruskal-Wallis test 24 for continuous ones.
To examine the associations between NAR and outcomes (30-day, 90-day and 365-day mortality), we used Cox proportional hazards models 25. The outcomes were respectively analysed according to the tertiles or the quintiles of the NAR level. The first tertile or quartile group was regarded as the reference group. The results were presented as hazard ratios (HRs) with 95% confidence intervals (CIs). To further identify the association between NAR and mortality, multivariate analyses were performed using two adjusted models. The confounders selected in our models were based on their associations with mortality or a mutation exceeding 10% 26. In model I, we adjusted covariates for age, gender and ethnicity. In model II, covariates were adjusted further for length of stay in ICU, HR, SBP, DBP, RR, SPO2, anion gap, serum bicarbonate, serum potassium, SCr, BUN, hematocrit, platelet count, WBC count, PTT, PT, INR, stroke, pneumonia, COPD, chronic liver disease, chronic renal disease, RRT, malignancy, vasoactive agent, SOFA score and SAPSII score. Building further on this foundation, we performed stratified analysis to confirm whether the effect of NAR differed across each of the subgroups that were classified by stay in ICU, vital signs (HR, SBP, DBP, RR, temperature, SPO2), laboratory parameters (anion gap, serum bicarbonate, serum sodium, serum potassium, serum chloride, serum bilirubin, serum glucose, SCr, BUN, hematocrit, hemoglobin, platelet count, WBC count, PTT, PT, INR), comorbidities (CAD, CHF, stroke, pneumonia, COPD, respiratory failure, chronic liver disease, chronic renal disease, RRT, malignancy), vasoactive drug use and scoring systems (SOFA and SAPSII scores).
A two-tailed P value < 0.05 was deemed statistically significant. EmpowerStats version 2.17.8 (http://www.empowerstats.com/cn/) and R software version 3.42 were used for all statistical analysis.