We analyzed the Nationwide Inpatient Sample database from 2002 to 2010 to examine the temporal trends in incidence of acute kidney injury (AKI), AKI requiring dialysis, and associated in-hospital mortality in patients ≥75 years of age hospitalized with acute myocardial infarction and undergoing early (within 24 hours) percutaneous coronary intervention. Of 2,225,707 patients ≥75 years of age with acute myocardial infarction, 233,508 (10.5%) underwent early percutaneous coronary intervention, of which 21,961 (9.4%) developed AKI and 1,257 (0.54%) developed AKI requiring dialysis. From 2002 to 2010, the incidence of AKI increased from 5.6% to 14.2% (p for trend <0.001) and that for AKI requiring dialysis decreased (0.6% to 0.4%; p for trend 0.018). Compared with 2002, multivariable-adjusted odds ratios and 95% confidence intervals for AKI, AKI requiring dialysis, and in-hospital mortality in 2010 were 1.87 (1.71 to 2.05), 0.20 (0.15 to 0.27) and 0.74 (0.60 to 0.90), respectively. In conclusion, among hospitalized adults ≥75 years of age, from 2002 to 2010, there was an increase in AKI, but there was paradoxical decrease in AKI requiring dialysis and in-hospital mortality, potentially reflecting increased health-care provider awareness resulting in early recognition and implementation of renal-protective strategies and diagnosis-related group creep.
Contrast use, compromised hemodynamic reserve, co-morbidities, and underlying age-related decrease in glomerular filtration rate result in acute kidney injury (AKI) in patients ≥75 years of age after percutaneous coronary intervention (PCI) for acute myocardial infarction (AMI). AKI is defined as an abrupt (within 48 hours) reduction in kidney function currently defined as an absolute increase in serum creatinine of >0.3 mg/dl, a percentage increase of 50%, or a reduction in urine output (documented oliguria of <0.5 ml/kg/h for >6 hours). Early PCI is the treatment of choice in appropriately selected patients presenting with AMI. Development of AKI in patients admitted with AMI is associated with poorer short- and long-term outcomes. The United States census data from 2010 indicate that persons ≥65 years of age grew faster than the general population (15.1% vs 9.7%) in the past decade. Previous studies have analyzed the trends of AKI in various hospitalized patient populations. Analysis of Medicare claims data from 1996 to 2001 showed an 11% increase in AKI per year in hospitalized Medicare beneficiaries. However, there remains a paucity of data on the temporal trends in AKI and subsequent in-hospital mortality after early PCI in patients ≥75 years of age. The primary objective of this study was to analyze the contemporary trends in AKI, AKI requiring dialysis, and associated outcomes (in-hospital mortality and length of stay in survivors) in patients ≥75 years of age undergoing early PCI for AMI using a large, well-validated national database.
Methods
Data were obtained using the Nationwide Inpatient Sample databases for years 2002 to 2010. The Nationwide Inpatient Sample is the largest publicly available all-payer inpatient care database in the United States. It is sponsored by the Agency for Healthcare Research and Quality as a part of the Healthcare Cost and Utilization Project. The database contains discharge-level data from approximately 8 million hospital stays from about 1,000 hospitals each year designed to approximate a 20% stratified sample of all community hospitals in the United States. A discharge weight is provided for each patient discharge record and was used to obtain national estimates of all hospitalizations.
We used the Healthcare Cost and Utilization Project’s Clinical Classifications Software code “100,” corresponding to the International Classification of Diseases, Ninth Edition, Clinical Modification diagnosis code “410.xx,” to identify all patients ≥75 years of age with the principal diagnosis of AMI admitted to the hospital from 2002 to 2010 (n = 2,225,707). We chose the principal diagnosis because it is considered the primary reason for hospital admission. In administrative databases, the diagnosis of AMI using these codes has been shown to have a specificity of 99.5% with a sensitivity of 72.4%, a negative predictive value of 96.1%, and a positive predictive value of 95.9%. We then selected patients who underwent PCI (procedure codes 00.66, 36.01, 36.02, 36.05, 36.06, and 36.07) within day 0 of admission, that is, early PCI (n = 239,836). The Nationwide Inpatient Sample database provides the day since admission PCI was performed. Patients with the diagnosis of end-stage renal disease requiring chronic dialysis (code 585.6) were excluded (n = 2,313). Similarly, discharges with codes associated with dialysis (codes 39.95, 54.98, 39.27, 39.42, 39.43, 39.93, V45.1x, V56.x, or 792.5) but without a diagnosis code of AKI (code 584.x) were also excluded (n = 917), assuming that these patients were on dialysis for end-stage renal disease but were not coded as 585.6. Waikar et al demonstrated that the coding for AKI has a sensitivity of 35.4% and a specificity of 97.7% in the administrative databases by comparing administrative codes with review of medical records. The higher specificity of the coding for AKI in administrative databases makes it a useful and a reliable research tool, although low sensitivity is an important caveat. Lastly, we excluded patients who died within 24 hours of admission (n = 3,099) because they would not have had sufficient time to develop AKI. This gave us a final study cohort of 233,508 patients aged ≥75 years who were not on dialysis before admission, were admitted for AMI, underwent early PCI, and had a length of stay of ≥1 day. We initially studied the temporal trends in AKI and AKI requiring dialysis among patients ≥75 years of age undergoing early PCI for AMI. We then examined the trends in outcomes among patients developing AKI. Our primary outcome of interest was all-cause in-hospital mortality, defined as “died” during the hospitalization encounter in the Nationwide Inpatient Sample database. Average length of stay (in survivors) was used as a secondary outcome.
Baseline patient characteristics used included demographics (age, gender, and race), primary expected payer, weekday versus weekend admission, Agency for Healthcare Research and Quality’s Elixhauser co-morbidities (acquired immunodeficiency syndrome, alcohol abuse, deficiency anemia, rheumatoid arthritis/collagen vascular diseases, chronic blood loss anemia, congestive heart failure, chronic pulmonary disease, coagulopathy, depression, diabetes [uncomplicated], diabetes [with chronic complications], drug abuse, hypertension [uncomplicated and complicated, hypothyroidism, liver disease, lymphoma, fluid and electrolyte disorders, metastatic cancer, other neurological disorders, obesity, paralysis, peripheral vascular disorders, psychoses, pulmonary circulation disorders, solid tumor without metastasis, peptic ulcer disease [excluding bleeding], valvular disease, and weight loss), other clinically relevant co-morbidities (smoking, dyslipidemia, known coronary artery disease, family history of coronary artery disease, previous AMI, carotid artery disease, chronic kidney disease [excluding end-stage renal disease on chronic dialysis] and dementia), ST elevation AMI versus non-ST elevation AMI, in-hospital complications (gastrointestinal bleeding, acute cerebrovascular disease, cardiogenic shock, and sepsis), and in-hospital procedures (thrombolysis, blood transfusion, pulmonary artery catheter placement, intra-aortic balloon pump placement, multivessel PCI, coronary artery bypass grafting, and mechanical ventilation). A list of International Classification of Diseases, Ninth Edition, Clinical Modification codes and Clinical Classifications Software codes used to identify co-morbidities and in-hospital procedures is provided in the Supplementary Table . We also studied hospital characteristics such as hospital region (northeast, midwest, south, and west), bed size (small, medium, and large), rural versus urban location, and teaching versus nonteaching status.
We used the Mantel-Haenszel test of linear association or linear regression with polynomial contrast for analysis of trends in baseline characteristics in patients ≥75 years of age undergoing early PCI for AMI. We used unadjusted and multivariable-adjusted logistic regression models to estimate the odds of AKI, AKI requiring dialysis, and in-hospital mortality (among patients with AKI) for patients discharged in each year relative to 2002. Models were adjusted for demographics, co-morbidities, hospital characteristics, in-hospital complications, and procedures as aforementioned. Major clinically relevant interaction terms were tested and accounted for in the regression model. We graphically displayed unadjusted and adjusted odds ratios and 95% confidence intervals for AKI and each outcome over time.
Statistical analyses were performed using IBM SPSS Statistics 20.0 (IBM Corp., Armonk, New York). We used a 2-sided p value of <0.05 to assess for statistical significance for all analyses. Categorical variables are expressed as percentage and continuous variables as mean ± SD. Odds ratio and 95% confidence intervals are used to report the results of logistic regression models.
Results
Table 1 depicts the changes in baseline and clinical characteristics of the overall population at risk, that is, patients ≥75 years of age undergoing early PCI for AMI from 2002 to 2010 (n = 233,508). Mean age of the cohort increased minimally from 2002 to 2010. There was an almost equal distribution of men and women throughout the 9-year study period. Smoking, dyslipidemia, carotid artery disease, diabetes mellitus, hypertension, peripheral vascular disease, and chronic kidney disease increased substantially in the patient population over the study period (p trend for all <0.001). In the overall cohort (2002 to 2010), 62.2% of patients presented with ST elevation AMI and 37.3% with non-ST elevation AMI. In-hospital procedures and complications are listed in Table 2 . Drug-eluting stent use increased exponentially from 2003 (United States Food and Drug Administration approval) to 2005, declining afterward to a plateau, as increased risk of in-stent thrombosis gained recognition (p trend <0.001). Overall, the utilization of drug-eluting stents surpassed the bare-metal stent utilization in our cohort. Rates of thrombolysis increased from 2002 to 2010 (p trend <0.001). On comparison of in-hospital complications, rates of sepsis and cardiogenic shock increased, whereas the rates of gastrointestinal bleeding decreased during the study period (p trend for all <0.001). Rates of acute cerebrovascular disease and coronary artery bypass graft surgery utilization remained nonsignificant on statistical analysis. In-hospital mortality in the overall cohort was 6.3% (p = 0.022). Mean length of stay in survivors decreased from 4.7 days in 2002 to 4.4 days in 2010 (p trend <0.001).
| Variable | 2002 (%) | 2003 (%) | 2004 (%) | 2005 (%) | 2006 (%) | 2007 (%) | 2008 (%) | 2009 (%) | 2010 (%) | Total (%) | p |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Number of cases (weighted) | 22,433 | 21,849 | 22,875 | 25,272 | 28,138 | 25,772 | 27,911 | 30,839 | 28,419 | 233,508 | — |
| Mean age (yrs) | 80.5 | 80.5 | 80.5 | 80.8 | 80.9 | 81.1 | 81.3 | 81.4 | 81.3 | 80.9 | <0.001 |
| Women | 50.0 | 50.5 | 51.2 | 50.1 | 50.8 | 49.3 | 49.7 | 49.3 | 48.3 | 49.9 | <0.001 |
| Race | <0.001 | ||||||||||
| Caucasian | 84.9 | 86.1 | 87.3 | 86.8 | 85.9 | 85.6 | 83.9 | 83.3 | 85.2 | 85.3 | |
| African-American | 3.3 | 4.3 | 4.3 | 3.0 | 3.5 | 4.2 | 4.3 | 4.3 | 5.2 | 4.1 | |
| Hispanic | 6.0 | 5.4 | 4.5 | 5.6 | 5.3 | 5.1 | 5.0 | 6.2 | 5.2 | 5.4 | |
| Asian or Pacific Islander | 1.9 | 1.9 | 1.5 | 1.5 | 1.4 | 1.7 | 2.4 | 2.3 | 1.7 | 1.8 | |
| Native American | 0.0 | 0.1 | 0.2 | 0.2 | 0.1 | 0.4 | 0.7 | 0.5 | 0.7 | 0.3 | |
| Other | 3.9 | 2.3 | 2.2 | 2.9 | 3.9 | 3.0 | 3.7 | 3.4 | 2.1 | 3.1 | |
| Primary expected payer | <0.001 | ||||||||||
| Medicare | 92.0 | 93.5 | 89.4 | 90.9 | 91.7 | 88.6 | 90.7 | 90.2 | 88.8 | 90.6 | |
| Medicaid | 0.8 | 0.8 | 0.8 | 1.0 | 0.7 | 0.9 | 1.1 | 0.6 | 1.2 | 0.9 | |
| Private insurance | 6.1 | 4.7 | 8.7 | 6.9 | 6.3 | 8.7 | 6.3 | 7.6 | 8.4 | 7.1 | |
| Self-pay | 0.4 | 0.2 | 0.3 | 0.6 | 0.4 | 0.5 | 1.0 | 0.8 | 0.6 | 0.6 | |
| No charge | 0.0 | 0.0 | 0.1 | 0.1 | 0.0 | 0.2 | 0.1 | 0.1 | 0.0 | 0.1 | |
| Other | 0.7 | 0.9 | 0.7 | 0.5 | 0.8 | 1.2 | 0.8 | 0.7 | 0.9 | 0.8 | |
| Weekend admission | 17.3 | 16.2 | 18.8 | 17.1 | 17.4 | 19.1 | 21.0 | 21.8 | 23.3 | 19.3 | <0.001 |
| Co-morbidities ∗ | |||||||||||
| Smoking | 14.2 | 11.7 | 14.7 | 15.8 | 16.2 | 19.1 | 17.9 | 20.2 | 22.1 | 17.2 | <0.001 |
| Dyslipidemia | 35.2 | 37.2 | 41.6 | 48.2 | 50.1 | 53.7 | 56.2 | 59.1 | 61.5 | 50.1 | <0.001 |
| Coronary artery disease | 87.6 | 86.5 | 88.0 | 87.1 | 88.7 | 88.2 | 89.7 | 89.9 | 90.4 | 88.6 | <0.001 |
| Family history of coronary artery disease | 2.3 | 2.1 | 3.0 | 3.6 | 3.0 | 3.7 | 4.0 | 4.6 | 5.0 | 3.6 | <0.001 |
| Previous myocardial infarction | 6.1 | 6.5 | 6.9 | 6.1 | 6.5 | 7.7 | 7.8 | 7.8 | 8.9 | 7.2 | <0.001 |
| Carotid artery disease | 0.8 | 0.9 | 1.1 | 1.0 | 1.2 | 1.3 | 1.9 | 1.3 | 1.7 | 1.3 | <0.001 |
| Chronic kidney disease | 0.4 | 0.4 | 0.4 | 2.1 | 6.6 | 12.5 | 13.6 | 15.3 | 17.1 | 8.2 | <0.001 |
| Dementia | 3.3 | 4.2 | 4.1 | 4.6 | 4.9 | 5.3 | 6.2 | 7.6 | 6.8 | 5.4 | <0.001 |
| AIDS | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | <0.1 | 0.0 | 0.0 | <0.1 | — |
| Alcohol abuse | 0.7 | 0.5 | 0.7 | 0.6 | 0.6 | 1.0 | 0.8 | 0.7 | 0.7 | 0.7 | 0.010 |
| Deficiency anemia | 9.6 | 11.1 | 10.7 | 10.5 | 12.3 | 15.8 | 16.0 | 15.5 | 17.2 | 13.5 | <0.001 |
| RA/collagen vascular diseases | 2.1 | 1.6 | 2.3 | 2.0 | 2.2 | 2.3 | 3.1 | 2.0 | 2.6 | 2.3 | <0.001 |
| Chronic blood loss anemia | 1.4 | 1.3 | 1.4 | 1.2 | 1.4 | 1.7 | 0.9 | 1.1 | 0.9 | 1.3 | <0.001 |
| Congestive heart failure | 25.8 | 25.1 | 25.9 | 26.9 | 24.8 | 25.5 | 24.4 | 26.2 | 26.0 | 25.6 | 0.987 |
| Chronic pulmonary disease | 16.3 | 15.1 | 16.1 | 15.7 | 17.3 | 17.1 | 16.5 | 15.8 | 16.3 | 16.3 | 0.071 |
| Coagulopathy | 2.9 | 3.0 | 2.8 | 3.7 | 3.4 | 3.1 | 3.8 | 4.0 | 4.6 | 3.6 | <0.001 |
| Depression | 2.2 | 2.7 | 2.7 | 3.3 | 3.5 | 4.0 | 4.5 | 4.9 | 4.5 | 3.7 | <0.001 |
| Diabetes (uncomplicated) | 19.9 | 21.2 | 21.5 | 21.6 | 21.5 | 23.5 | 23.9 | 24.3 | 24.0 | 22.6 | <0.001 |
| Diabetes (complicated) | 1.6 | 1.6 | 1.7 | 1.8 | 1.8 | 2.6 | 2.7 | 2.7 | 2.7 | 2.2 | <0.001 |
| Drug abuse | <0.1 | <0.1 | <0.1 | <0.1 | 0.1 | 0.1 | 0.1 | 0.2 | 0.1 | 0.1 | <0.001 |
| Hypertension | 56.0 | 60.4 | 62.2 | 63.5 | 66.7 | 67.9 | 70.6 | 72.6 | 72.3 | 66.5 | <0.001 |
| Hypothyroidism | 9.2 | 9.2 | 9.5 | 10.3 | 10.6 | 12.0 | 13.7 | 13.3 | 13.6 | 11.5 | <0.001 |
| Liver disease | 0.2 | 0.2 | 0.2 | 0.3 | 0.2 | 0.4 | 0.3 | 0.3 | 0.5 | 0.3 | <0.001 |
| Lymphoma | 0.5 | 0.4 | 0.5 | 0.2 | 0.4 | 0.5 | 0.6 | 0.7 | 0.5 | 0.5 | <0.001 |
| Fluid and electrolyte disorder | 9.9 | 9.6 | 10.2 | 11.6 | 12.3 | 13.2 | 15.1 | 15.5 | 17.3 | 13.0 | <0.001 |
| Metastatic cancer | 0.5 | 0.5 | 0.5 | 0.6 | 0.7 | 0.7 | 0.5 | 0.5 | 0.6 | 0.6 | 0.285 |
| Obesity | 2.9 | 3.3 | 3.4 | 3.4 | 3.6 | 4.3 | 5.0 | 5.3 | 5.8 | 4.2 | <0.001 |
| Paralysis | 1.2 | 1.0 | 1.0 | 0.8 | 1.0 | 1.1 | 1.3 | 1.1 | 1.4 | 1.1 | <0.001 |
| Other neurological disorders | 5.4 | 3.5 | 4.1 | 4.1 | 4.2 | 5.0 | 5.7 | 5.8 | 5.5 | 4.9 | <0.001 |
| Peripheral vascular disease | 7.9 | 8.1 | 9.6 | 9.9 | 10.2 | 11.1 | 11.6 | 11.7 | 11.7 | 10.4 | <0.001 |
| Psychoses | 0.6 | 1.0 | 0.7 | 0.6 | 0.8 | 0.9 | 1.1 | 1.1 | 1.3 | 0.9 | <0.001 |
| Pulmonary circulation disorders | 0.0 | 0.0 | 0.0 | <0.1 | <0.1 | 0.0 | <0.1 | <0.1 | 0.1 | <0.1 | — |
| Solid tumor without metastasis | 9.5 | 1.8 | 1.4 | 1.7 | 1.5 | 1.8 | 1.5 | 1.5 | 1.8 | 2.3 | <0.001 |
| Peptic ulcer (nonbleeding) | 1.8 | 0.0 | 0.1 | 0.0 | 0.0 | 0.0 | 0.0 | 0.1 | 0.0 | 0.2 | <0.001 |
| Valvular disease | 0.1 | 0.0 | 0.1 | 0.1 | 0.1 | 0.0 | 0.1 | 0.1 | 0.2 | 0.1 | 0.007 |
| Weight loss | 0.5 | 0.4 | 0.5 | 0.8 | 1.2 | 0.8 | 1.5 | 1.8 | 2.1 | 1.2 | <0.001 |
| Hospital characteristics | |||||||||||
| Region | <0.001 | ||||||||||
| Northeast | 28.8 | 24.3 | 24.9 | 23.8 | 21.8 | 24.7 | 22.3 | 21.8 | 22.0 | 23.6 | |
| Midwest | 14.1 | 19.3 | 16.7 | 19.7 | 20.1 | 18.2 | 18.7 | 20.1 | 25.0 | 19.3 | |
| South | 39.8 | 41.1 | 39.7 | 35.0 | 41.3 | 36.3 | 40.2 | 38.0 | 31.6 | 38.0 | |
| West | 17.4 | 15.2 | 18.7 | 21.5 | 16.8 | 20.8 | 18.8 | 20.1 | 21.3 | 19.0 | |
| Bed size † | <0.001 | ||||||||||
| Small | 7.3 | 6.3 | 7.6 | 3.4 | 8.8 | 6.3 | 5.0 | 6.5 | 8.7 | 6.7 | |
| Medium | 17.5 | 19.4 | 19.8 | 19.6 | 21.0 | 26.4 | 23.3 | 18.9 | 19.4 | 20.7 | |
| Large | 75.2 | 74.3 | 72.5 | 76.9 | 70.2 | 67.4 | 71.6 | 74.6 | 72.0 | 72.7 | |
| Urban location | 95.0 | 93.3 | 95.3 | 95.7 | 95.7 | 95.2 | 93.8 | 94.4 | 92.4 | 94.5 | <0.001 |
| Teaching hospital | 59.6 | 56.0 | 59.0 | 52.9 | 57.3 | 57.1 | 51.5 | 53.2 | 49.8 | 54.9 | <0.001 |
| ST elevation myocardial infarction | 66.7 | 64.5 | 61.8 | 61.2 | 59.6 | 60.1 | 62.7 | 59.7 | 64.6 | 62.2 | <0.001 |
∗ Co-morbidities (including the 29 Elixhauser co-morbidities) were extracted from the database using International Classification of Diseases, Ninth Edition, Clinical Modification Diagnosis codes and Clinical Classifications Software.
† Bed size categories are specific to hospital location and teaching status, available at http://www.hcup-us.ahrq.gov/db/vars/hosp_bedsize/nisnote.jsp .
| Variable | 2002 (%) | 2003 (%) | 2004 (%) | 2005 (%) | 2006 (%) | 2007 (%) | 2008 (%) | 2009 (%) | 2010 (%) | Total (%) | p |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Number of cases (weighted) | 22,433 | 21,849 | 22,875 | 25,272 | 28,138 | 25,772 | 27,911 | 30,839 | 28,419 | 233,508 | — |
| In-hospital procedures | |||||||||||
| Mechanical ventilation | 7.6 | 5.7 | 6.4 | 7.0 | 6.6 | 7.7 | 7.9 | 7.9 | 8.6 | 7.3 | <0.001 |
| IABP | 8.5 | 7.2 | 7.3 | 7.1 | 7.4 | 7.6 | 8.9 | 8.1 | 8.7 | 7.9 | <0.001 |
| CABG | 2.2 | 1.9 | 1.7 | 1.8 | 2.1 | 1.9 | 2.0 | 1.9 | 2.0 | 1.9 | 0.381 |
| MVPCI | 13.3 | 12.8 | 16.2 | 17.7 | 17.1 | 15.9 | 15.9 | 15.0 | 13.2 | 15.3 | 0.435 |
| Blood transfusion | 5.4 | 5.9 | 7.0 | 6.7 | 5.8 | 7.7 | 6.9 | 7.2 | 6.5 | 6.6 | <0.001 |
| Pulmonary artery catheter | 2.0 | 1.6 | 1.4 | 1.0 | 1.2 | 1.0 | 1.0 | 0.8 | 1.0 | 1.2 | <0.001 |
| Thrombolysis | 1.5 | 1.1 | 1.1 | 1.6 | 1.9 | 2.0 | 2.4 | 3.2 | 3.0 | 2.1 | <0.001 |
| Bare-metal stent | 88.9 | 70.3 | 27.8 | 11.6 | 16 | 40.2 | 43.8 | 35.3 | 39.5 | 40.2 | <0.001 |
| Drug-eluting stent | 0.1 | 20.9 | 65.3 | 82 | 76.6 | 51 | 47.5 | 55.9 | 51 | 51.4 | <0.001 |
| Balloon angioplasty alone | 11.1 | 8.9 | 6.9 | 6.5 | 7.4 | 8.8 | 8.8 | 8.8 | 9.5 | 8.5 | <0.001 |
| In-hospital complications | |||||||||||
| Gastrointestinal bleeding | 4.5 | 4.1 | 3.4 | 3.4 | 3.6 | 3.0 | 3.2 | 2.9 | 2.9 | 3.4 | <0.001 |
| Acute cerebrovascular disease | 1.3 | 1.4 | 1.1 | 1.5 | 1.5 | 1.0 | 1.3 | 1.2 | 1.7 | 1.3 | 0.158 |
| Cardiogenic shock | 8.0 | 7.2 | 7.2 | 7.9 | 7.4 | 8.8 | 9.0 | 9.1 | 10.8 | 8.5 | <0.001 |
| Sepsis | <0.1 | 0.1 | 0.6 | 1.1 | 1.1 | 1.4 | 1.5 | 1.5 | 1.6 | 1.0 | <0.001 |
| In-hospital mortality | 7.1 | 6.2 | 6.5 | 6.2 | 5.7 | 6.2 | 6.4 | 6.1 | 6.5 | 6.3 | 0.022 |
| Mean length of stay in survivors (days) | 4.7 | 4.6 | 4.7 | 4.6 | 4.6 | 4.4 | 4.5 | 4.4 | 4.4 | 4.5 | <0.001 |
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