The Effect of Readmission to the ICU on 60-Day Hospital Mortality in Patients With and Without Frailty: A Binational Registry-Based Study

OBJECTIVES: Frailty and readmission to the ICU are common, and both are associated with worse outcomes. However, there is limited literature that assesses how frailty impacts those patients who require readmission to the ICU during a hospitalization. Therefore, we sought to assess whether the association between ICU readmission and death differ by frailty state. DESIGN: A registry-based study used the Australian and New Zealand Intensive Care Society Adult Patient Database. SETTING AND PATIENTS: All adult patients (age ≥ 18 yr) admitted to 203 ICUs in Australia and New Zealand between January 2017 and December 2022 with a documented Clinical Frailty Scale (frailty defined as a score ≥ 5) were included. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: The primary outcome was 60-day mortality. A Cox proportional hazards model, treating time to readmission as a time-dependent covariate and including an interaction term between frailty state and readmission, was used. Regression standardization was used to estimate absolute risk and risk differences, with 95% CIs calculated using a nonparametric bootstrap. A competing risk analysis was conducted, treating in-hospital death without ICU readmission as a competing risk. Secondary outcomes included length of hospital stay and discharge location. Six hundred fifteen thousand seven hundred nineteen ICU admission episodes were analyzed. Of the entire cohort, 19% (115,453) were frail, and 4.1% (25,329) were readmitted to the ICU. By day 60, 2.7% patients had died (16,353) in the hospital. Patients with frailty were at increased risk of both ICU readmission (subdistribution hazard ratio [SHR], 1.10; 95% CI, 1.07-1.14) and death without readmission (SHR, 2.83; 95% CI, 2.72-2.94). Observed 60-day mortality was greatest in frail, readmitted patients (22.7%). The standardized risk increase in 60-day mortality associated with ICU readmission was similar between patients with and without frailty (14.6% [95% CI, 13.7-15.6%] vs. 14.9% [95% CI, 13.4-16.6%]), respectively. CONCLUSIONS: This large, multicenter, retrospective study found that ICU readmission was associated with increased 60-day mortality in patients with and without frailty. Readmitted patients with frailty had the greatest risk of 60-day mortality; however, frailty state did not modify the incremental absolute risk of death relative to nonreadmitted patients.