Serum Calcium and Respiratory Mechanics as Independent Correlates of Extubation Beyond 24 Hours in Postoperative Surgical ICU Patients: A Two-Cohort Retrospective Study

Timely airway extubation is central to recovery in the surgical intensive care unit (SICU), but its objective correlates remain incompletely defined, and the role of metabolic factors such as calcium is unclear. This two-cohort retrospective study included 204 postoperative SICU patients in Taiwan and 6,016 patients from the Medical Information Mart for Intensive Care IV (MIMIC-IV) database, grouped by extubation within or beyond 24 hours, a local workflow benchmark. Candidate variables defined a priori and associated with the outcome in univariable analysis were entered into multivariable logistic regression. Multicollinearity was assessed with the variance inflation factor (VIF). Extubation beyond 24 hours occurred in 82 (40.2%) Taiwan and 4,599 (76.4%) MIMIC-IV patients. After adjustment, serum calcium and the rapid shallow breathing index (RSBI) were the only variables independently associated with the outcome in both cohorts, with odds ratios of 2.564 (95% CI 1.169–5.622) and 1.063 (95% CI 1.041–1.084) in Taiwan and 2.171 (95% CI 1.851–2.547) and 1.005 (95% CI 1.001–1.009) in MIMIC-IV, respectively. In MIMIC-IV, the calcium association was nonlinear (p < 0.001), with odds increasing across calcium quartiles in an exploratory model adjusted only for RSBI, so the per-unit odds ratio reflects an average trend. All VIF values were below 5. Calcium and RSBI were associated with the outcome in two cohorts with different surgical case mix. Because calcium was measured close to extubation, as total rather than ionized calcium, and showed a nonlinear association, these findings are hypothesis-generating and require prospective evaluation before clinical use.