Factors Associated with Invasive Mechanical Ventilation Requirement in Older ICU Patients: The Roles of Frailty, Comorbidity, and Acute Illness Severity.

Background/Objectives: Invasive mechanical ventilation (IMV) in older intensive care unit (ICU) patients represents a critical turning point in disease trajectory. While acute illness severity is a key determinant of IMV, the independent contributions of frailty and comorbidity remain uncertain. This study evaluated whether integrating geriatric domains with acute physiological parameters provides additional information about the risk of IMV requirement, beyond acute severity alone. Methods: In this single-center retrospective cohort study, patients aged ≥65 years admitted to a tertiary ICU between January 2024 and December 2025 were included. Variables obtained within the first 24 h included Clinical Frailty Scale (CFS), Charlson Comorbidity Index (CCI), Sequential Organ Failure Assessment (SOFA), Acute Physiology and Chronic Health Evaluation II (APACHE II), and laboratory parameters. The primary outcome was IMV requirement. Multivariable logistic regression models were constructed, and performance was assessed using discrimination (AUC) and calibration. Results: Among 285 patients (median age 77 years [IQR 71-86]), 161 (56.5%) required IMV. These patients had higher frailty, comorbidity burden, and illness severity on univariable comparison. In multivariable analysis, SOFA score showed the strongest independent association with IMV requirement (adjusted OR 1.91; 95% CI 1.63-2.23; p < 0.001), followed by serum lactate (adjusted OR 1.30; 95% CI 1.08-1.58; p = 0.006). Older age was independently associated with lower odds of IMV (adjusted OR 0.96 per year; 95% CI 0.92-1.00; p = 0.034). Neither frailty (CFS) nor comorbidity burden (CCI) remained independently associated after adjustment. The final SOFA- and APACHE II-based models demonstrated AUCs of 0.899 (95% CI 0.864-0.933) and 0.911 (95% CI 0.879-0.944), respectively. Conclusions: Acute organ dysfunction and early lactate remain the dominant determinants of IMV requirement in older ICU patients. Neither comorbidity burden nor frailty retained an independent association once acute severity was accounted for; instead, advancing age was independently associated with a lower likelihood of IMV, a finding that may reflect unmeasured treatment-limitation decisions and warrants further investigation. These findings suggest that frailty and comorbidity indices should not be used in isolation as triggers for ventilatory decision-making and instead support a framework centered on acute severity and perfusion markers, with age considered mainly as a marker of care-goal context.