Responsiveness and the minimal clinically important difference of the Chelsea Critical Care Physical Assessment tool (CPAx) in critically ill, mechanically ventilated adults: a study protocol for a prospective, multicentre, cohort study.

introductionThe use of invasive life support in patients with a prolonged critical illness clearly saves lives but carries substantial risks, including intensive care unit-acquired weakness (ICUAW) and long-term disability. Early mobilisation might improve outcomes, yet the evidence is conflicting and complicated by the lack of a responsive outcome measurement to detect change in critically ill patients' physical function and activity. The Chelsea Critical Care Physical Assessment tool (CPAx) is a valid and reliable instrument for patients at risk of ICUAW. However, its ability to measure change over time (responsiveness) and the minimal clinically important difference (MCID) have not yet been rigorously investigated. METHODS AND ANALYSIS: The primary objective of this prospective, international, multicentre, longitudinal cohort study is to investigate responsiveness and to establish the MCID of the CPAx during the 'intensive care unit (ICU) period', from ICU baseline to ICU discharge, and 'hospital period', from ICU to hospital discharge. Adults with any critical illness who are mechanically ventilated for at least 72 hours, expected to remain in ICU (≥48 hours) and being treated by a physiotherapist are eligible for study inclusion. Functional measurements, including the CPAx and a global rating of change (GRC) scale, will be collected during routine physiotherapy. Responsiveness will be evaluated primarily using the GRC as an anchor to distinguish changed from unchanged/deteriorated patients (criterion validity). As such, the magnitude of change will be analysed with receiver operating characteristics. Additionally, construct validity will be explored with correlation coefficients and effect sizes to confirm/reject a priori formulated hypotheses. MCID will be investigated with anchor-based and distribution-based methods. We plan to recruit 120 patients across three sites in Australia and Switzerland. ETHICS AND DISSEMINATION: Ethical approval has been obtained from each local ethics committee (Canton of Bern, Switzerland (2024-00346), Monash Health, Australia (HREC/106143/MonH-2024-438474(v3)), the Alfred, Australia (490/24)). The results will be disseminated through international/national conferences, peer-reviewed journals and social media. The high quality, rigorous testing of the CPAx could benefit researchers, clinicians and patients. TRIAL REGISTRATION NUMBER: NCT06419699.