Intensive care unit (ICU) acquired weakness is a common complication associated with long-term physical impairments in survivors of a critical illness. The Chelsea Critical Care Physical Assessment tool (CPAx) is a valid and reliable instrument for physical function and activity in critically ill patients at risk for muscle weakness. However, its ability to measure change over time (responsiveness) and the minimal clinically important difference (MCID) have not yet been rigorously investigated. This multi-centre, mixed-methods, longitudinal cohort study therefore aims to establish responsiveness and the MCID of the CPAx in the target population from ICU baseline to ICU and hospital discharge. The study uses routine data from standard physiotherapy sessions like mobility, function and activity with no additional burden for critically ill adults. The investigators expect the CPAx to be responsive allowing its use as a primary outcome in future effectiveness trials for the treatment of ICU-acquired weakness using the newly established MCID for sample size calculation. A high quality, rigorously tested measurement tool for physical function and activity in the ICU should benefit researchers, clinicians and patients.

The use of invasive life support in critically ill patients clearly saves lives but carries substantial risks, including intensive care unit (ICU) acquired weakness and long-term disability. The investigators urgently need a valid, reliable, and responsive measurement tool for this population to use in clinical practice and trials. The Chelsea Critical Care Physical Assessment tool (CPAx) is a promising measurement instrument to measure change in critically ill patients' physical function and activity. After several studies have confirmed its validity and excellent reliability, it is time to confirm responsiveness and to establish the MCID in a large, international sample of the target population. This multi-centre, mixed-methods, longitudinal cohort study will include critically ill, mechanically ventilated (>72h) adults at risk for muscle weakness and collect their mobility, physical function and activity with the CPAx and other relevant measures at ICU baseline, to ICU and hospital discharge. Responsiveness will be determined by the ability of the CPAx to identify change according to a prespecified anchor (criterion validity) and by testing prospective hypotheses about the expected magnitude of change between the CPAx and other relevant measures (construct validity). The MCID will be established with anchor- and distribution-based methods, whereby a seven-point global rating of change scale obtained from treating ICU physiotherapists will serve as anchor to distinguish improved from unchanged patients.

Inclusion Criteria:

• Age ≥ 18 years
• Mechanical ventilation ≥ 72 hours
• Expected to remain for ≥ 48 hours in the ICU
• Physiotherapy referral

Exclusion Criteria:

• Not expected to survive to hospital discharge (imminent to death)
• Second or subsequent ICU admission for this hospital stay
• Transfer from external ICU (with an ICU stay of >72 hours)
• Primary neurological admission diagnosis (i.e., of the central nervous system including stroke, intracerebral haemorrhage, traumatic brain injury)
• Known pregnancy
• Living in a care facility pre-admission (severe pre-existing mental or physical disability)
• Local regulations (i.e. Switzerland: refusal of general consent)

The target population are critically ill adults who are mechanically ventilated for a prolonged period (≥ 72 hours) and who have an increased risk for ICUAW and long-term impairment.

Solely anonymised main outcome data