Validation of the Rehabilitation Activity Time Score in Mechanically Ventilated Intensive Care Unit Patients

Introduction Quantifying the mobilization dose in the intensive care unit (ICU) remains a major challenge. Although the Mobilization Quantification Score (MQS) is a validated and widely used index, its calculation requires separate quantification of the time spent at multiple mobilization levels, making it complex and less feasible for routine clinical use. To address this limitation, we developed the Rehabilitation Activity Time score (RATs), which is a simple, time-based bedside tool designed to efficiently quantify the rehabilitation dose. This pilot study primarily aimed to evaluate the feasibility and convergent validity of RATs in mechanically ventilated ICU patients, and secondarily to explore their association with clinical outcomes. Methods This prospective single-center observational pilot study included adult ICU patients expected to require invasive mechanical ventilation for ≥48 hours and to receive rehabilitation as part of usual care. Daily RATs and MQS were recorded by physical therapists, and physical activity was continuously measured using a wrist-worn triaxial accelerometer (ActiGraph wGT3X-BT; Ametis, Pensacola, Florida). Feasibility outcomes included data completion rates, valid accelerometer wear days, and adverse events. Convergent validity among RATs, MQS, and accelerometer-derived activity metrics was assessed using Spearman's rank correlation. Exploratory analyses examined associations between activity measures and clinical outcomes. Results Twelve patients were screened, and 10 were enrolled (median age 68, IQR 61-81). Across 59 ICU patient-days, the RATs and MQS were recorded on 56 (95%) and 53 (90%) days, respectively, whereas accelerometry provided 55 valid days (83%). No device-related adverse events were observed. RATs was strongly correlated with the MQS (r=0.94) and accelerometer activity counts (r=0.76). Exploratory analyses indicated that higher mean RATs were associated with a higher discharge Barthel Index (r=0.56) and earlier ventilator liberation (median four vs. six days; Cliff delta =0.47). Conclusion The RATs demonstrated high feasibility and strong validity, reflecting objective activity levels while requiring minimal bedside effort. As a simple and reliable measure, the RATs may enable the standardized quantification of rehabilitation doses in the ICU and inform future dose-response research.