Feasibility and challenges of ambulance-delivered blood pressure lowering in hyperacute stroke

Introduction: Stroke trials conducted in prehospital settings face inherent complexities in patient screening, timely intervention delivery, and outcome assessment. The INTEnsive ambulance-delivered blood pressure Reduction in hyper-acute stroke Trial (INTERACT4) investigated the effect of prehospital blood pressure (BP) lowering initiated within 2 hours of symptom onset in suspected acute stroke across multiple sites in China. This process evaluation aimed to examine implementation outcomes and contextual mechanisms influencing delivery. Methods: A mixed-methods approach with a convergent parallel design. Quantitative data on fidelity, dose, and reach were obtained from case report forms and monitoring logs, including protocol deviations and BP target achievement. Qualitative data on acceptability, appropriateness, and adoption were collected through semi-structured interviews with ambulance and hospital staff from eight sampled hospitals, and a focus group discussion with project implementers. Data were analyzed descriptively and thematically, guided by the Medical Research Council complex intervention framework for process evaluation and normalization process theory. Results: A total of 34 participants (19 ambulance staff and 15 hospital doctors) across 8 sites were interviewed. Fidelity was generally acceptable, with nearly 90% of eligible patients receiving the first urapidil dose. Dose achievement was modest, with 31.7% of patients achieving the prehospital systolic BP target of <140 mmHg. Qualitative findings provided further explanation. Staff regarded the intervention as meaningful, safe, and compatible with workflows, and reported increasing proficiency through training and quality control. System-level differences were evident: in centralized dispatch systems, very short transfer times and frequent handovers with rotating junior physicians contributed to protocol deviations. Conversely, hospital-based systems, with stable doctor-nurse teams and longer transfer times, facilitated better adherence. Conclusions: BP lowering in the prehospital setting is feasible and acceptable, but fidelity varied by the ambulance model. Hospital-based systems achieved higher adherence, highlighting the role of system design and training in prehospital stroke interventions.