## 1 Introduction  
Emergency department (ED) visits in Spain have seen a significant increase, reaching 31.15 million in 2022. This figure represents the highest recorded since 2010, highlighting a growing demand for emergency healthcare services [1]. The sharp rise in ED visits has a ripple effect on various aspects of the healthcare system, including patient outcomes, the overall cost of care, and the workload of healthcare professionals [2,3]. These challenges underscore the importance of efficient time management in ensuring the delivery of high-quality care [4, 5, 6]. A crucial aspect of this growing demand is the impact on complex patients [2], a group that includes older adults, individuals with multiple comorbidities, and those reliant on mobility aids [7]. These patients face unique challenges in the ED, often experiencing longer waiting times and requiring more extensive resources to address their needs [7]. As a result, their presence contributes significantly to the congestion and operational strain of emergency departments [7]. Addressing the needs of such complex patient populations is essential for improving both individual outcomes and the overall efficiency of emergency care systems. Tailored interventions, designed specifically for these vulnerable groups, can help mitigate delays, optimize resource allocation, and ultimately enhance the quality of care provided in emergency settings [8]. ED overcrowding poses a significant and ongoing problem for healthcare systems worldwide. Length of stay, the time from patient arrival to discharge, is a key indicator of ED congestion. Increased LOS is linked to poorer patient outcomes, including higher rates of illness and death, and reduced satisfaction for both patients and staff [9, 10, 11]. Because extended LOS can lead to treatment delays, increased complications, and more patients leaving without being seen [2,12, 13], it serves as an indirect measure of quality of care, having healthcare systems, including Spain's, often aim to discharge or admit at least 90% of ED patients within 4 h [13]. Elderly patients (65 and older) are particularly vulnerable, with prolonged LOS increasing their risk of adverse events like delirium [14,15]. Factors such as care flows, patient characteristics (e.g., age, comorbidities, triage level), or the reason for consultation influence prolonged LOS [16, 17, 18]. Advanced age, high triage levels (II or III), arrival by stretcher or ambulance [19,20], chronic disease, hemodynamic instability, and functional deterioration all contribute to greater complexity in care and increased resource utilisation. Identifying and treating these patients as early as possible is crucial to mitigating delays and complications [21, 22, 23]. One common strategy to address ED overcrowding is the implementation of fast-track (FT) pathways, designed to expedite care for patients with less urgent needs, specific complaints, or defined care pathways [24,25]. Although the literature typically associates the concept of 'fast track' with the treatment of less complex patients, we propose a novel approach by introducing the idea of 'pre-admission' for more complex cases. While we acknowledge that this terminology might lead to confusion, as these patients often require longer stays, we chose the term 'fast track' to emphasise the fact that their pre-admission process is expedited, saving critical time for their recovery. While FTs have demonstrated success in improving overall ED throughput for low-acuity patients, their effectiveness for complex patients at high risk of prolonged LOS remains limited [26]. Existing FT protocols often lack the flexibility to adapt resource allocation to the specific needs of these patients, potentially leading to bottlenecks and suboptimal outcomes. Triage nurses, based on their clinical experience and initial assessment, can often identify patients at risk for prolonged LOS, considering factors such as the patient's overall appearance, level of distress, and perceived need for immediate intervention.