Persistent Critical Illness (PCI) is a condition that affects some patients who remain in the Intensive Care Unit (ICU) for a long time, usually more than 10-14 days. It is estimated to occur in 5-20% of critically ill patients. A recent Portuguese study found that more than 14% of ICU patients stayed longer than 14 days. PCI is often associated with ongoing need for life support, such as mechanical ventilation or medications to maintain blood pressure. However, patients may also experience severe muscle weakness, repeated infections, or other complications, which makes this group very diverse.

One of the main risk factors for prolonged ICU stay is sepsis, a severe infection that affects the whole body. Other factors-such as prior health conditions, use of corticosteroids, sedation practices, early versus late mobilization, fluid and antibiotic management, and delirium treatment-may also influence the development and course of PCI.

This study aims to identify different clinical patterns ("clusters") among critically ill patients who remain in the ICU for more than 10 days. Patients will be followed until hospital discharge, and up to one year if data are available. Understanding these different patterns will help develop more personalized and effective care strategies for each patient profile.

The study is a multicenter retrospective cohort including adult patients (≥18 years) admitted to participating ICUs for more than 5 days between 2021 and 2023. Data collected will include demographic, clinical, and laboratory information, details of organ support (such as mechanical ventilation or vasopressors), medications, nutrition, and rehabilitation practices.

Statistical and machine learning methods will be used to identify groups of patients with similar clinical trajectories and to assess how these groups are related to outcomes such as survival, recovery of organ function, or long-term disability.

Expected results are the identification of distinct clinical clusters of PCI that combine clinical and laboratory data, and the development of tailored management strategies to improve recovery and outcomes for patients with PCI.

Inclusion Criteria:

• Adult patients aged 18 years or older;
• ICU length of stay equal to or greater than 5 days.

Exclusion Criteria:

• Patients with an ICU stay < 5 days;
• Patients discharged from the ICU early due to lack of ward availability, rather than clinical recovery;
• Patients who do not survive the early phase of critical illness (i.e., early ICU deaths).

This study will include adult patients (≥18 years old) who are consecutively admitted to participating intensive care units (ICUs) and remain in the ICU for 5 or more days. The focus will be on patients who survive the early phase of critical illness, allowing the formation of a relatively homogeneous cohort of early ICU survivors.

The population will comprise patients with a wide range of critical illnesses, including sepsis, respiratory failure, cardiovascular instability, and multi-organ dysfunction, who require ongoing organ support such as invasive mechanical ventilation or vasopressors. Patients with prolonged ICU stays exceeding 10 days who continue to require organ support will be further characterized as having Persistent Critical Illness.

Individual participant data collected in this study will be made available to other researchers. All data will undergo a thorough anonymization process prior to sharing to prevent direct or indirect identification of participants. Each patient will be assigned an internal numeric code, consisting of two digits representing the center of origin and four sequential digits assigned by order of admission. No personal identifiers (e.g., name, national ID number, full date of birth, address) are collected.

The anonymized dataset will be accompanied by a data dictionary, describing each variable and its format, to allow full interpretation and reuse of the data. This approach ensures participant confidentiality and complies with ethical and legal requirements, while enabling secondary analyses, replication studies, or meta-analyses by other researchers.

The individual participant data and accompanying data dictionary will be made available after the completion of dat of interest for the study publication, starting December 1, 2028, and will remain accessible for years thereafter.

De-identified individual participant data and the corresponding data dictionary will be made available to qualified researchers whose proposed use of the data has been reviewed and approved by the study's coordinating center or ethics committee. Before sharing, all datasets will be verified, curated, and securely stored in accordance with applicable data protection regulations and current ethical and scientific recommendations.

Researchers will have access to fully anonymized datasets containing all study variables, excluding any directly or indirectly identifiable information. Data will be shared upon reasonable request through a secure, controlled-access repository, following submission of a research proposal and a data use agreement that ensures confidentiality and appropriate data use.

The anonymized dataset and related documentation will be deposited in a certified open research repository, such as Zenodo, Dryad, or Figshare, and will be assigned a Digital Object Identifier (DOI).