Interests: None  Paediatric Critical Care (PCC) supports the recovery of children with severe illness. Nationally, there are 30 PCC units with a total of approximately 400 beds. There is constant demand for these beds with an average five-day length of stay and admissions increasing at a greater rate than age-specific population growth[1, 2]. Prolonged stay patients account for approximately half of all PCC patient bed days[3]. Complex critically ill (CCI) patients need input from multiple different teams alongside support for their family [4, 5]. CCI patients often become prolonged PCC-stay patients too [6]. Internationally, there is variation in the definition of CCI patients [4, 8], this creates service variation and tensions around what resources can be provided and how quickly to support home discharge. Objective: The face of Paediatric Critical Care, in the UK and internationally has changed over the course of the last ten years with a growing cohort of complex critically ill patients. This integrative review aims to look at current nomenclature, definitions, and outcome measures of priority in this undefined patient population. Inclusion criteria: All types of studies looking at complex critically ill children (age<18 years) admitted to any paediatric intensive care unit (PICU). Methods: The review will be registered on Prospero. Medline, Embase, Maternity and Infant care, The Cochrane library, the Cumulative Index to Nursing and Allied Health literature (CINAHL), Web of Science, and NHS evidence will be searched from 2014 to April 2024. Search limits will include all languages, exclude the setting of neonatal intensive care and age>18 years old. Four independent authors will screen citations for eligible studies and perform data extraction. The final search strategy will be developed in Medline and peer-reviewed by a health research librarian not involved in the study. This will be then translated to other databases as appropriate.

The authors have declared no competing interest.

This study did not receive any funding

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Yes

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Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

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Yes

All data produced in the present study are available upon reasonable request to the authors