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Introduction
Patients who are critically ill, those who are in need of critical care, can be found all over hospitals. Some, but not all, receive care in ICUs (intensive care units). Medical specialties usually define themselves by organ system, disease process or procedure, however, critical care has struggled to define its identity. (1) 
 
Critical care medicine, when used synonymous to intensive care medicine, emerged as a result of multiple historical events. Knowledge regarding trauma, shock and infectious medicine grew as the World War II and the conflicts in Korea and Vietnam occurred. The poliomyelitis epidemic during the 1950s led to the development of invasive mechanical ventilation for patients with respiratory failure. Technological developments enabled critical care medicine to emerge and develop. (2) Today, critical care medicine is considered to be resource-intensive and the National Health Services in England spends approximately £1 billion annually. (3)
 
However, as initially alluded to, critical care can also be understood as something broader than critical care medicine or intensive care medicine. Critical care can be given to critically ill patients admitted to the ICU or wherever else there are patients who are critically ill, meanwhile critical care medicine or intensive care medicine is usually only received in the ICU.
 
A retrospective study by Halpern et al published in 2004 investigated the role, use and costs of critical care in the Unites States between 1985 and 2000. Critical care medicine (CCM) beds in the United States increased by 26.2% between 1985 and 2000, during the same period non-CCM beds decreased by 30.9%. (4) 
 
There is an absence of a consistent definition of critical care. Some countries focus on the capability to support failing organ systems, or on the ratio between nurses and patients, where other countries focus on the need for close monitoring. There is a need for a universal consensus regarding the definition regarding of what constitutes critical care. A universal definition would greatly benefit the increasing need for critical care research and clinical discussion. (5) 
 
Aim
The aim of this study is to operationally define critical care. The review will attempt to answer the question: What are the main elements of existing definitions of critical care and can these be homogenized to form a common definition?

Design
This scoping review will be conducted to examine how the literature defines critical care. The scoping review complemented by a thematic content analysis. A scoping review methodology is an approach to synthesizing available evidence and is particularly suited for questions not answerable by a systematic review because the scope is too broad, or when not much has been published on the topic. Both these conditions apply in this case. The review will be conducted using the Arksey and O’Malley framework and hence first relevant studies will be identified, second studies will be selected for inclusion, third data will be charted and finally, the data will be summarized. (6, 7) This protocol is structured following the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) checklist (8).

References
1. Adhikari N, Fowler R, Bhagwanjee S, Rubenfeld G. Critical care and the global burden of critical illness in adults. The Lancet. 2010;376(9749):1339-1346.
 
2. Kelley M. Critical Care Medicine — A New Specialty?. New England Journal of Medicine. 1988;318(24):1613-1617.
 
3. Amiruddin N, Prescott G, Coventry D, Jansen J. Evaluating service development in critical care: The impact of establishing a medical high dependency unit on intensive care unit workload, case mix, and mortality. Journal of the Intensive Care Society. 2018;19(3):226-235.

4. Halpern N, Pastores S, Greenstein R. Critical care medicine in the United States 1985–2000: An analysis of bed numbers, use, and costs*. Critical Care Medicine. 2004;32(6):1254-1259.
 
5. Wunsch H, Angus D, Harrison D, Collange O, Fowler R, Hoste E et al. Variation in critical care services across North America and Western Europe*. Critical Care Medicine. 2008;36(10):2787-e8.

6. Arksey H, O'Malley L. Scoping studies: towards a methodological framework. International Journal of Social Research Methodology. 2005;8(1):19-32.
  
7. Levac D, Colquhoun H, O'Brien K. Scoping studies: advancing the methodology. Implementation Science. 2010;5(1).
  
8. Tricco A, Lillie E, Zarin W, O'Brien K, Colquhoun H, Levac D et al. PRISMA Extension for Scoping Reviews (PRISMA-ScR): Checklist and Explanation. Annals of Internal Medicine. 2018;.
 


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SECTION: Protocol and registration
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SECTION: Eligibility criteria
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SECTION: Information sources
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SECTION: Search strategy
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SECTION: Selection of sources of evidence
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SECTION: Data charting process
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SECTION: Data items
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SECTION: Synthesis of results
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